Excitonic Effects Research Articles

Recent Advances in 2D Wearable Flexible Sensors

AbstractA variety of wearable technology for detecting human safety as well as environmental conditions have new research directions owing to 2D heterostructured transition metal dichalcogenides (TMDs), graphene (Gr), and reduced graphene‐oxide (rGrO). This in‐depth review article compiles the most recent and effective methods for ultrathin, large‐area flexible sensors with conformal adhesion to human skin that are essential for the development of wearable electronic skins. The results presented in this review aim to design flexible, comfortable wearable devices to monitor users’ continuous health conditions and to show the essential parameters (vital human activity signals such as heartbeat and pulse rate, expressions, and voice recognition) on smartphones in real‐time, allowing users to monitor their health issues in a convenient setting. When used to detect gases, higher selectivity biomolecules, and other organic and inorganic molecules, 2D TMDs can produce superior sensitivities and stability due to their unique and fascinating properties that result from the close interaction between the components. The understanding and control of excitonic effects and associated wearable applications in van der Waals heterostructures over a wide spectral response range is also encouraged. The review concludes by summarizing the present difficulties and opportunities.

Hydrogen Evolution, Piezoelectricity, and Optical Properties of Monolayer Semimetal (α,β)–S2C2N2 and Janus (α,β)–SCN/CN

Herein, the density functional theory is used to study the structural, mechanical, electrical, optical, and hydrogen evolution reaction (HER) properties of monolayer structures and . They are claimed to be stable monolayer structures by phonon dispersion and ab initio molecular dynamics (AIMD). Compared to graphene (≈340 N m−1), the shows a higher Young's modulus (≈429, 414 N m−1). Moreover, the piezoelectric stress coefficient of α‐ (=2.65 pm V−1) is comparable with bulk piezoelectric α‐quartz (=2.3 pm V−1). Exciton effect at M point causes redshift and enhancement of light absorption of α‐ and . The overpotential of is only –0.072 V. Besides, the gives a small effective electron mass of about 0.2(m0). It is worth mentioning that the monolayer structures show the semimetal phases. Due to the unique semimetal phase, the absorption of has an excellent performance in the near‐infrared (NIR) region. It can be enhanced at the NIR region and extended to the middle infrared region when applying tensile stress. These results provide more choices for 2D materials in electrocatalysis, piezoelectric sensors, visible light devices, and infrared photodetectors.

Epitaxial hexagonal boron nitride with high quantum efficiency

Two-dimensional (2D) hexagonal boron nitride (h-BN) is one of the few materials showing great promise for light emission in the far ultraviolet (UV)-C wavelength, which is more effective and safer in containing the transmission of microbial diseases than traditional UV light. In this report, we observed that h-BN, despite having an indirect energy bandgap, exhibits a remarkably high room-temperature quantum efficiency (∼60%), which is orders of magnitude higher than that of other indirect bandgap material, and is enabled by strong excitonic effects and efficient exciton-phonon interactions. This study offers a new approach for the design and development of far UV-C optoelectronic devices as well as quantum photonic devices employing 2D semiconductor active regions.

Unveiling the Linear Photogalvanic Effect in a Two-Dimensional Organic Tin Halide Perovskite: (CH3)2NH2SnI3

Hybrid organic–inorganic halide perovskites (HOIPs) with intrinsic noncentrosymmetry are an emerging class of semiconducting materials that has been revolutionizing the field of optoelectronics including second-order nonlinear optics (NLO). Following an upsurge in three-dimensional (3D) perovskites, more recently, low-dimensional perovskite materials have displayed desirable NLO responses attributed to structural diversity, strong quantum confinement, and remarkable exciton effect. Herein, using first-principles methods, we elucidate a linear photogalvanic effect (LPGE) photocurrent (shift photocurrent, SPC) with a maximum amplitude of ∼430 μAV–2 in a quasi-two-dimensional (2D) lead-free HOIP, (CH3)2NH2SnI3 (DMASnI3). The simulated SPC whose prominent peaks occur predominantly in the ultraviolet (UV) range is about 8-fold larger in magnitude compared to its 3D counterpart. From the orbital characteristics of the initial and final states, the calculated SPC is attributed to the contributions from the electronic properties of Sn and I whose p-states control the band edges. The organic-cation orientation indirectly influences the bandgap size, valence and conduction band edge behavior, as well as the position of the Fermi level, slightly altering the LPGE photocurrent spectrum. The findings of this work demonstrate the quasi-2D organic tin triiodide perovskite as a promising lead-free alternative for NLO devices.

