Yia Chang

• Academia Sinica

We present a Semiempirical Pseudopotential Method for accurately computing the band structures and Bloch states of monolayer transition-metal dichalcogenides (TMDCs), including MoS2, MoSe2, WS2, and WSe2. Our approach combines local and non-local pseudopotentials, carefully fitted to replicate fully self-consistent density-functional theory results...

The Semiempirical Pseudopotential Method (SEPM) has emerged as a valuable tool for accurately determining band structures, especially in the realm of low-dimensional materials. SEPM operates by utilizing atomic pseudopotentials, which are derived from DFT calculations. SEPM calculations offer a unique advantage compared to DFT as they eliminate the...

The physics of nonlinear optical materials is incredibly versatile, with the design of novel materials and structures offering numerous degrees of freedom. Nevertheless, weak inherent nonlinearity of conventional optical materials continues to hinder the progress of a number of important applications. In this study, we delve into the realm of broad...

The Rabi-splitting energy represents the strength of light–matter interaction. This quantity is a good benchmark for evaluating the performance of light-modulation devices. Herein, we adopt ZnO microrods as microcavities for whispering gallery modes and propose a convenient method for estimating the light–matter coupling strength based on the shift...

We implemented a semi-empirical pseudopotential (SEP) method for calculating the band structures of graphene and graphene nanoribbons. The basis functions adopted are two-dimensional plane waves multiplied by several B-spline functions along the perpendicular direction. The SEP includes both local and non-local terms, which were parametrized to fit...

We have developed a highly efficient computation method based on density functional theory (DFT) within a set of fully symmetrized basis functions for the C60 buckyball, which possesses the icosahedral (Ih) point-group symmetry with 120 symmetry operations. We demonstrate that our approach is much more efficient than the conventional approach based...

The soft and polar nature of quasi‐2D (PEA)2PbBr4 perovskite, and robust photo‐generated excitons lead exciton‐polaritons and exciton‐polarons as the important phenomena near the band edge for application in the lighting aspect. In this work, a convenient methodology is proposed based on the polariton resonant modes in temperature‐dependent (77 K t...

We propose a mechanism to explain the quantum anomalous Hall (QAH) effect and the electric-field-induced topological phase transition in AB-stacked MoTe2/WSe2 moiré heterobilayers at ν=1 hole filling. We suggest that the Chern band of the QAH state is generated from an intrinsic band inversion composed of the highest two moiré hole bands with oppos...

In this work, we investigated both spontaneous and stimulated whispering gallery mode (WGM) emissions of 2 mol.% Li+ doped ZnO (Li-ZnO) microspheres with different sizes under 325 nm, and 488 nm wavelength laser excitations respectively. It was found that all the microspheres exhibit stimulated emissions under a visible laser excitation source of 4...

The transport and thermoelectric properties of finite textured graphene nanoribbons (t-GNRs) connected to electrodes with various coupling strengths are theoretically studied in the framework of the tight-binding model and Green’s function approach. Due to quantum constriction induced by the indented edges, such t-GNRs behave as serially coupled gr...

Transport and thermoelectric properties of finite textured graphene nanoribbons (t-GNRs) connected to electrodes with various coupling strengths are theoretically studied in the framework of the tight-binding model and Green's function approach. Due to quantum constriction induced by the indented edges, such t-GNRs behave like serially-coupled grap...

In this work, we present a theoretical method to study the effect of magnetic field on trions in two-dimensional materials. The trion is modeled by a three-particle Schrödinger equation and the magnetic-field interaction is included by means of a vector potential in symmetric gauge. By using a coordinate transformation and a unitary transformation,...

This work reports comprehensive theoretical modeling of photocurrent spectra generated by an In0.91Ga0.09N/ In0.4Ga0.6N disk-in-wire photodiode. The strain distribution is calculated by the valence-force-field model, while a realistic band structure of the InxGa1−xN/InyGa1−yN heterostructure is incorporated using an eight-band effective bond-orbita...

In this letter, we propose a new mechanism to explain the unexpected quantum anomalous Hall (QAH) effect and the electric-field-induced Mott-to-QAH phase transition without bulk gap closure in AB-stacked MoTe2/WSe2 moire heterobilayers. We suggest that a hole-occupied band carrying a non-zero Chern number can be generated by an intrinsic band inver...

