arXiv subjects

Diana Y. Qiu

Publications and source records attributed to Diana Y. Qiu.

51 records · Page 3Linked to original sources

arXiv2020

Strongly-bound excitons and trions in anisotropic 2D semiconductors

Monolayer and few-layer phosphorene are anisotropic quasi-two-dimensional (quasi-2D) van der Waals (vdW) semiconductors with a linear-dichroic light-matter interaction and a widely-tunable direct-band gap in the infrared frequency range. Despite recent theoretical predictions of strongly-bound excitons with unique properties, it remains experimentally challenging to probe the excitonic quasiparticles due to the severe oxidation during device fabrication. In this study, we report observation of strongly-bound excitons and trions with highly-anisotropic optical properties in intrinsic bilayer phosphorene, which are protected from oxidation by encapsulation with hexagonal boron nitride (hBN), in a field-effect transistor (FET) geometry. Reflection contrast and photoluminescence spectroscopy clearly reveal the linear-dichroic optical spectra from anisotropic excitons and trions in the hBN-encapsulated bilayer phosphorene. The optical resonances from the exciton Rydberg series indicate that the neutral exciton binding energy is over 100 meV even with the dielectric screening from hBN. The electrostatic injection of free holes enables an additional optical resonance from a positive trion (charged exciton) ~ 30 meV below the optical bandgap of the charge-neutral system. Our work shows exciting possibilities for monolayer and few-layer phosphorene as a platform to explore many-body physics and novel photonics and optoelectronics based on strongly-bound excitons with two-fold anisotropy.

cond-mat.mes-hall↗

arXiv2020

Bright excitons with negative-mass electrons

Bound electron-hole excitonic states are generally not expected to form with charges of negative effective mass. We identify such excitons in a single layer of the semiconductor WSe2, where they give rise to narrow-band upconverted photoluminescence in the UV, at an energy of 1.66 eV above the first band-edge excitonic transition. Negative band curvature and strong electron-phonon coupling result in a cascaded phonon progression with equidistant peaks in the photoluminescence spectrum, resolvable to ninth order. Ab initio GW-BSE calculations with full electron-hole correlations unmask and explain the admixture of upper conduction-band states to this complex many-body excitation: an optically bright, bound exciton in resonance with the semiconductor continuum. This exciton is responsible for atomic-like quantum-interference phenomena such as electromagnetically induced transparency. Since band curvature can be tuned by pressure or strain, synthesis of exotic quasiparticles such as flat-band excitons with infinite reduced mass becomes feasible.

cond-mat.mes-hall↗

arXiv2020

Comparison of GW band structure to semi-empirical approach for an FeSe monolayer

We present the G$_0$W$_0$ band structure, core levels, and deformation potential of monolayer FeSe in the paramagnetic phase based on a starting mean field of the Kohn Sham density functional theory (DFT) with the PBE functional. We find the GW correction increases the bandwidth of the states forming the $M$ pocket near the Fermi energy, while leaving the $Γ$ pocket roughly unchanged. We then compare the G$_0$W$_0$ quasiparticle band energies with the band structure from a simple empirical +A approach, which was recently proposed to capture the renormalization of the electron-phonon interaction going beyond DFT in FeSe, when used as a starting point in density functional perturbation theory (DFPT). We show that this empirical correction succeeds in approximating the GW non-local and dynamical self energy in monolayer FeSe and reproduces the GW band structure near the Fermi surface, the core energy levels, and the deformation potential (electron-phonon coupling).

cond-mat.mtrl-sci↗

arXiv2020

Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides with experiment and theory

Chalcogen vacancies are considered to be the most abundant point defects in two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors, and predicted to result in deep in-gap states (IGS). As a result, important features in the optical response of 2D-TMDs have typically been attributed to chalcogen vacancies, with indirect support from Transmission Electron Microscopy (TEM) and Scanning Tunneling Microscopy (STM) images. However, TEM imaging measurements do not provide direct access to the electronic structure of individual defects; and while Scanning Tunneling Spectroscopy (STS) is a direct probe of local electronic structure, the interpretation of the chemical nature of atomically-resolved STM images of point defects in 2D-TMDs can be ambiguous. As a result, the assignment of point defects as vacancies or substitutional atoms of different kinds in 2D-TMDs, and their influence on their electronic properties, has been inconsistent and lacks consensus. Here, we combine low-temperature non-contact atomic force microscopy (nc-AFM), STS, and state-of-the-art ab initio density functional theory (DFT) and GW calculations to determine both the structure and electronic properties of the most abundant individual chalcogen-site defects common to 2D-TMDs. Surprisingly, we observe no IGS for any of the chalcogen defects probed. Our results and analysis strongly suggest that the common chalcogen defects in our 2D-TMDs, prepared and measured in standard environments, are substitutional oxygen rather than vacancies.

cond-mat.mtrl-sci↗

arXiv2020

The bright side of defects in MoS$_2$ and WS$_2$ and a generalizable chemical treatment protocol for defect passivation

