Search query: Valley-polarized exciton dynamics in a 2D semiconductor heterostructure

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abstract: Stacking to prolong valley lifetime In the material MoSe2, which, like graphene, has a two dimensional honeycomb crystal lattice, the electronic structure has two "valleys." Electrons can be distinguished by the valley they reside in, making them act as potential information carriers. However, electrons easily lose this information by scattering into the other valley. Rivera et al. placed single layers of MoSe2 and WSe2 on top of each other and shone circularly polarized light on the structure. The light caused excitons pairs of electrons and holes to form so that the hole and electron came from the same valley but different layers. The valley specific character of such excitons persisted far longer than would be possible in a single layer of either material. Science, this issue p. 688 Photoluminescence measurements are used to deduce a valley lifetime of 40 nanoseconds in heterostructures of MoSe2 and WSe2. Heterostructures comprising different monolayer semiconductors provide an attractive setting for fundamental science and device technologies, such as in the emerging field of valleytronics. We realized valley specific interlayer excitons in monolayer WSe2 MoSe2 vertical heterostructures. We created interlayer exciton spin valley polarization by means of circularly polarized optical pumping and determined a valley lifetime of 40 nanoseconds. This long lived polarization enables the visualization of the expansion of a valley polarized exciton cloud over several micrometers. The spatial pattern of the polarization evolves into a ring with increasing exciton density, a manifestation of valley exciton exchange interactions. Our work introduces van der Waals heterostructures as a promising platform from which to study valley exciton physics.
author_names: Pasqual Rivera

Kyle L Seyler

Hongyi Yu

John R Schaibley

Jiaqiang Yan

David G Mandrus

Wang Yao

Xiaodong Xu
title: Valley polarized exciton dynamics in a 2D semiconductor heterostructure
venue: Science
year: 2016

abstract: Van der Waals heterobilayers based on 2D transition metal dichalcogenides have been recently shown to support robust and long lived valley polarization for potential valleytronic applications. However, the roles of the chemical composition and geometric alignment of the constituent layers in the underlying dynamics remain largely unexplored. Here we study spin valley relaxation dynamics in heterobilayers with different structures and optical properties engineered via the use of alloyed monolayer semiconductors. Through a combination of time resolved Kerr rotation spectroscopic measurements and theoretical modeling for Mo1 x W x Se2/WSe2 samples with different chemical compositions and stacking angles, we uncover the contributions of the interlayer exciton recombination and charge carrier spin depolarization to the overall valley dynamics. We show that the corresponding decay rates can be tuned in a wide range in transitions from a misaligned to an aligned structure, and from a hetero to a homo bilayer. Our results provide insights into the microscopic spin valley polarization mechanisms in van der Waals heterostructures for the development of future 2D valleytronic devices.
author_names: Vasily Kravtsov

Aleksey D Liubomirov

Roman V Cherbunin

Alessandro Catanzaro

Armando Genco

Daniel Gillard

Evgeny M Alexeev

Tatiana Yu Ivanova

Ekaterina Khestanova

Ivan Andreevich Shelykh

Alexander I Tartakovskii

M S Skolnick

Dmitry Krizhanovskii

Ivan V Iorsh
title: Spin valley dynamics in alloy based transition metal dichalcogenide heterobilayers
year: 2020

abstract: Photoinduced interfacial charge transfer is at the heart of many applications, including photovoltaics, photocatalysis, and photodetection. With the emergence of a new class of semiconductors, i.e. monolayer two dimensional transition metal dichalcogenides (2D TMDs) charge transfer at the 2D/2D heterojunctions has attracted several efforts due to the remarkable optical and electrical properties of 2D TMDs. Unfortunately, in 2D/2D heterojunctions, for a given combination of two materials, the relative energy band alignment and the charge transfer efficiency are locked. Due to their large variety and broad size tunability, semiconductor quantum dots (0D QDs) interfaced with 2D TMDs may become an attractive heterostructure for optoelectronic applications. Here, we incorporate femtosecond pump probe spectroscopy to reveal the sub 45 fs charge transfer at a 2D/0D heterostructure composed of tungsten disulfide monolayers (2D WS2) and a single layer of cadmium selenide/zinc sulfide core/shell 0D QDs. Furthermore, ultrafast dynamics and steady state measurements suggested that, following electron transfer from the 2D to the 0D, hybrid excitons, wherein the electron resides in the 0D and the hole resides in the 2D TMD monolayer, are formed with a binding energy on the order of ~140 meV, which is several times lower than that of tightly bound excitons in 2D TMDs.
author_names: Abdelaziz Boulesbaa