Heterojunctions made by laterally stitching two different transition metal dichalcogenide monolayers create a unique one-dimensional boundary with intriguing local optical properties that can only be characterized by nanoscale-spatial-resolution spectral tools. Here, we use near-field photoluminescence (NF-PL) to reveal the narrowest region (105 nm...

Through polarized light interacting with samples, imaging ellipsometry is capable of aiding in the study of semitransparent biological cells microscopically; it is also possible to find applications in marker-free nondestructive disease diagnosis. Often a living biological cell is sensitive to environmental conditions, and fast measurement is prefe...

We have implemented a total-energy minimization scheme to allow for relaxation of atomic positions in density functional calculations for two-dimensional (2D) materials using a mixed basis set. The basis functions consist of products of 2D plane waves in the plane of the material and localized B-splines along the perpendicular direction. By using t...

Broadband perfect absorbers in the visible region have attracted considerable attention in many fields, especially in solar thermophotovoltaic and energy harvesting systems. However, developing light absorbers with high absorptivity, thermal stability, and a broad bandwidth remains a great challenge. Here, we theoretically and experimentally demons...

The CsPbBr3 microwires with unique isosceles right triangle cross-sections are commonly observed via chemical vapor deposition method. In this work, we study the correlations between measured multi-mode lasing behaviors and the simulation of the mode patterns inside the triangular-rod microcavity. We confirm that lasing action with higher-order tra...

Exciton polaron is a hypothetical many-body quasiparticle that involves an exciton dressed with a polarized electron-hole cloud in the Fermi sea. It has been evoked to explain the excitonic spectra of charged monolayer transition metal dichalcogenides, but the studies were limited to the ground state. Here we measure the reflection and photolumines...

In this work, we provide a detailed derivation of Foldy-Wouthuysen (FW) transformation for two-dimensional (2D) gapped Dirac fermions under external fields and apply the formalism to study valley excitons in 2D semiconducting materials. Similar to relativistic quantum few-body problem, the gapped Dirac equation can be transformed into a Schr\"{o}di...

In this work, trions in two-dimensional (2D) space are studied by the variational method with trial wavefunctions being constructed by 2D slater-type orbitals. Via this method, trion energy levels and wavefunctions can be calculated efficiently with fairly good accuracy. We first apply this method to study trion energy levels in a 2D hydrogen-like...

Moiré superlattices formed by van der Waals materials can support a wide range of electronic phases, including Mott insulators1–4, superconductors5–10 and generalized Wigner crystals2. When excitons are confined by a moiré superlattice, a new class of exciton emerges, which holds promise for realizing artificial excitonic crystals and quantum optic...

This work reports comprehensive theoretical modeling of photocurrent spectra generated by an In$_{0.91}$Ga$_{0.09}$N/In$_{0.4}$Ga$_{0.6}$N disk-in-wire photodiode. The strain distribution is calculated by valence-force-field (VFF) model, while a realistic band structure of the InN/InGaN heterostructure is incorporated using an eight-band effective...

Photoluminescence (PL) spectra of high-Q ZnO microspheres of various degrees of inhomogeneity are studied. Multiple sharp resonance features with distinct patterns associated with whispering gallery modes (WGM) of these microspheres are observed. Relative WGM peak strengths in these PL spectra are found to be closely related to defects and the surf...

We report the direct observation of intervalley exciton between the Q conduction valley and $\Gamma$ valence valley in bilayer WSe$_2$ by photoluminescence. The Q$\Gamma$ exciton lies at ~18 meV below the QK exciton and dominates the luminescence of bilayer WSe$_2$. By measuring the exciton spectra at gate-tunable electric field, we reveal differen...

We resolved some controversial issues on the Bloch-Grüneisen (BG) temperature in doped graphene via analytical and numerical calculations based on full inelastic electron-acoustic-phonon (EAP) scattering rate and various approximation schemes. Analytic results for BG temperature obtained by semi-inelastic (SI) approximation (which gives scattering...

In this work, we use slater-type orbitals (STOs) as basis functions to construct exciton and trion wavefunctions in two dimensions. The shielding constants of the STOs are treated as variational parameters. Via this method, exciton and trion energy levels and wavefunctions can be calculated efficiently with fairly accurate results. We apply the met...