Structural defects are widely regarded as detrimental to the optoelectronic properties of monolayer transition metal dichalcogenides, leading to concerted efforts to eliminate defects via improved materials growth or post-growth passivation. Here, using steady-state and ultrafast optical spectroscopy, supported by ab initio calculations, we demonstrate that sulfur vacancy defects act as exciton traps. Current chemical treatments do not passivate these sites, leading to decreased mobility and trap-limited photoluminescence. We present a generalizable treatment protocol based on the use of passivating agents such as thiols or sulfides in combination with a Lewis acid to passivate sulfur vacancies in monolayer MoS$_2$ and WS$_2$, increasing photoluminescence up to 275 fold, while maintaining mobilities. Our findings suggest a route for simple and rational defect engineering strategies, where the passivating agent varies the electronic properties, thereby allowing the design of new heterostructures.

cond-mat.mtrl-sci↗

arXiv2019

Large spin-orbit splitting of deep in-gap defect states of engineered sulfur vacancies in monolayer WS2

Structural defects in 2D materials offer an effective way to engineer new material functionalities beyond conventional doping in semiconductors. Specifically, deep in-gap defect states of chalcogen vacancies have been associated with intriguing phenomena in monolayer transition metal dichalcogenides (TMDs). Here, we report the direct experimental correlation of the atomic and electronic structure of a sulfur vacancy in monolayer WS2 by a combination of CO-tip noncontact atomic force microscopy (nc-AFM) and scanning tunneling microscopy (STM). Sulfur vacancies, which are absent in as-grown samples, were deliberately created by annealing in vacuum. Two energetically narrow unoccupied defect states of the vacancy provide a unique fingerprint of this defect. Direct imaging of the defect orbitals by STM and state-of-the-art GW calculations reveal that the large splitting of 252 meV between these defect states is induced by spin-orbit coupling. The controllable incorporation and potential decoration of chalcogen vacancies provide a new route to tailor the optical, catalytic and magnetic properties of TMDs.

cond-mat.mtrl-sci↗

arXiv2018

Defect-induced modification of low-lying excitons and valley selectivity in monolayer transition metal dichalcogenides

We study the effect of point-defect chalcogen vacancies on the optical properties of monolayer transition metal dichalcogenides using ab initio GW and Bethe-Salpeter equation calculations. We find that chalcogen vacancies introduce unoccupied in-gap states and occupied resonant defect states within the quasiparticle continuum of the valence band. These defect states give rise to a number of strongly-bound defect excitons and hybridize with excitons of the pristine system, reducing the valley-selective circular dichroism. Our results suggest a pathway to tune spin-valley polarization and other optical properties through defect engineering.

cond-mat.mtrl-sci↗

arXiv2018

Orbital symmetry and the optical response of single-layer monochalcogenides

The absorption spectra of single-layer GaSe and GaTe in the hexagonal phase feature exciton peaks with distinct polarization selectivity. We investigate these distinct features from first-principle calculations using the GW-BSE formalisms. We show that the brightness of the exciton absorption peaks is tunable with the polarization of the light. Due to the symmetry of the bands under z-axis mirror symmetry, the bound exciton states selectively couple to either in-plane or out-of-plane polarization of the light. In particular, for a p-polarized light absorption experiment, the absorption peaks of the s-like excitons emerge at large angle of incidence, while the overall absorbance reduces over the rest of the spectrum.

cond-mat.mtrl-sci↗

arXiv2017

Gate Switchable Transport and Optical Anisotropy in 90° Twisted Bilayer Black Phosphorus

Anisotropy describes the directional dependence of a material's properties such as transport and optical response. In conventional bulk materials, anisotropy is intrinsically related to the crystal structure, and thus not tunable by the gating techniques used in modern electronics. Here we show that, in bilayer black phosphorus with an interlayer twist angle of 90°, the anisotropy of its electronic structure and optical transitions is tunable by gating. Using first-principles calculations, we predict that a laboratory-accessible gate voltage can induce a hole effective mass that is 30 times larger along one Cartesian axis than along the other axis, and the two axes can be exchanged by flipping the sign of the gate voltage. This gate-controllable band structure also leads to a switchable optical linear dichroism, where the polarization of the lowest-energy optical transitions (absorption or luminescence) is tunable by gating. Thus, anisotropy is a tunable degree of freedom in twisted bilayer black phosphorus.

cond-mat.mtrl-sci↗

arXiv2016

Non-uniform sampling schemes of the Brillouin zone for many-electron perturbation-theory calculations in reduced dimensionality

First principles calculations based on many-electron perturbation theory methods, such as the \textit{ab initio} GW and GW plus Bethe-Salpeter equation (GW-BSE) approach, are reliable ways to predict quasiparticle and optical properties of materials, respectively. However, these methods involve more care in treating the electron-electron interaction and are considerably more computationally demanding when applied to systems with reduced dimensionality, since the electronic confinement leads a slower convergence of sums over the Brillouin zone due to a much more complicated screening environment that manifests in the "head" and "neck" elements of the dielectric matrix. Here, we present two new schemes to sample the Brillouin zone for GW and GW-BSE calculations: the non-uniform neck subsampling method and the clustered sampling interpolation method, which can respectively be used for a family of single-particle problems, such as GW calculations, and for problems involving the scattering of two-particle states, such as when solving the BSE. We tested these methods on several few-layer semiconductors and graphene and show that they perform a much more efficient sampling of the Brillouin zone and yield two to three orders of magnitude reduction in the computer time. These two methods can be readily incorporated into several \textit{ab initio} packages that compute electronic and optical properties through the GW and GW-BSE approaches.