Kai Wang

Masoud Mahjouri-Samani

Mengkun Tian

Alexander A Puretzky

Ilia N Ivanov

Christopher M Rouleau

Kai Xiao

Bobby G Sumpter

David B Geohegan
title: Ultrafast Charge Transfer and Hybrid Exciton Formation in 2D/0D Heterostructures.
venue: Journal of the American Chemical Society
year: 2016

abstract: CdSe/CdTe core crown type II nanoplatelet heterostructures are two dimensional semiconductors that have attracted interest for use in light emitting technologies due to their ease of fabrication, outstanding emission yields, and tunable properties. Despite this, the exciton dynamics of these complex materials, and in particular how they are influenced by phonons, is not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperature resolved photoluminescence (PL) and temperature dependent structural measurements to investigate CdSe/CdTe nanoplatelets with a thickness of four monolayers. We show that charge transfer (CT) excitons across the CdSe/CdTe interface are formed on two distinct time scales: initially from an ultrafast ~70 fs) electron transfer and then on longer time scales ~5 ps) from the diffusion of domain excitons to the interface. We find that the CT excitons are influenced by an interfacial phonon mode at ~120 cm 1, which localizes them to the interface. Using low temperature PL spectroscopy we reveal that this same phonon mode is the dominant mechanism in broadening the CT PL. On cooling to 4 K, the total PL quantum yield reaches close to unity, with an ~85% contribution from CT emission and the remainder from an emissive sub band gap state. At room temperature, incomplete diffusion of domain excitons to the interface and scattering between CT excitons and phonons limit the PL quantum yield to ~50% Our results provide a detailed picture of the nature of exciton phonon interactions at the interfaces of 2D heterostructures and explain both the broad shape of the CT PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that to maximize the PL quantum yield both improved engineering of the interfacial crystal structure and diffusion of domain excitons to the interface, e.g. by altering the relative core/crown size, are required.
author_names: Rajal Pandya

Richard Y S Chen

Alexandre Cheminal

Marion Dufour

Johannes M Richter

Tudor H Thomas

Shahab Ahmed

Aditya Sadhanala

Edward P Booker

Giorgio Divitini

Felix Deschler

Neil C Greenham

Sandrine Ithurria

Akshay Rao
title: Exciton Phonon Interactions Govern Charge Transfer State Dynamics in CdSe/CdTe Two Dimensional Colloidal Heterostructures.
venue: Journal of the American Chemical Society
year: 2018

abstract: Semiconductor heterostructures provide a powerful platform to engineer the dynamics of excitons for fundamental and applied interests. However, the functionality of conventional semiconductor heterostructures is often limited by inefficient charge transfer across interfaces due to the interfacial imperfection caused by lattice mismatch. Here we demonstrate that MoS(2)/WS(2) heterostructures consisting of monolayer MoS(2) and WS(2) stacked in the vertical direction can enable equally efficient interlayer exciton relaxation regardless the epitaxy and orientation of the stacking. This is manifested by a similar 2 orders of magnitude decrease of photoluminescence intensity in both epitaxial and nonepitaxial MoS(2)/WS(2) heterostructures. Both heterostructures also show similarly improved absorption beyond the simple superimposition of the absorptions of monolayer MoS(2) and WS(2) Our result indicates that 2D heterostructures bear significant implications for the development of photonic devices, in particular those requesting efficient exciton separation and strong light absorption, such as solar cells, photodetectors, modulators, and photocatalysts. It also suggests that the simple stacking of dissimilar 2D materials with random orientations is a viable strategy to fabricate complex functional 2D heterostructures, which would show similar optical functionality as the counterpart with perfect epitaxy.
author_names: Yifei Yu