In this work, we resolved some controversial issues on the Bloch-Grüneisen (BG) temperature in doped graphene via analytical and numerical calculations based on full inelastic electron-acoustic-phonon (EAP) scattering rate and various approximation schemes. Analytic results for BG temperature obtained by semi-inelastic (SI) approximation (which giv...

Transfer characteristics of p-doped graphene/monolayer-MoS2 heterostructure at 300 K are measured experimentally and analyzed based on a model calculation. In the model, we first discretize the Poisson equation into multiple zones. In each zone the charge density is assumed constant and the Poisson equation can be transformed into a linear equation...

The electronic exciton polaron is a hypothetical many-body quasiparticle formed by an exciton dressed with a polarized electron-hole cloud in the Fermi sea (FS). It is predicted to display rich many-body physics and unusual roton-like dispersion. Exciton polarons were recently evoked to explain the excitonic spectra of doped monolayer transition me...

Excitons and trions (or exciton polarons) in transition metal dichalcogenides (TMDs) are known to decay predominantly through intravalley transitions. Electron-hole recombination across different valleys can also play a significant role in the excitonic dynamics, but intervalley transitions are rarely observed in monolayer TMDs, because they violat...

Excitons and trions (or exciton-polarons) in transition metal dichalcogenides (TMDs) are known to decay predominantly through intravalley transitions. Electron-hole recombination across different valleys can also play a significant role in the excitonic dynamics, but intervalley transitions are rarely observed in monolayer TMDs, because they violat...

The optical emission from type-II semiconductor nanostructures is influenced by the long carrier lifetime and can exhibit remarkable thermal stability. In this study, utilizing a high quality photonic crystal circular nanobeam cavity with a high quality factor and a sub-micrometer mode volume, we demonstrated an ultra-compact semiconductor laser wi...

We investigate Landau-quantized excitonic absorption and luminescence of monolayer WSe2 under magnetic field. We observe gate-dependent quantum oscillations in the bright exciton and trions (or exciton polarons) as well as the dark trions and their phonon replicas. Our results reveal spin- and valley-polarized Landau levels (LLs) with filling facto...

DOI:https://doi.org/10.1103/PhysRevApplied.13.029901

We investigate Landau-quantized excitonic absorption and luminescence of monolayer WSe$_2$ under magnetic field. We observe gate-dependent quantum oscillations in the bright exciton and trions (or exciton-polarons) as well as the dark trions and their phonon replicas. Our results reveal spin- and valley-polarized Landau levels (LLs) with filling fa...

The in-plane acoustic phonon scattering in graphene is solved by considering fully inelastic acoustic phonon scatterings in two-dimensional (2D) Dirac materials for large range of temperature (T) and chemical potential (μ). Rigorous analytical solutions and symmetry properties of Fermionic and Bosonic functions are obtained. We illustrate how dopin...

A simple theoretical method for deducing the effective bond-orbital model (EBOM) of III-nitride wurtzite (WZ) semiconductors is presented. In this model, the interaction parameters for zinc-blende (ZB) structures are used as an initial guess for WZ structure based on the two-center approximation. The electronic band structure of III-nitride WZ semi...

The high spatial resolution of tip-enhanced Raman spectroscopy may be due to the interaction between tip electrons and molecular vibrations. The resolution can be as low as 1.62 nm, closed to the experimental result.

Transfer characteristics of p-doped graphene/monolayer-MoS2 heterostructure at 300 K are measured experimentally and analyzed based on a model calculation. In the model, we first discretize the Poisson equation (PE) into multiple zones. In each zone the charge density is assumed constant and the PE can be transformed into a linear equation to deter...

We observe a set of three replica luminescent peaks at ∼21.4 meV below the dark exciton, and negative and positive dark trions (or exciton polarons) in monolayer WSe2. The replica redshift energy matches the energy of the zone-center E′′-mode optical phonons. The phonon replicas exhibit a parallel gate dependence and the same g-factors as the dark...

We present a full analytical consideration of inelastic acoustic phonon scatterings in two-dimensional (2D) Dirac materials for large range of temperature ($T$) and chemical potential ($\mu$). Rigorous analytical solutions and symmetry properties of Fermionic and Bosonic functions are obtained. We illustrate how doping alters the temperature depend...