cond-mat.mtrl-sci↗

arXiv2016

Screening and Many-Body Effects in Two-Dimensional Crystals: Monolayer MoS$_{2}$

We present a systematic study of the variables affecting the electronic and optical properties of two-dimensional(2D) crystals within \textit{ab initio} GW and GW plus Bethe Salpeter Equation (GW-BSE) calculations. As a prototypical 2D transition metal dichalcogenide material, we focus our study on monolayer MoS${}_2$. We find that the reported variations in GW-BSE results in the literature for monolayer MoS${}_2$ and related systems arise from different treatments of the long-range Coulomb interaction in supercell calculations and convergence of k-grid sampling and cutoffs for various quantities such as the dielectric screening. In particular, the quasi-2D nature of the system gives rise to fast spatial variations in the screening environment, which are computationally challenging to resolve. We also show that common numerical treatments to remove the divergence in the Coulomb interaction can shift the exciton continuum leading to false convergence with respect to k-point sampling. Our findings apply to GW-BSE calculations on any low-dimensional semiconductors.

cond-mat.mtrl-sci↗

arXiv2016

Direct Observation of Layer-Dependent Electronic Structure in Phosphorene

Phosphorene, a single atomic layer of black phosphorus, has recently emerged as a new twodimensional (2D) material that holds promise for electronic and photonic technology. Here we experimentally demonstrate that the electronic structure of few-layer phosphorene varies significantly with the number of layers, in good agreement with theoretical predictions. The interband optical transitions cover a wide, technologically important spectrum range from visible to mid-infrared. In addition, we observe strong photoluminescence in few-layer phosphorene at energies that match well with the absorption edge, indicating they are direct bandgap semiconductors. The strongly layer-dependent electronic structure of phosphorene, in combination with its high electrical mobility, gives it distinct advantages over other twodimensional materials in electronic and opto-electronic applications.

cond-mat.mtrl-sci↗

arXiv2015

Probing the Role of Interlayer Coupling and Coulomb Interactions on Electronic Structure in Few-Layer MoSe2 Nanostructures

Despite the weak nature of interlayer forces in transition metal dichalcogenide (TMD) materials, their properties are highly dependent on the number of layers in the few-layer two-dimensional (2D) limit. Here, we present a combined scanning tunneling microscopy/spectroscopy and GW theoretical study of the electronic structure of high quality single- and few-layer MoSe2 grown on bilayer graphene. We find that the electronic (quasiparticle) bandgap, a fundamental parameter for transport and optical phenomena, decreases by nearly one electronvolt when going from one layer to three due to interlayer coupling and screening effects. Our results paint a clear picture of the evolution of the electronic wave function hybridization in the valleys of both the valence and conduction bands as the number of layers is changed. This demonstrates the importance of layer number and electron-electron interactions on van der Waals heterostructures, and helps to clarify how their electronic properties might be tuned in future 2D nanodevices.

cond-mat.mes-hall↗

arXiv2014

Observation of giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor

Two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibit novel electrical and optical properties and are emerging as a new platform for exploring 2D semiconductor physics. Reduced screening in 2D results in dramatically enhanced electron-electron interactions, which have been predicted to generate giant bandgap renormalization and excitonic effects. Currently, however, there is little direct experimental confirmation of such many-body effects in these materials. Here we present an experimental observation of extraordinarily large exciton binding energy in a 2D semiconducting TMD. We accomplished this by determining the single-particle electronic bandgap of single-layer MoSe2 via scanning tunneling spectroscopy (STS), as well as the two-particle exciton transition energy via photoluminescence spectroscopy (PL). These quantities yield an exciton binding energy of 0.55 eV for monolayer MoSe2, a value that is orders of magnitude larger than what is seen in conventional 3D semiconductors. This finding is corroborated by our ab initio GW and Bethe Salpeter equation calculations, which include electron correlation effects. The renormalized bandgap and large exciton binding observed here will have a profound impact on electronic and optoelectronic device technologies based on single-layer semiconducting TMDs.

cond-mat.mes-hall↗

arXiv2013

Optical Spectrum of MoS${}_2$: Many-body Effects and Diversity of Exciton States

We present first-principles calculations of the optical response of monolayer molybdenum disulfide employing the GW-Bethe Salpeter equation (GW-BSE) approach including self-energy, excitonic, and electron-phonon effects. We show that monolayer MoS2 possesses a large and diverse number of strongly bound excitonic states with novel k-space characteristics that were not previously seen experimentally or theoretically. The absorption spectrum is shown to be dominated by excitonic states with binding energy close to $1$~eV and by strong electron-phonon broadening in the visible to ultra-violet range. Our results explain recent experimental measurements and resolve inconsistencies between previous GW-BSE calculations.

cond-mat.mtrl-sci↗