Hole Excitations Research Articles

Low‐Temperature Thermophotovoltaic Cells by Bonding Bi2Te3 and Si Wafers

The thermophotovoltaic cells working under 300–500 K heat sources are fabricated by bonding Bi2Te3 and Si wafers. The performance of the cells is seriously affected by the strong intrinsic excitation in Bi2Te3, which leads to the slight split of the Fermi energy level and the weak absorption in the very narrow depletion region of Bi2Te3/Si heterojunction. The valence band offset at the interface between Bi2Te3 and Si, which limits the transportation of excited holes and the thickness of Bi2Te3 sheet are also important to affect the performance of the cells. Although the conversion efficiency of the thermophotovoltaic cells is low, this work provides a method to fabricate Bi2Te3/Si low‐temperature thermophotovoltaic cells.

Photoplastic effect in II–VI semiconductors: The role of hole redistribution in dislocation glide

AbstractPhotoplasticity in semiconductors, characterized by the modification of plastic deformation under light, has long been recognized but its underlying mechanisms remain elusive. Here, we integrated quantum mechanics (QM) simulations with nanoindentation experiments to investigate the impact of light‐induced electron‐hole pairs on the plastic deformation in two typical II–IV semiconductors: ZnS and CdTe. For ZnS, the QM simulations indicate an increase in the energy for breaking the Zn‐S bond within the S‐core dislocation in its glide process, due to the redistribution of excited holes. This enhancement in bond energy elevates the energy barrier for dislocation glide, eventually saturating at high electron‐hole concentrations and leading to reduced dislocation mobility in ZnS under light. These findings align well with the nanoindentation experiments, which demonstrate an increase followed by a plateau in the hardness of ZnS under light. In contrast, CdTe exhibits a reduced energy barrier for the glide of Te‐core dislocation due to the lack of hole redistribution, likely causing an enhanced dislocation mobility.

Quantum Flutter from Nonlinear Luttinger Liquid perspective

Abstract Quantum flutter is a ubiquitous phenomenon which can be observed from the fast moving impurity
injected into a fermionic or bosonic medium of quantum liquid. In this scenario, one usually consider
a medium of a fully polarized state and inject a spin-flipped impurity as the initial state. When the
initial velocity of the impurity is beyond the intrinsic sound velocity of the medium, the impurity
momentum dramatically exhibits a long-lived periodic oscillation with the periodicity remaining
invariant with respect to the initial velocity. In this letter, we show that such a novel phenomenon
can be explained by a linear Luttinger liquid coupled to a deep hole in the Fermi sea. Once the
deep hole excitations are involved and the impurity momentum surpass the Fermi momentum,
the propagator thus displays a periodic oscillation after a quick relaxation decay. The oscillation
periodicity is solely determined by the energy of the deepest hole excitation. Our result provides
deep insights into the dynamical behavior of quantum impurities immersed into one-dimensional
quantum liquids.

Strongly enhanced shift current at exciton resonances in a noncentrosymmetric wide-gap semiconductor

Excitons are fundamental quasiparticles that are ubiquitous in photoexcited semiconductors and insulators. Despite causing a sharp and strong photoabsorption near the interband absorption edge, charge-neutral excitons do not yield photocurrent in conventional photovoltaic processes unless dissociated into free charge carriers. Here, we experimentally demonstrate that excitons can directly contribute to photocurrent generation through a nonlinear light−matter interaction in a noncentrosymmetric semiconductor CuI. Epitaxial thin films of CuI exhibit a substantial enhancement of photocurrent at exciton resonance energies even below the bandgap. From the light polarization dependence, this photocurrent is identified to be shift current, a nonlinear photocurrent driven by the change in the geometric Berry phase of electron wave functions upon the optical transition. The shift current at the exciton resonance is much larger than that induced above the band gap by free electron−hole excitation, and their signs are opposite. First-principles calculations elucidate that the sign and magnitude of the exciton shift current are strongly dependent on the strain in the thin film. The present study reveals the crucial role of excitons in enhancing the shift current magnitude and its strain sensitivity, and will open an unprecedented route for efficient manipulation of nonlinear optical effects.