Heat engines (HEs) made of low dimensional structures offer promising applications in energy harvesting due to their reduced phonon thermal conductance. Many efforts have been devoted to the design of HEs made of quantum-dot (QD) superlattice nanowire (SLNW), but only SLNWs with uniform energy levels in QDs were considered. Here we propose a HE mad...

We present theoretical calculations of the line shapes of emission spectra of ZnO micro spheres (MSs), including the exciton-polariton and Purcell effect. Our calculation explains the red shift of emission peaks of whispering gallery modes (WGMs) in UV range commonly observed in ZnO MSs. We show that the red shift of the UV emission peak is caused...

Monolayer WSe2 hosts long-lived dark excitonic states with robust valley polarization, but thus far lacks an experimental signature to identify their valley pseudospin. Here we reveal a set of three replica luminescent peaks at ~21.4 meV below the dark exciton, negative and positive dark exciton-polarons (or trions) in monolayer WSe2. The redshift...

A theoretical model for describing the emission spectra of microsphere cavities is presented and its predictions of detailed line shapes of emission spectra associated with whispering gallery modes (WGMs) of various orders in ZnO microspheres (MSs) are verified experimentally by photoluminescence (PL) spectroscopy. The interplay of the Purcell effe...

Monolayer WSe2 hosts a series of exciton Rydberg states denoted by the principal quantum number n=1, 2, 3, etc. While most research focuses on their absorption properties, their optical emission is also important but much less studied. Here we measure the photoluminescence from the 1s–5s exciton Rydberg states in ultraclean monolayer WSe2 encapsula...

Heat engines (HEs) made of low dimensional structures offer promising applications in energy harvesting due to their reduced phonon thermal conductance. Many efforts have been devoted to the design of HEs made of quantum-dot (QD) superlattice nanowire (SLNW), but only SLNWs with uniform energy levels in QDs were considered. Here we propose a HE mad...

A novel coral-like perovskite nanostructured layer was grown on a compact perovskite foundation layer by the facile surface modification with dimethylformamide/isopropanol (DMF/IPA) as co-solvent. Surface morphological characterizations with SEM and XRD analyses revealed a growing mechanism of the new morphology, which was composed of the perovskit...

Monolayer WSe$_2$ hosts a series of exciton Rydberg states denoted by the principal quantum number n = 1, 2, 3, etc. While most research focuses on their absorption properties, their optical emission is also important but much less studied. Here we measure the photoluminescence from the 1s - 5s exciton Rydberg states in ultraclean monolayer WSe$_2$...

A theoretical model for describing the emission spectra of microsphere cavities is presented, and its prediction of detailed lineshapes of emission spectra associated with whispering gallery modes (WGMs) of various orders in ZnO microspheres (MSs) are verified experimentally by photoluminescence (PL) spectroscopy. The interplay of Purcell effect, q...

We present a theoretical study of photoabsorption in n-doped two-dimensional (2D) and quasi-2D semiconductors that takes into account the interaction of the photocreated exciton with Fermi-sea (FS) electrons through (i) Pauli blocking, (ii) Coulomb screening, and (iii) excitation of FS electron-hole pairs—that we here restrict to one. The system we...

An efficient method for the theoretical investigation of optical properties of semiconductor core-shell quantum dots (CSQDs) is introduced within the multi-band approach, which takes the advantage of the symmetry of the system. The heteroepitaxial strain and excitonic effect are included in the calculation of energy levels, envelope wave-functions,...

We present a theoretical study of photo-absorption in n-doped two-dimensional (2D) and quasi-2D semiconductors that takes into account the interaction of the photocreated exciton with Fermi-sea (FS) electrons through (i) Pauli blocking, (ii) Coulomb screening, and (iii) excitation of FS electron-hole pairs---that we here restrict to one. The system...

Experimental observation and theoretical analysis of the whispering gallery mode (WGM) related emission spectra of ZnO microrods with a hexagonal cross-section are reported. ZnO microrods of various sizes are fabricated, and their individual geometric information (including the inner radius, edge profile modification, and tilt angle) are characteri...