Shi Hu

Liqin Su

Lujun Huang

Yanzhao Liu

Zhenghe Jin

Alexander A Purezky

David B Geohegan

Ki Wook Kim

Yue-fei Zhang

Linyou Cao
title: Equally efficient interlayer exciton relaxation and improved absorption in epitaxial and nonepitaxial MoS2/WS2 heterostructures.
venue: Nano letters
year: 2015

abstract: Semiconductor heterostructures provide a powerful platform to engineer the dynamics of excitons for fundamental and applied interests. However, the functionality of conventional semiconductor heterostructures is often limited by inefficient charge transfer across interfaces due to the interfacial imperfection caused by lattice mismatch. Here we demonstrate that MoS2/WS2 heterostructures consisting of monolayer MoS2 and WS2 stacked in the vertical direction can enable equally efficient interlayer exciton relaxation regardless the epitaxy and orientation of the stacking. This is manifested by a similar 2 orders of magnitude decrease of photoluminescence intensity in both epitaxial and nonepitaxial MoS2/WS2 heterostructures. Both heterostructures also show similarly improved absorption beyond the simple superimposition of the absorptions of monolayer MoS2 and WS2. Our result indicates that 2D heterostructures bear significant implications for the development of photonic devices, in particular those requesting efficient exciton separation and strong light absorption, such as solar cells, photodetectors, modulators, and photocatalysts. It also suggests that the simple stacking of dissimilar 2D materials with random orientations is a viable strategy to fabricate complex functional 2D heterostructures, which would show similar optical functionality as the counterpart with perfect epitaxy.
author_names: Yifei Yu

Shi Hu

Liqin Su

Lujun Huang

Yanzhao Liu

Zhenghe Jin

Alexander A Purezky

David B Geohegan

Ki Wook Kim

Yong Zhang

Linyou Cao
title: Equally E ffi cient Interlayer Exciton Relaxation and Improved Absorption in Epitaxial and Nonepitaxial MoS 2 WS 2 Heterostructures
year: 2015

abstract: In recent years, the study of exciton dynamics in semiconductor heterostructures has advanced considerably due to its important applications in electronic industries. As excitons have larger binding energies in quantum wells, they can be observed easily at room temperature absorption spectra. However the theoretical study of excitons in quantum wells is a difficult area as the space quantization in a quantum well destroys the translational symmetry of the crystal in the direction perpendicular to the quantum well plane. In this project, processes like the generation, scattering and decay of excitons are theoretically examined. First, we extend the theory of excitons developed in 3D systems to Quasi 2D systems. Then we present an approach to calculate the generation rate of heavy hole exciton at various quantum well widths and depths. A Quasi 2D exciton phonon interaction operator applicable in quantum wells is derived and is used to calculate the transition rate of an electron hole pair to an exciton. We obtain an expression for the transition rate as a function of the quantum well width and binding energy. To obtain the binding energies of exciton at various well widths, we follow a method which uses the model of an exciton in a fractional dimensional space. The rate of exciton generation is plotted as a function of the well width for different values of aluminium concentration, x, in GaAs A1 x Ga 1 x As quantum wells. The rate of exciton generation increases from 5 x 10 8 s 1 to 1 x 10 10 s 1 when quantum well widths decreases from 200A to 25A, which agrees well with recent experimental results. Results indicate that the rate of exciton generation decreases sharply with quantum well width for the case of an infinite well, but the decrease becomes gradual for finite potential wells. Next we study the scattering of delocalised excitons due to acoustic phonons in quantum wells. We assume that the exciton scattering takes place due to exciton phonon interactions. The rates of HH exciton scattering are obtained in the range of about 2 x 10 11 s 1 to 20 x 10 12 s 1 at temperatures ranging from10K to 100K and for well widths varying from 25A to 200A. These results compare well with the experimental rates. At a given temperature our results indicate that the excitonic linewidths from wider wells are smaller than those from narrower wells. The results also predict scattering of the heavy hole (HH) exciton to be greater than the light hole (LH) exciton. We have also extended the study of scattering of an exciton to include its decay into an electron hole pair by absorbing an acoustic phonon. Our results show that at small well widths, the rate of LH exciton decay is apparently larger than that of HH decay, but the difference diminishes at large well widths. This is consistent with recent experimental measurements which show that the decay time for the LH excitons is twice that for the HH exciton. Next, we present the theory of Quasi 2D polarons in semiconductor quantum wells. We have derived analytical expressions for the energy gap shift and effective polaron mass in the small well width limit. The analytical results agree exactly with those obtained for an ideal 2D system in the limit of zero well width. For larger quantum well widths, we have numerically evaluated the values for the energy gap shift and effective mass of a polaron. The theory used for electrons interacting with phonons in quasi 2D systems is extended to study excitonic polarons in quantum wells. Numerical values for the energy gap shift and effective mass of an excitonic polaron in a GaAs A1 x Ga 1 x As system are obtained. We find that the polaronic effect is more pronounced for excitonic polarons, particularly in the case of the LH exciton in quantum wells of small widths. Our results also show the importance of the inclusion of the exciton phonon interaction in the calculation of the exciton ground state energy in semiconductor quantum wells. In conclusion, we have presented a study of the dynamics of excitons due to their interaction with acoustic and optical phonons in this thesis. All theories put forward here have been supported by known experimental results. This agreement validates the theory while offering an explanation for recent experiments. Hence we expect our results to have importance in the quantitative understanding of the present and future experimental work on semiconductor devices.
author_names: A Thilagam
title: Dynamics of excitons in semiconductor heterostructures
year: 1993