Embedded Many‐Body Green's Function Methods for Electronic Excitations in Complex Molecular Systems

ABSTRACTMany‐body Green's function theory in the GW approximation with the Bethe–Salpeter equation (BSE) provides a powerful framework for the first‐principles calculations of single‐particle and electron–hole excitations in perfect crystals and molecules alike. Application to complex molecular systems, for example, solvated dyes, molecular aggregates, thin films, interfaces, or macromolecules, is particularly challenging as they contain a prohibitively large number of atoms. Exploiting the often localized nature of excitation in such disordered systems, several methods have recently been developed in which GW‐BSE is applied to a smaller, tractable region of interest that is embedded into an environment described with a lower‐level method. Here, we review the various strategies proposed for such embedded many‐body Green's functions approaches, including quantum–quantum and quantum–classical embeddings, and focus in particular on how they include environment screening effects either intrinsically in the screened Coulomb interaction in the GW and BSE steps or via extrinsic electrostatic couplings.

Efficient hydrogen production performance of 1T-2H MoSe2/ MnxCd1-xS Z-type heterojunction photocatalysts under visible light

Finding suitable co-catalysts to help metal sulfides achieve effective bulk phase and surface charge separation is the key to improving their photocatalytic performance. In this paper, two-phase 1T-2H MoSe2 was loaded onto the surface of Mn0.4Cd0.6S solid solution with a twin crystal structure, and the 1T-2H MoSe2/Mn0.4Cd0.6S Z-type heterojunction structure was successfully constructed to realize the high efficiency of hydrogen production in the visible light. The metallic properties of the 1T phase can significantly improve the catalytic activity of MoSe2. The transition from the 2H to the 1T phase can lead to the creation of extra defects, such as unsaturated Mo and Se, thereby increasing the number of active sites on the photocatalyst surface. The two crystalline phases are staggered and form homogeneous junctions within them in the twin crystalline structure Mn0.4Cd0.6S prepared under an alkaline environment, which gives it excellent bulk and charge separation properties. After loading, the 1T phase MoSe2 at the interface of MoSe2 and Mn0.4Cd0.6S can accelerate the charge separation between the interfaces, and the Z-type heterojunctions formed by the two and the better retention of useful excitation electrons and holes. The 1% 1T-2H MoSe2/Mn0.4Cd0.6S could achieve a notable hydrogen production rate of 12.34 mmol ∙ g−1∙ h−1, surpassing that of the Mn0.4Cd0.6S solid solution. This non-precious metal-loaded homojunction photocatalyst effectively harnesses solar energy to produce clean H2 under mild conditions, giving a promising prospect for practical applications and laying the groundwork for further exploration of MnxCd1-xS photocatalysts.

(Invited) Understanding Carrier-Selective Catalyst/Semiconductor Contacts in Water-Splitting Photoelectrodes and Photocatalyst Particles

Heterogeneous electrochemical processes, including photoelectrochemical water splitting to evolve hydrogen and oxygen using electrocatalyst-coated semiconductors, are driven by the accumulation of charge carriers and thus the interfacial electrochemical potential gradients that promote charge transfer. Conventional electrochemical techniques measure/control potentials at the conductive substrate or semiconductor ohmic contact, but are unable to isolate processes and electrochemical potentials at the surface during operation. I will discuss how the nanoelectrode tip of an atomic-force-microscope cantilever can effectively sense the surface electrochemical potential of electrocatalysts coating semiconductor photoelectrodes during operation. This data can be combined with ex-situ electrical measurements, spectroscopy, and materials characterization approaches to provide a comprehensive physical picture of how excited electrons and holes can be effectively separated and catalyst/semiconductor contacts. This new understanding is particularly important for the design of photocatalyst particles where direct electrical measurements of catalyst/semiconductor interface properties is not possible. In these systems we are applying spectroscopic techniques, for example ambient pressure x-ray photoelectron spectroscopy and Stark-effect spectroscopy to understand electric and electrochemical potential distribution and how nanoscale, adaptive, heterogeneous contacts can be controlled to design higher performance photochemical systems.