A simple theoretical method for deducing the effective bond-orbital model (EBOM) of III-nitride wurtzite (WZ) semiconductors is presented. In this model, the interaction parameters for zinc-blende (ZB) structures are used as initial guess of the ones for WZ structures base on the two-center approximation. The electronic band structures of III-nitri...

Further technological development of perovskite solar cells (PSCs) will require improvements in power conversion efficiency and stability while maintaining low material costs and simple fabrication. In this paper we describe top-illuminated ITO-free, stable PSCs featuring microcavity structures, wherein metal layers on both sides on the active laye...

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule (SMPV1) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), by utilizing ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The inverted SMPV1:PC71BM solar cells with ZnO NR...

It is known that the figure of merit ($ZT$) of thin nanowires can be significantly enhanced at room temperature due to the reduction of phonon thermal conductance arising from the increase of boundary scattering of phonons. It is expected that the phonon thermal conductance of nanowires filled with quantum dots (QDs) will be further reduced. Here w...

It is known that the figure of merit (ZT) of thin nanowires can be significantly enhanced at room temperature due to the reduction of phonon thermal conductance arising from the increase of boundary scattering of phonons. It is expected that the phonon thermal conductance of nanowires filled with quantum dots (QDs) will be further reduced. Here we...

A van der Waals (vdW) density functional was implemented in the mixed basis approach previously developed for studying two dimensional systems, in which the vdW interaction plays an important role. The basis functions here are taken to be the localized B-splines for the finite non-periodic dimension and plane waves for the two periodic directions....

A van der Waals (vdW) density functional was implemented in the mixed basis approach previously developed for studying two dimensional systems, in which the vdW interaction plays an important role. The basis functions here are taken to be the localized B-splines for the finite non-periodic dimension and plane waves for the two periodic directions....

Highly efficient electron transport layer (ETL) is an essential constituent for good performance and stability in planar perovskite solar cell. Among the n-type metal oxide materials, zinc oxide (ZnO) is a promising candidate for electron transport layer due to its relatively high electron mobility, high transparency, and versatile nanostructures....

Purpose: To demonstrate the power of microscopic imaging ellipsometry (MIE) to identify submicron-scale bacterial cells and track their surface topology variation. Methods: Microscopic imaging ellipsometry with rotating compensator configuration was used to measure the ellipsometric spectra for dried submicron-scale Streptococcus mutans cells cultu...

We use the composite boson (coboson) many-body formalism to tackle scattering lengths relevant to cold atoms. We show that bound dimers can be taken as elementary entities provided that fermion exchanges between them are treated exactly, as can be done through the coboson formalism. This alternative tool extended to cold atom many-body physics not...

We use the composite boson (coboson) many-body formalism to tackle scattering lengths for cold fermionic atoms. We show that bound dimers can be taken as elementary entities provided that fermion exchanges between them are treated exactly, as can be done through the coboson formalism. This alternative tool extended to cold atom physics not only mak...

We demonstrate numerically that one-dimensional metamaterials with a gain medium can exhibit a near-unity-refractive-index for TE-modes, which resembles the effect of electro-magnetically induced transparency. Our results are further supported by rigorous coupled wave analysis simulations. Additionally, we analyze the behavior of the modal gain and...

Among the n-type metal oxide materials used in the planar perovskite solar cells, zinc oxide (ZnO) is a promising candidate to replace titanium dioxide (TiO2) due to its relatively high electron mobility, high transparency, and versatile nanostructures. Here, we present the application of low temperature solution processed ZnO/Al-doped ZnO (AZO) bi...

We adopted the time-dependent density functional theory (TDDFT) within the linear augmented Slater-type orbitals basis and the cluster averaging method to compute the excitation spectra of III-V ternary alloys with arbitrary concentration x. The TDDFT was carried out with the use of adiabatic meta-generalized gradient approximation (mGGA), which co...

Using the composite boson many-body formalism that takes single-exciton states rather than free carrier states as a basis, we derive the integral equation fulfilled by the exciton-exciton effective scattering from which the role of fermion exchanges can be unraveled. For excitons made of $(\pm1/2)$-spin electrons and $(\pm3/2)$-spin holes, as in Ga...