Single-Atom Engineering of Covalent Organic Framework for Photocatalytic H2 Production Coupled with Benzylamine Oxidation.

In photocatalysis, reducing the exciton binding energy and boosting the conversion of excitons into free charge carriers are vital to enhance photocatalytic activity. This work presents a facile strategy of engineering Pt single atoms on a 2D hydrazone-based covalent organic framework (TCOF) to promote H2 production coupled with selective oxidation of benzylamine. The optimised TCOF-Pt SA photocatalyst with 3 wt% Pt single atom exhibited superior performance to TCOF and TCOF-supported Pt nanoparticle catalysts. The production rates of H2 and N-benzylidenebenzylamine over TCOF-Pt SA3 are 12.6 and 10.9 times higher than those over TCOF, respectively. Empirical characterisation and theoretical simulation showed that the atomically dispersed Pt is stabilised on the TCOF support through the coordinated N1 -Pt-C2 sites, thereby induing the local polarization and improving the dielectric constant to reach the low exciton binding energy. These phenomena led to the promotion of exciton dissociation into electrons and holes and the acceleration of the separation and transport of photoexcited charge carriers from bulk to the surface. This work provides new insights into the regulation of exciton effect for the design of advanced polymer photocatalysts.

Exploring chemical bonding nature, weak exciton effect, electronic, optical and thermoelectric properties of NaReN2 via DFT computations

We performed first principles calculations on the recently synthesized NaReN2. This compound is the main product of the reaction between Re and NaN3 in forming the super hard material ReN2. Theoretical calculations of sodium rhenium nitride, such as band structure and density of states (DOS) were performed. The partial density of states (PDOS) was also included to determine the contribution of individual atoms to the electronic states. The theoretical data reveal that the material possesses a rather narrow bandgap of 0.26 eV. Semi-conducting materials with such low bandgap values have interesting applications in various fields. The optical properties were also studied indicating possible application of NaReN2 in opto-electronic devices. Excitonic study reveals the weak excitonic behavior of this material. Thus, first principles calculations on this material provide necessary theoretical information on physicochemical properties of NaReN2.

Partial disorder structured BiOI atomic layers boosting excitons dissociation for photocatalytic CO2 reduction and pollutant removal

Owing to atomic-thin thickness, reduced dielectric screening will usually happen in 2D atomic layers and lead to the formation of strong exciton effect, which is not energetically for some photocatalytic reactions. In this work, a scalable fast synthesis strategy is developed to fabricate BiOI atomic layers with partial disorder area (BiOI-D) at room temperature, as observed by HAADF-STEM image. This order–disorder interface in BiOI atomic layers favors boosting excitons dissociation to free charge carriers. Due to the atomic-thin thickness for rapid bulk charge diffusion and order–disorder interface for excitons dissociation, this BiOI-D atomic layers display greatly improved charge separation efficiency compared to BiOI nanosheets, thus contribute to enhanced photocatalytic activity for pollutants degradation and CO2 reduction. Compared to BiOI nanosheets, a 12.5 times increased CO2 photoreduction activity can be realized over BiOI-D atomic layers, with the CO generation rate of 15.1 μmol g−1 in 5 h. Profiting from the order–disorder interface, CO2 activation energy barrier can be lowered and rate-limiting step energy barrier for COOH* formation is decreased from 0.67 to 0.34 eV.