Strain-Engineered Quasiparticle Band Structure and Electron–Hole Excitation in Hittorf’s Phosphorene for Efficient Photon-to-Electricity Conversion

Strain-Engineered Quasiparticle Band Structure and Electron–Hole Excitation in Hittorf’s Phosphorene for Efficient Photon-to-Electricity Conversion

Electron–Hole Excitation and Optical Response in Two-Dimensional Fibrous Phosphorene

Electron–Hole Excitation and Optical Response in Two-Dimensional Fibrous Phosphorene

Introducing Ag Dopants into CdSe Nanoplatelets (NPLs) Leads to Effective Charge Separation for Better Photodetector Performance.

Solution-processed colloidal cadmium chalcogenide nanoplatelets (NPLs)-based photodetectors (PD) are promising materials for next-generation optoelectronic devices due to their excellent optical properties. Here, we report on ultrafast carrier relaxation dynamics of four monolayer (4 ML) Ag-doped CdSe (Ag: CdSe) NPLs using ultrafast transient absorption spectroscopy and their photodetectors applications. A broad dopant emission is observed at around 650 nm with a large FWHM of ~431 meV and band edge emission at 515 nm. The intragap dopant state acts as a hole acceptor, which leads to better charge separation. The ultrafast transient absorption spectroscopy study shows faster carrier recombination dynamics with a hole transfer time scale of ~10 ps in Ag-doped CdSe NPLs. This supports the excited hole capture phenomenon at the dopant state. Ag-doped CdSe NPLs-based PD performed better than undoped CdSe NPLs with detectivity and responsivity values of 1.3×1010 Jones and 2.4 mA/W, respectively.

Substituent effects in Benzo[c][1,2,5]thiadiazole-based donor–acceptor conjugated polymers: A density functional theory study

In the development of high-efficient organic photovoltaics (OPVs), theoretical screening methods for donor–acceptor conjugated polymers (D-A CPs) usually requires high experimental costs. In this study, we employed a time-dependent density functional theory, to investigate the electron–hole excitation characteristics of D-A CPs with substituent effects of methoxy (OCH3), fluorine (F) on the poly(4,7-bis(4′,5′-dimethyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (PBTD4T). Our findings revealed that the OCH3 substituent increases the highest occupied molecular orbital energy level, decreases the band gap, and enhances charge transfer and electron density in the D fragments. This approach highlights the potential of substituents for manipulating the electronic properties of CPs, offering a pathway for future development.

Ultrafast Photoexcitation Induced Passivation for Quasi-2D Perovskite Photodetectors.

Quasi-2D perovskites exhibit great potential in photodetectors due to their exceptional optoelectronic responsivity and stability, compared to their 3D counterparts. However, the defects are detrimental to the responsivity, response speed, and stability of perovskite photodetectors. Herein, an ultrafast photoexcitation-induced passivation technique is proposed to synergistically reduce the dimensionality at the surface and induce oxygen doping in the bulk, via tuning the photoexcitation intensity. At the optimal photoexcitation level, the excited electrons and holes generate stretching force on the Pb─I bonds at the interlayered [PbI6]-, resulting in low dimensional perovskite formation, and the absorptive oxygen is combined with I vacancies at the same time. These two induced processes synergistically boost the carrier transport and interface contact performance. The most outstanding device exhibits a fast response speed with rise/decay time of 201/627ns, with a peak responsivity/detectivity of 163 mA W-1/4.52 × 1010 Jones at 325nm and the enhanced cycling stability. This work suggests the possibility of a new passivation technique for high performance 2D perovskite optoelectronics.