In this work, we demonstrate that whispering gallery mode (WGM) resonances can be observed in small ZnO microspheres embedded with Au nanoparticles (AuNPs). In general, one can enhance the WGM resonances by decreasing air fraction in porous ZnO microsphere resonators grown by the hydrothermal method. We demonstrate that embedding AuNPs in a porous...

Microscopic Imaging Ellipsometry is an optical technique that uses an objective and sensing procedure to measure the ellipsometric parameters {\Psi} and {\Delta} in the form of microscopic maps. This technique is well known for being non-invasive and label-free. Therefore it can be used to detect and characterize biological species without any impa...

Microscopic Imaging Ellipsometry is an optical technique that uses an objective and sensing procedure to measure the ellipsometric parameters {\Psi} and {\Delta} in the form of microscopic maps. This technique is well known for being non-invasive and label-free. Therefore it can be used to detect and characterize biological species without any impa...

Using the composite boson (coboson) many-body formalism, we determine under which conditions "trion-polariton" can exist. Dipolar attraction can bind an exciton and an electron into a trion having an energy well separated from the exciton energy. Yet, the existence of long-lived "trion-polariton" is a priori implausible not only because the photon-...

One-dimensional CdZnS nanostructures have been synthesized through the sublimation. Effect of high substrate temperature on morphology, structural and optical properties of these nanostructures has been studied. X-Ray diffraction peak intensity, lattice parameters, crystallite size decreased with an increase in substrate temperature. The morphology...

The implementation of lossless plasmonic modes with very high or very low wave vectors, which are associated with extraordinary large or small nonlocal effective permittivity, could bring a breakthrough in the development of high-performance plasmonic devices. We consider the possibility of obtaining volume plasmon-polariton modes with an extraordi...

It is theoretically demonstrated that the figure of merit ($ZT$) of quantum dot (QD) junctions can be significantly enhanced when the degree of degeneracy of the energy levels involved in electron transport is increased. The theory is based on the the Green-function approach in the Coulomb blockade regime by including all correlation functions resu...

It is theoretically demonstrated that the figure of merit ($ZT$) of quantum dot (QD) junctions can be significantly enhanced when the degree of degeneracy of the energy levels involved in electron transport is increased. The theory is based on the the Green-function approach in the Coulomb blockade regime by including all correlation functions resu...

The nonlinear electron and heat currents of quantum dot molecules (QDMs) under a temperature bias are theoretically investigated, including all correlation functions arising from electron Coulomb interactions in QDMs. Unlike the case of double QDs, the maximum efficiency of serially coupled triple QDs (SCTQD) occurs in the orbital depletion regime...

Using the composite boson (coboson) many-body formalism, we determine under which conditions "trion-polariton" can exist. Dipolar attraction can bind an exciton and an electron into a trion having an energy well separated from the exciton energy. Yet, the existence of long-lived "trion-polariton" is a priori implausible not only because the photon-...

Using the composite boson many-body formalism that takes single-exciton states rather than free carrier states as a basis, we derive the integral equation fulfilled by the exciton-exciton effective scattering from which the role of fermion exchanges can be unraveled. For excitons made of $(\pm1/2)$-spin electrons and $(\pm3/2)$-spin holes, as in Ga...

Effect of vacuum annealing on structural and optical properties of CdZnS thin films is reported. Nano-crystalline CdZnS films have been deposited through sublimation technique and subsequently vacuum annealed at 300 0 C and 400 0 C for two hours. The samples have been characterized through X-ray diffraction, Scanning Electron Microscopy, Raman Scat...

We derive the scattering length of composite bosons (cobosons) within the framework of the composite boson many-body formalism that uses correlated-pair states as a basis, instead of free fermion states. The integral equation constructed from this physically relevant basis makes transparent the role of fermion exchange in the coboson-coboson effect...

We derive the scattering length of composite bosons (cobosons) within the framework of the composite boson many-body formalism that uses correlated-pair states as a basis, instead of free fermion states. The integral equation constructed from this physically relevant basis makes transparent the role of fermion exchange in the coboson-coboson effect...

We use Richardson-Gaudin exact equations to derive the ground-state energy of N composite bosons (cobosons) interacting via a potential which acts between fermion pairs having zero center-of-mass momentum, that is, a potential similar to the reduced BCS potential used in conventional superconductivity. Through a density expansion, we show that whil...