Excitonic effects in time-dependent density functional theory from zeros of the density response

We show that the analytic structure of the dynamical xc kernels of semiconductors and insulators can be sensed in terms of its poles which mark physically relevant frequencies of the system where the counter-phase motion of discrete collective excitations occurs: if excited, the collective modes counterbalance each other, making the system to exhibit none at all or extremely weak density response. This property can be employed to construct simple and practically relevant approximations of the dynamical xc kernel for time-dependent density functional theory (TDDFT). Such kernels have simple analytic structure, are able to reproduce dominant excitonic features of the absorption spectra of monolayer semiconductors and bulk solids, and promise high potential for future uses in efficient real-time calculations with TDDFT.

Large‐Area Structure‐Selective Synthesis of Symmetry‐Broken MoSe2 and Their Broadband Nonlinear Optical Response

AbstractTransition metal dichalcogenides (TMDCs) have various electronic and optical properties depending on their structure, so they can be used as a fascinating material in various applications including photonics, electronics, optoelectronics, and valleytronics. In particular, spiral TMDCs grown through the formation of screw dislocations exhibit novel electronic and optical properties different from layer‐by‐layer TMDCs. However, large‐area structure‐selective synthesis of TMDCs remains challenging. Here, this work reports for the first time the large‐area structure‐selective synthesis of monolayer MoSe2 and spiral MoSe2 using a flux‐controlled chemical vapor deposition method. Under a low MoSe2 flux condition, monolayer MoSe2 is synthesized, whereas thick spiral MoSe2 is synthesized under a high flux condition. Under a medium flux condition, both monolayer and spiral MoSe2 are synthesized. In addition, through the nonlinear optical (NLO) signal analysis of monolayer MoSe2 and spiral MoSe2, the giant enhancement of NLO signals induced by the combined effect of breaking inversion symmetry and the excitonic resonance effects in the synthesized MoSe2 is confirmed. Monolayer MoSe2 and spiral MoSe2 synthesized using this method are expected to be used as advanced optical materials for novel electronics, optoelectronics, and NLO applications.

Boron nitride quantum dots modified carbon-defects ultra-thin porous carbon nitride: double channels and quantum size effects facilitate photogenerated carrier migration and exciton dissociation

Graphite carbon nitride possesses great promise for visible photocatalysis, but the bulk carbon nitride prepared from nitrogen-rich precursors such as melamine has inherent drawbacks such as retarded photogenerated carrier migration and exciton effects, which limit its application. Herein, we constructed a novel Boron nitride quantum dots modified carbon-defects ultra-thin porous carbon nitride (BNQDs/Vc-UPCN). The double channels were constructed by carbon-defects structure and Boron nitride quantum effect to overcome its inherent drawbacks and applied to the photodegradation of common persistent organic pollutants (methylene blue). The structure, porosity, elemental composition, optical properties, photoelectrochemical properties, and photocatalytic properties of the prepared BNQDs/Vc-UPCN composites were investigated using various characterization methods. Meanwhile, the results of radical trapping experiments and electron spin resonance characterization demonstrated that BNQDs/Vc-UPCN promote molecular oxygen activation more than Vc-UPCN did. In terms of degradation effect, the best sample (BC-1) is 10 times more effective than the initial sample (BCN). This study proposes an effective mechanism for constructing novel visible-light-driven photocatalysts using carbon-defects ultra-thin structures and quantum dots, which can be used for the treatment of organic pollutants.

On-the-Fly Nonadiabatic Dynamics Simulations of Single-Walled Carbon Nanotubes with Covalent Defects

Single-walled carbon nanotubes (SWCNTs) with covalent surface defects have been explored recently due to their promise for use in single-photon telecommunication emission and in spintronic applications. The all-atom dynamic evolution of electrostatically bound excitons (the primary electronic excitations) in these systems has only been loosely explored from a theoretical perspective due to the size limitations of these large systems (>500 atoms). In this work, we present computational modeling of nonradiative relaxation in a variety of SWCNT chiralities with single-defect functionalizations. Our excited-state dynamics modeling uses a trajectory surface hopping algorithm accounting for excitonic effects with a configuration interaction approach. We find a strong chirality and defect-composition dependence on the population relaxation (varying over 50-500 fs) between the primary nanotube band gap excitation E11 and the defect-associated, single-photon-emitting E11* state. These simulations give direct insight into the relaxation between the band-edge states and the localized excitonic state, in competition with dynamic trapping/detrapping processes observed in experiment. Engineering fast population decay into the quasi-two-level subsystem with weak coupling to higher-energy states increases the effectiveness and controllability of these quantum light emitters.