abstract: Molecular beam epitaxy is used to "spin engineer" an environment wherein quantum confined electronic states in a wide band gap II VI semiconductor quantum well (Zn1 xCdx Se) are strongly exchange coupled to systematic 2D distributions of localized spins (Mn2+ ions) Magneto optical spectroscopy of undoped structures demonstrates that such a scheme successfully produces well confined excitonic states whose Zeeman splitting in modest magnetic fields greatly exceeds the inhomogeneous line widths. In modulation doped structures, a combination of magneto transport and magneto optical measurements shows the formation of a "magnetic" two dimensional electron gas characterized by spin gaps which are much larger than Landau level gaps. This results in a novel quantum Hall system which can be highly spin polarized even at large filling factors. Time resolved Faraday/Kerr effect measurements in the Voigt geometry probe the electronic spin dynamics of the exciton/electron gas, revealing terahertz and gigahertz oscillations that originate from the coherent spin precession of electrons and local moments, respectively.
author_names: Ioulia Smorchkova

Frank S Flack

Nitin Samarth

James Makoto Kikkawa

Scott A Crooker

David D Awschalom
title: Spin transport and optically probed coherence in magnetic semiconductor heterostructures
year: 1998

abstract: We report on a comprehensive experimental analysis of the influence of the environment on the optical properties of monolayer WSe2. Different dielectric environmental configurations with hBN as buffer and encapsulant were studied at room temperature and 10 K using time integrated and time resolved photoluminescence spectroscopy. Furthermore, Raman signal was evaluated to highlight the influence of the surroundings on the monolayer system. Here, linewidth and intensity improvement for 2D excitons as a consequence of hBN encapsulation is demonstrated, and a change of strain and doping levels in our produced structures are observed. The increase of the exciton radius for hBN buffered samples in comparison to WSe2 on bare substrates is indicated via an exciton exciton annihilation study, while hBN as buffer is found to generally shorten the effective radiative lifetime. Furthermore, we report that trions experience a weaker interaction with the WSe2 phonons than excitons, while the coupling to phonons is consistently decreased when WSe2 is capped by hBN. Ultimately, our energy resolved analysis of the dynamics at 10 K shows that the individual excitonic modes exhibit different photoluminescence decay times. A comparison of the different hBN WSe2 configurations shows that the shortest measurable time constants, which are on the ps scale, generally increase when buffering or encapsulating the 2D semiconductor, while sandwiching caused the strongest lifetime increase on these short time scales. In contrast, our monolayer monolayer heterostructures indicate fast charge/energy transfer <1 ps) from MoSe2 to WSe2 and a comparably slow radiative recombination of the type I A excitons depending on the tilt angle between the two lattices.
author_names: Lorenz Maximilian Schneider