Simultaneously Utilizing Excited Holes and Electrons for Piezoelectric-Enhanced Photoproduction of H2O2 from S-Scheme 2D S-Doped VOx/g-C3N4 Nanostructures.

2D/2D step-scheme (S-scheme) piezo-photocatalysts for the production of fine chemicals, such as hydrogen peroxide (H2O2), have attracted significant attention of global scientists owing to the efficiency in utilizing surface piezoelectric effects from 2D materials to overcome rapid charge recombination in photocatalytic processes. In this research, we reported the fabrication of 2D S-doped VOx deposited on 2D g-C3N4 to produce H2O2 via the piezo-photocatalytic process with high production yields at 20.19 mmol g-1 h-1, which was 1.75 and 4.87 times higher than that from solely piezo-catalytic and photocatalytic H2O2 generation. The finding pointed out that adding sulfur (S) to VOx can help to improve the catalytic outcomes by modifying the electronic properties of pristine VOx. In addition, when coupled with g-C3N4, the presence of S limits the formation of graphene in the VOx/g-C3N4 composites, causing shielding effects and pushing the cascade reactions toward water generation in the materials. Besides, the research also sheds light on the charge transport between g-C3N4 and S-VOx under irradiation and how the composites work to trigger the formation of H2O2. The presence of S in the composite systems enhances charge transfer between two semiconductors by strengthening the internal electric fields (IEF) to drive electrons moving in one direction, as demonstrated by density functional theory (DFT) calculations. Moreover, the formation of H2O2 significantly relies on the reduction of oxygen to generate oxygenic radical species at the g-C3N4 sites. Meanwhile, S-VOx provides oxidative sites in the composites to oxidize water molecules to directly or indirectly generate H2O2 or O2, which will further participate in the reactions to produce the final products. This study confirms the validation of S-scheme piezo-photocatalysts, thus encouraging further research on developing heterojunction materials with high catalytic efficiency, which can be used in practical conditions.

Exact Results for a Boundary-Driven Double Spin Chain and Resource-Efficient Remote Entanglement Stabilization

We derive an exact solution for the steady state of a setup where two XX-coupled N-qubit spin chains (with possibly nonuniform couplings) are subject to boundary Rabi drives and common boundary loss generated by a waveguide (either bidirectional or unidirectional). For a wide range of parameters, this system has a pure entangled steady state, providing a means for stabilizing remote multiqubit entanglement without the use of squeezed light. Our solution also provides insights into a single boundary-driven dissipative XX spin chain that maps to an interacting fermionic model. The nonequilibrium steady state exhibits surprising correlation effects, including an emergent pairing of hole excitations that arises from dynamically constrained hopping. Our system could be implemented in a number of experimental platforms, including circuit QED. Published by the American Physical Society 2024

Study of rutile TiO2(110) single crystal by transient absorption spectroscopy in the presence of Ce4+ cations in aqueous environment. Implication on water splitting

Ce4+ cations are commonly used as electron acceptors during the water oxidation to O2 reaction over Ir- and Ru-based catalysts. They can also be reduced to Ce3+ cations by excited electrons from the conduction band of an oxide semiconductor with a suitable energy level. In this work, we have studied their interaction with a rutile TiO2(110) single crystal upon band gap excitation by femtosecond transient absorption spectroscopy (TAS) in solution in the 350–900 nm range and up to 3.5 ns. Unlike excitation in the presence of water alone the addition of Ce4+ resulted in a clear ground-state bleaching (GSB) signal at the band gap energy of TiO2 (ca. 400 nm) with a time constant t = 4–5 ps. This indicated that the Ce4+ cations presence has quenched the e-h recombination rate when compared to water alone. In addition to GSB, two positive signals are observed and are attributed to trapped holes (in the visible region, 450–550 nm) and trapped electrons in the IR region (>700 nm). Contrary to expectation, the lifetime of the positive signal between 450 and 550 nm decreased with increasing concentrations of Ce4+. We attribute the decrease in the lifetime of this signal to electrostatic repulsion between Ce4+ at the surface of TiO2(110) and positively charged trapped holes. It was also found that at the very short time scale (<2–3 ps) the fast decaying TAS signal of excited electrons in the conduction band is suppressed because of the presence of Ce4+ cations. Results point out that the presence of Ce4+ cations increases the residence time (mobility) of excited electrons and holes at the conduction band and valence band energy levels (instead of being trapped). This might provide further explanations for the enhanced reaction rate of water oxidation to O2 in the presence of Ce4+ cations.