Electron-Exciton Coupling in 1T-TiSe2 Bilayer

Excitons in solid state are bosons generated by electron-hole pairs as the Coulomb screening is sufficiently reduced. The exciton condensation can result in exotic physics such as super-fluidity and insulating state. In charge density wave (CDW) state, 1T-TiSe2 is one of the candidates that may host the exciton condensation. However, to envision its excitonic effect is still challenging, particularly at the two-dimensional limit, which is applicable to future devices. Here, we realize the epitaxial 1T-TiSe2 bilayer, the two-dimensional limit for its 2 × 2 × 2 CDW order, to explore the exciton-associated effect. By means of high-resolution scanning tunneling spectroscopy and quasiparticle interference, we discover an unexpected state residing below the conduction band and right within the CDW gap region. As corroborated by our theoretical analysis, this mysterious phenomenon is in good agreement with the electron-exciton coupling. Our study provides a material platform to explore exciton-based electronics and opto-electronics.

Nature of excitons in the Ti L and O K edges of x-ray absorption spectra in bulk SrTiO3 from a combined first principles and many-body theory approach

Based on density functional theory calculations, we model the x-ray absorption spectra of the $\mathrm{O} K$ edge and the $\mathrm{Ti} {L}_{2,3}$ edge in bulk ${\mathrm{SrTiO}}_{3}$. Taking into account excitonic effects by solving the Bethe-Salpeter equation is found to be pivotal for obtaining concurrence with the experimental data with respect to the energetic positions and relative intensity of the peaks. Moreover, analysis of the underlying interband transitions in reciprocal space reveals the origin of the prominent peaks and features in the spectra, and provides a deeper understanding of the electronic structure. For example, the characteristic four-peak structure of the $\mathrm{Ti} {L}_{2,3}$ edge results from transitions from $\mathrm{Ti} 2{p}_{3/2}$ states to the unoccupied $\mathrm{Ti} 3d\phantom{\rule{4pt}{0ex}}{t}_{2g}$ (456.1 eV) and $\mathrm{Ti} 3d\phantom{\rule{4pt}{0ex}}{e}_{g}$ states (458.2 eV), followed by transitions from $\mathrm{Ti} 2{p}_{1/2}$ states to $\mathrm{Ti} 3d\phantom{\rule{4pt}{0ex}}{t}_{2g}$ (461.8 eV) and $\mathrm{Ti} 3d\phantom{\rule{4pt}{0ex}}{e}_{g}$ states (463.7 eV). The first bound exciton is strongly localized in real space, and is confined to essentially one unit cell with $3{d}_{xz}$ character near the Ti sites. On the other hand, the first bound exciton of the O K edge is identified as a charge-transfer type with a dominant contribution from the $\mathrm{Ti} 3{d}_{xy}$ states hybridized with O $p$ states. Moreover, the spatial distribution of the exciton wave function shows an intriguing two-dimensional spread in the $x\text{\ensuremath{-}}y$ plane, despite the three-dimensional nature of the material.

Fine Structure Splitting of Phonon-Assisted Excitonic Transition in (PEA)2PbI4 Two-Dimensional Perovskites

Two-dimensional van der Waals materials exhibit particularly strong excitonic effects, which causes them to be an exceptionally interesting platform for the investigation of exciton physics. A notable example is the two-dimensional Ruddlesden-Popper perovskites, where quantum and dielectric confinement together with soft, polar, and low symmetry lattice create a unique background for electron and hole interaction. Here, with the use of polarization-resolved optical spectroscopy, we have demonstrated that the simultaneous presence of tightly bound excitons, together with strong exciton-phonon coupling, allows for observing the exciton fine structure splitting of the phonon-assisted transitions of two-dimensional perovskite (PEA)2PbI4, where PEA stands for phenylethylammonium. We demonstrate that the phonon-assisted sidebands characteristic for (PEA)2PbI4 are split and linearly polarized, mimicking the characteristics of the corresponding zero-phonon lines. Interestingly, the splitting of differently polarized phonon-assisted transitions can be different from that of the zero-phonon lines. We attribute this effect to the selective coupling of linearly polarized exciton states to non-degenerate phonon modes of different symmetries resulting from the low symmetry of (PEA)2PbI4 lattice.