Sina Lippert

Jan Kuhnert

Obafunso A Ajayi

Dylan L Renaud

Saleh Firoozabadi

Quyet T Ngo

R Guo

Young Jung R Kim

Wolfram Heimbrodt

James C Hone

Arash Rahimi-Iman
title: The influence of the environment on monolayer tungsten diselenide photoluminescence
year: 2017

abstract: 1 Electronic Structure of Abrupt Heterojunctions. 1.1 Band Diagrams of Heterostructures. 1.2 k p Model for Heterojunctions. 1.3 Shallow Electronic States at Heterojunctions. 1.4 Field Induced Interface States. 1.5 Intervalley Mixing at Heterojunctions. 1.6 Numerical Methods for the Description of Heterostructures 27 Bibliography Notes. 2 Electrons in Low Dimensional Structures. 2.1 Formation of Confined Electronic States. 2.2 Electronic States in Quantum Wells, Wires, and Dots. 2.3 Self Consistent Electronic States in Quantum Wells. 2.4 More Complex Quantum Wells. 2.5 Mixing of Hole States in Heterostructures. 2.6 Multiband k p Approximation. 2.7 Electronic States in the Presence of Strain. Bibliography Notes. 3 Tunneling in Heterostructures. 3.1 Tunneling Transmission. 3.2 Tunnel Coupled Levels. 3.3 Superlattices. 3.4 Superlattices Formed by States of Different Origin. Bibliography Notes. 4 Impurity States and Excitons in Heterostructures. 4.1 Electron Localization on Imperfections. 4.2 Impurity States in Quantum Wells. 4.3 Quantum Well Excitons. 4.4 Excitons in Other Heterostructures. Bibliography Notes. 5 Interband Optical Transitions in Heterostructures. 5.1 Absorption of Light by a 2D Layer. 5.2 Polarization Dependence of the Interband Transitions. 5.3 Interband Absorption Spectra in Heterostructures. 5.4 Excitonic Absorption. 5.5 Electrooptics of Heterostructures. 5.6 Modulation Spectroscopy of Heterostructures. Bibliography Notes. 6 Radiative Processes in Heterostructures. 6.1 Theory of Luminescence in 2D Systems. 6.2 Spectral and Polarization Dependencies of Luminescence. 6.3 Luminescence from Complex Heterostructures. 6.4 Radiative Recombination. Bibliography Notes. 7 Scattering of Light on Low Dimensional Electrons. 7.1 Scattering Cross Section. 7.2 Raman Spectroscopy of Intersubband Excitations. 7.3 Scattering on Collective Electronic Excitations. Bibliography Notes. 8 Intersubband Optical Transitions. 8.1 Resonant Transitions and Excitation into the Continuum. 8.2 Intersubband Transitions for In Plane Electric Field. 8.3 Depolarization Shift and Coulomb Renormalization. 8.4 Submillimeter Intersubband Transitions. 8.5 Radiative Intraband Transitions in Heterostructures. Bibliography Notes. 9 Nonlinear Optics of Heterostructures. 9.1 Nonlinear Response. 9.2 Nonlinear Susceptibilities. 9.3 Photoelectric Phenomena. 9.4 Nonlinearities Induced by Electron Hole Pairs. Bibliography Notes. 10 Ultrafast Processes in Heterostructures. 10.1 Ultrafast Optical Excitation. 10.2 Carrier Relaxation Processes. 10.3 Coherent Optics of Heterostructures. 10.4 Ultrafast Charge Dynamics in Heterostructures. Bibliography Notes. 11 Heterostructure Based Optoelectronic Devices. 11.1 Heterostructure Lasers. 11.2 Electrooptic Modulators. 11.3 Photodetectors. 11.4 Intersubband Optoelectronic Devices. 11.5 Optical Characterization of Heterostructures. Bibliography Notes. A k p Method for Bulk Semiconductors. A.I Nondegenerate Band. A.2 Two Band Model. A.3 Luttinger Model. A.4 Kane Model. A.5 Effects of External Fields. A.6 Effects of Deformation. Bibliography Notes. B Electromagnetic Waves in Layered Media. B.I Modes in a Layered Medium. B.2 Second Quantization of the Field. Bibliography Notes. C Kinetic Equations for Electrons and Photons. C.I Kinetic Equations Approach. C.2 Wigner Function for Photons. C.3 Electron Response. C.4 Conductivity and Generation Rate. Bibliography Notes. D Coulomb Effects in Heterostructures. D.I Mean Field Treatment of Coulomb Effects. D.2 Matrix Elements for 3D, 2D, and ID States. D.3 Intraband Density Matrix Equations. D.4 Semiconductor Bloch Equations. D.5 Beyond the Mean Field Approximation. Bibliography Notes. References.
author_names: F T Vas'ko

Alexander V Kuznetsov
title: Electronic states and optical transitions in semiconductor heterostructures
year: 1999