Spontaneous polarization dynamics in V-doped monolayer MoS2

Valleytronics has drawn attention both for its fundamental aspects and potential applications in device technology. Lifting the valley degeneracy is an attractive route to achieve stable valley polarization and spontaneous valley polarization. Our first-principles calculations on V-doped monolayer MoS2 reveal a substantial permanent valley splitting of 174 meV, driven by the combined effects of spin-orbit coupling (SOC) and magnetic coupling with the doped atom. Non-adiabatic molecular dynamics simulations show that the spontaneous valley polarization dynamics depends on the SOC, electron-phonon (e-ph) coupling and the scattering with the impurity, and it varies notably with different initial spin orientations of excited holes. For spin-up holes, the process involves e-ph scattering and spin-flip induced by spinor-phonon scattering over several picoseconds. On the other hand, spin-down hole excitation leads to spontaneous polarization through e-ph and impurity scattering on a timescale shorter by an order of magnitude. This study provides insights into the distinct valley polarization dynamics of carriers with different spins, contributing to the design of ultrafast valleytronic and spintronic devices.

Constructing Interfacial Charge Transfer Channels and Electric Field in Violet Phosphorus-Based van der Waals Heterojunction for Phototherapy of Periodontitis.

Periodontitis, a chronic oral disease instigated by bacteria, severely compromises human oral health. The prevailing clinical treatment for periodontitis involves mechanical scraping in conjunction with antibiotics. Phototherapy is employed to rapidly remove the bacteria and achieve periodontitis treatment, effectively circumventing the adverse effects associated with traditional therapies. Constructing 2D/2D van der Waals (VDW) heterojunctions is a key strategy for obtaining excellent photocatalytic activity. Herein, a 2D/2D violet phosphorus (VP)/Ti3C2 VDW heterojunction is designed using an interfacial engineering strategy. By constructing an electron transport "bridge" (P-Ti bond) at the heterogeneous interface as an effective transfer channel for photogenerated carriers, a compact monolithic structure between the VP and Ti3C2 phases is formed, and the spatial barrier for electron transfer at the interface is eliminated. Meanwhile, the strong directional built-in electric field induced by the intensive electron-coupling effect at the heterogeneous interface served as an internal driving force, which greatly accelerates the exciton dissociation and charge transfer in the photocatalytic process. These excited photogenerated electrons and holes are trapped by O2 and H2O on the surfaces of Ti3C2 and VP, respectively, and are subsequently catalytically converted to antibacterial reactive oxygen species (ROS). The VP/Ti3C2 VDW heterojunction eradicated 97.5% and 98.48% of Staphylococcus aureus and Escherichia coli, respectively, by photocatalytic and photothermal effects under visible light for 10 min. The VP/Ti3C2 nanoperiodontal dressing ointment effectively attenuated inflammatory response, reduced alveolar bone resorption, and promoted periodontal soft and hard tissue repair. Its periodontitis therapeutic effect outperforms the clinically used Periocline.