Reducing dielectric confinement effect in ionic covalent organic nanosheets to promote the visible-light-driven hydrogen evolution

Ultra-thin two-dimensional (2D) organic semiconductors are promising candidates for photocatalysts because of the short charge diffusion pathway and favorable exposure of active sites plus the versatile architecture. Nonetheless, the inherent dielectric confinement of 2D materials will induce a strong exciton effect hampering the charge separation. Herein, we demonstrated an effective way to reduce the dielectric confinement effect of 2D ionic covalent organic nanosheets (iCONs) by tailoring the functional group via molecular engineering. Three ultra-thin CONs with different functional groups and the same ionic moieties were synthesized through Schiff base condensation between ionic amino monomer triaminoguanidinium chloride (TG) and aldehyde linkers. The integration of the hydroxyl group was found to significantly increase the dielectric constant by enhancing the polarizability of ionic moieties, and thus reduced the dielectric confinement and the corresponding exciton binding energy (Eb). The champion hydroxyl-functional iCON exhibited promoted exciton dissociation and in turn a high photocatalytic hydrogen production rate under visible-light irradiation. This work provided insights into the rationalization of the dielectric confinement effect of low-dimensional photocatalysts.

External field regulation strategies for exciton dynamics in 2D TMDs

Two-dimensional (2D) transition metal chalcogenides (TMDs) are regarded as promising materials for micro-optoelectronic devices and next-generation logic devices due to their novel optoelectronic properties, such as strong excitonic effects, tunable direct bandgap from visible to near-infrared regions, valley pseudospin degree of freedom, and so on. Recently, triggered by the growing demand to optimize the performance of TMDs devices, external field regulation engineering has attracted great attention. The goal of this operation is to exploit the external fields to control exciton dynamics in 2D TMDs, including exciton formation and relaxation, and to finally achieve high-performance 2D TMDs devices. Although the regulation strategies of exciton dynamics in 2D TMDs have been well explored, the underlying mechanisms of different regulation strategies need to be further understood due to the complex many-body interactions in exciton dynamics. Here, we first give a brief summary of the fundamental processes of exciton dynamics in 2D TMDs and then summarize the main field-regulation strategies. Particular emphasis is placed on discussing the underlying mechanisms of how different field-regulation strategies control varied fundamental processes. A deep understanding of field regulation provides direct guidelines for the integrated design of 2D TMDs devices in the future.

Layer-dependent optically induced spin polarization in InSe

Optical control of spin in semiconductors has been pioneered using nanostructures of III-V and II-VI semiconductors, but the emergence of two-dimensional van der Waals materials offers an alternative low-dimensional platform for spintronic phenomena. Indium selenide (InSe), a group-III monochalcogenide van der Waals material, has shown promise for opto-electronics due to its high electron mobility, tunable direct bandgap, and quantum transport. There are predictions of spin-dependent optical selection rules suggesting potential for all-optical excitation and control of spin in a two-dimensional layered material. Despite these predictions, layer-dependent optical spin phenomena in InSe have yet to be explored. Here, we present measurements of layer-dependent optical spin dynamics in few-layer and bulk InSe. Polarized photoluminescence reveals layer-dependent optical orientation of spin, thereby demonstrating the optical selection rules in few-layer InSe. Spin dynamics are also studied in many-layer InSe using time-resolved Kerr rotation spectroscopy. By applying out-of-plane and in-plane static magnetic fields for polarized emission measurements and Kerr measurements, respectively, the $g$-factor for InSe was extracted. Further investigations are done by calculating precession values using a $\textbf{k} \cdot \textbf{p}$ model, which is supported by \textit{ab-initio} density functional theory. Comparison of predicted precession rates with experimental measurements highlights the importance of excitonic effects in InSe for understanding spin dynamics. Optical orientation of spin is an important prerequisite for opto-spintronic phenomena and devices, and these first demonstrations of layer-dependent optical excitation of spins in InSe lay the foundation for combining layer-dependent spin properties with advantageous electronic properties found in this material.