Synthesis and characterization of bi-functional Cu and Ni co-doped ZnO photocatalysts for organic pollutant degradation and antimicrobial activity

The study of visible light active photocatalysts for organic pollutants degradation and antimicrobial activity has gained significant attention in recent years. Herein, we report the synthesis of pure ZnO, Ni-doped ZnO (Ni-ZnO), Cu-doped ZnO (Cu-ZnO), and Cu and Ni co-doped ZnO (CuNi-ZnO) photocatalysts via a facile coprecipitation method. These photocatalyst materials were characterized using various analytical techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscope (HR-TEM), Fourier transform infrared spectroscopy (FTIR), UV–visible spectroscopy (UV–Vis), photoluminescence (PL), and total organic carbon (TOC) analysis. XRD patterns confirm the presence of a mixed phase in the CuNi-ZnO sample. The presence of Cu, Ni, and Zn in CuNi-ZnO was verified through the XPS spectra. The nanoparticle-like morphology of the CuNi-ZnO photocatalyst was confirmed using HRTEM analysis. The FTIR spectra reveal the chemical bonds present in the materials. UV–visible spectroscopy was employed to determine the band gap of the materials, revealing a value of 3.10 eV for CuNi-ZnO. The mitigation in recombination of excited electrons (e−) and holes (h+) in CuNi-ZnO is validated through photoluminescence (PL) studies. The synthesized materials were used for the degradation of Indigo carmine (IC) dye and antimicrobial activity. The results showed that the optimized CuNi-ZnO sample displayed a better visible light photocatalytic performance (93.32 % IC dye degradation efficiency in 60 min) than the other samples. A notable degree of mineralization was observed with a reduction in total organic carbon (TOC) reaching 65.91 %. Additionally, the photocatalyst showed antimicrobial activity against both S. aureus and E. coli. Furthermore, a plausible visible light photocatalytic mechanism was proposed for the degradation of IC dye using the CuNi-ZnO photocatalyst.

In-situ electron capture by surface Co-Ci to facilitate the solar water splitting of Fe2O3

Surface co-catalyst has been widely used to improve the performance of hematite for solar water splitting. However, the specific working mechanism for co-catalyst has not yet been extensively studied, especially lacking for in-situ information. Here we report a Co-Ci/Zr-Fe2O3 (LV) photoanode for efficient water splitting and probe the working mechanism of surface Co-Ci by in-situ X-ray absorption spectroscopy (XAS) under illumination. The low vacuum treatment and the Zr-doping in Co-Ci/Zr-Fe2O3 (LV) can significantly increase the donor density and then facilitate the bulk charge transfer, while the surface Co-Ci co-catalyst can greatly reduce the electron-hole recombination and then lower the onset potential. In-situ XAS reveals that under illumination, the photon-excited electrons will be quickly transferred to the Co sites in Co-Ci, while the excited holes will be left on Fe to form the high-valence Fe4+ to enhance the oxidation capability. As a result, the combined Co-Ci/Zr-Fe2O3 (LV) photoanode shows a quick bulk charge transfer and a low surface charge recombination, leading to a high photocurrent density of 4.10 mA/cm2 (1.23 VRHE) and a low onset potential of 0.85 VRHE with good stability.

Beyond single-molecule chemistry for electrified interfaces using molecule polaritons

Abstract This review introduces the preparation, observation, and characterization of systems of molecule polaritons. We summarize recent progress by our group on the control of molecular properties in the spaces of various optical cavities, such as plasmonic metal nanostructures and Fabry–Perot mirrors. After stating our motivations, we introduce topics on single-molecule observations based on surface-enhanced Raman scattering (SERS) spectroscopy. Our original findings are not limited to the ultrasensitive detection of molecules/materials, but include unique resonant enhancements of SERS through exotic electronic excitation processes at electrified interfaces. Comprehensive analyses of SERS spectra provide novel routes to electronic excitation using plasmonic metal nanostructures. This behavior leads to the distinct photoenergy conversion based on excited electrons and holes with anomalous electrochemical potentials. Such unique resonant electronic excitation emerges as a large optical force that can be used to manipulate small single molecules on surfaces in solution, even at room temperature. We also introduce systems for examining unique interactions between molecules and the optical modes of cavities, from single molecules to molecular ensembles. Plasmonic surface lattices and Fabry–Perot mirrors allow the formation of electronic and vibrational strong coupling states, respectively, showing unique properties of molecule polaritons, even under dark conditions, i.e. without photoillumination.