Electronic and Optical Properties of CH3NH3SnI3 and CH(NH2)2SnI3 Perovskite Solar Cell

Using first‐principles calculations, a study on the electronic and optical characteristics of perovskite solar cells containing the orthorhombic phases CH3NH3SnI3 and CH(NH2)2SnI3 is conducted. The analysis includes the examination of relaxed geometry structures, electronic band structures, charge density distributions, and van Hove singularities in the density of states to thoroughly examine the orbital hybridizations in chemical bonds. The optical properties of the materials with and without excitonic effects by analyzing dielectric functions, energy loss functions, absorption coefficients, and reflectance spectra are also studied. The findings identify the close connections between the initial and final orbital hybridizations, as well as prominent optical excitations. Based on the computational predictions, It is believed that lead‐free materials such as CH3NH3SnI3 and CH(NH2)2SnI3 are promising candidates for photovoltaic applications and are worth experimental testing.

Excitonic Effects in the Linear and Nonlinear Optical Response of Rhenium Disulfide Regulated by Re Vacancy

AbstractRhenium disulfide (ReS2) with low structural symmetry and highly spontaneous, exhibits some unique optical and electrical anisotropy properties for developing optoelectronic devices. This work studies the hydrothermal synthesis and the excitonic effects in the linear and nonlinear optical properties of 1T′ phase ReS2. The growth is controlled by adjusting experimental conditions, and its growth mechanism is revealed. The varying degrees of Re vacancy are characterized. They cause the growth direction of ReS2 nanosheets from lateral to agglomeration and the stacks into self‐assembly micro‐nanospheres. The density functional theory explains the electronic and bonding properties of Re vacancy, where the existent defects reduce the band gap and change the states density. Re vacancy increases the conductivity and results in the metallization of ReS2. The photoluminescence intensity of defect emission peak is negatively correlated with the defect level for the enriching vacancies. However, compared with intrinsic peak, it will increase monotonically with the number of bound excitons in defects. Furthermore, the nonlinear absorption properties are obtained by picosecond Z‐scan technique. By adjusting temperature, different degrees of saturation absorption and reverse saturation absorption characteristics appear due to the vacancy effect. The synthesized ReS2 has broad application in photodetectors and optical devices with optical amplitude limiting.

Robustness of Momentum-Indirect Interlayer Excitons in MoS2/WSe2 Heterostructure against Charge Carrier Doping

Monolayer transition-metal dichalcogenide (TMD) semiconductors exhibit strong excitonic effects and hold promise for optical and optoelectronic applications. Yet, electron doping of TMDs leads to the conversion of neutral excitons into negative trions, which recombine predominantly non-radiatively at room temperature. As a result, the photoluminescence (PL) intensity is quenched. Here we study the optical and electronic properties of a MoS2/WSe2 heterostructure as a function of chemical doping by Cs atoms performed under ultra-high vacuum conditions. By PL measurements we identify two interlayer excitons and assign them to the momentum-indirect Q-Gamma and K-Gamma transitions. The energies of these excitons are in a very good agreement with ab initio calculations. We find that the Q-Gamma interlayer exciton is robust to the electron doping and is present at room temperature even at a high charge carrier concentration. Submicrometer angle-resolved photoemission spectroscopy (micro-ARPES) reveals charge transfer from deposited Cs adatoms to both the upper MoS2 and the lower WSe2 monolayer without changing the band alignment. This leads to a small (10 meV) energy shift of interlayer excitons. Robustness of the momentum-indirect interlayer exciton to charge doping opens up an opportunity of using TMD heterostructures in light-emitting devices that can work at room temperature at high densities of charge carriers.