Electron-hole Pair Density Research Articles

Quantum Size-Driven Spectral Variations in Pillar[n]arene Systems: A Density Functional Theory and Wave Function Assessment.

This study explores the quantum size effects on the optical properties of pillar[n]arene (n = 5, 6, 7, 8) utilizing density functional theory (DFT) and wave function analysis. The mechanisms of electron transitions in one-photon absorption (OPA) and two-photon absorption (TPA) spectra are investigated, alongside the calculation of electron circular dichroism (ECD) for these systems. Transition Density Matrix (TDM) and electron-hole pair density maps are employed to study the electron excitation characteristics, unveiling a notable size dependency. Analysis of the transition electric dipole moment (TEDM) and the transition magnetic dipole moment (TMDM) reveals the electromagnetic interaction mechanism within pillar[n]arene. Raman spectra computations further elucidate vibrational modes, while interactions with external environments are studied using electrostatic potential (ESP) analysis, and electron delocalization is assessed under an external magnetic field, providing insights into the magnetically induced current phenomena within these supramolecular structures. The thermal stability of pillar[n]arene was investigated by ab initio molecular dynamics (AIMD).

Development of diamond-based diagnostics for next-generation XFELs

The composition and performance of three different diamond sensor based pass-through diagnostics are presented. A position-sensitive quadrant sensor was designed and characterized using the XPP beam at the SLAC LCLS, demonstrating a generated-charge resolution of 0.025 pC and a position sensitivity of 2 μm, measurable pulse-by-pulse at a repetition rate of up to 50 MHz. A compact signal path capable of repetition rates up to 1 GHz was used to characterize the charge collection properties of diamond sensors as a function of the density of electron-hole pairs created within the sensor, finding a limit of approximately 1016 electron-hole pairs per cm3 beyond which the collection efficiency and time began to degrade. Finally, a proposed scheme for measuring ionizing radiation at multi-GHz rates is presented, along with results preliminarily demonstrating signal transport capability in excess of 5 GHz.

Photoexcitation induced magnetic phase transition and spin dynamics in antiferromagnetic MnPS3 monolayer

Antiferromagnetic spin dynamics is the key issue to develop spintronic devices. We adopt ab initio nonadiabatic molecular dynamics with spin–orbit-coupling (SOC) to investigate photoinduced spin dynamics in an antiferromagnetic semiconductor MnPS3 monolayer. Optical doping triggers MnPS3 from Néel antiferromagnetic to ferromagnetic phase at an experimentally achievable electron–hole pair density of 1.11 × 1014 cm−2. This phase transition can be ascribed to the light-induced mid-gap states of S-p orbitals, which lower the electron excitation energy and strengthen the SOC effect between S-p and Mn-d orbitals. The excited S-p electrons first decay to the mid-gap states due to p–p electron–phonon-coupling and then relax to the spin-down Mn-d orbitals via SOC. Such a dramatic relaxation process prolongs the photogenerated carrier lifetime up to 648 fs, providing an explanation for the unusual optoelectronic performance of MnPS3. The reversible switching of magnetic order via optical means gives an important clue for information storage and highly efficient photocatalysts utilizing antiferromagnetic semiconductors.

Quantum criticality of excitonic Mott metal-insulator transitions in black phosphorus

Quantum phase transition refers to the abrupt change of ground states of many-body systems driven by quantum fluctuations. It hosts various intriguing exotic states around its quantum critical points approaching zero temperature. Here we report the spectroscopic and transport evidences of quantum critical phenomena of an exciton Mott metal-insulator-transition in black phosphorus. Continuously tuning the interplay of electron-hole pairs by photo-excitation and using Fourier-transform photo-current spectroscopy as a probe, we measure a comprehensive phase diagram of electron-hole states in temperature and electron-hole pair density parameter space. We characterize an evolution from optical insulator with sharp excitonic transition to metallic electron-hole plasma phases featured by broad absorption and population inversion. We also observe strange metal behavior that resistivity is linear in temperature near the Mott transition boundaries. Our results exemplify an ideal platform to investigating strongly-correlated physics in semiconductors, such as crossover between superconductivity and superfluity of exciton condensation.

VS2 wrapped Si nanowires as core-shell heterostructure photocathode for highly efficient photoelectrochemical water reduction performance.

Interfacing an electrocatalyst with photoactive semiconductor surfaces is an emerging strategy to enhance the photocathode performance for the solar water reduction reaction. Herein, a core-shell heterostructure photocathode consisting of vanadium disulfide (VS2) as a 2D layered electrocatalyst directly deposited on silicon nanowire (Si NWs) surface is realized via single-step chemical vapor deposition towards efficient hydrogen evolution under solar irradiation. In an electrochemical study, 2D VS2/Si NWs photocathode exhibits a saturated photocurrent density (17mAcm-2) with a maximal photoconversion efficiency of 10.8% at -0.53V vs. RHE in neutral electrolyte condition (pH∼7). Under stimulated irradiation, the heterostructure photocathode produces a hydrogen gas evolution around 23μmolcm-2 h-1 (at 0V vs. RHE). Further, electrochemical impedance spectroscopy (EIS) analysis reveals that the high performance of the core-shell photocathode is associated with the generation of the high density of electron-hole pairs and the separation of photocarriers with an extended lifetime. Density functional theory calculations substantiate that core-shell photocathodes are active at very low Gibbs free energy (ΔGH*) with abundant hydrogen evolution reaction (HER) active sulphur sites. The charge density difference plot with Bader analysis of heterostructure reveals the accumulation of electrons on the sulphur sites via modulating the electronic band structure near the interface. Thus, facilitates the barrier-free charge transport owing to the synergistic effect of Si NWs@2D-VS2 core-shell hybrid photocatalyst for enhanced solar water reduction performance.

Encapsulated layer induces boundary effect and twist angle adjust absorption in h-BN/graphene/h-BN heterostructure

This work studied the optical absorption properties and electronic properties of graphene nanosheets encapsulated in h-BN at different twisted angles based on first principles. UV–Vis spectrum shows that compared with the monolayer graphene nanosheets, the absorption’s peak of the encapsulated graphene nanosheets show a red shift and its molar absorption coefficient decreased significantly. The molar absorption coefficients of h-BN/graphene/h-BN heterostructures show an increasing trend when the graphene nanosheets are twisted relative to the packaging layer (twisted angles are 0°, 15°, 21.8°, 27.8° and 30°, respectively). More curiously, charge density difference (CDD) indicates that the region of electron aggregation in excited state mainly occurs at the centre of graphene nanosheets due to the introduction of h-BN packaging layer. The smoothed isosurface diagram of electron hole pairs shows that the distribution breadth of electron hole pair density decreases compared with that before encapsulation, and the distribution breadth of electron increase is larger than that of hole. With the increase of twisted angles, the distribution breadth of electron hole pair density increases gradually. Both CDD and smoothed isosurface diagram show that the system has strong boundary effect. This provides a method to adjust the photoelectric properties of graphene nanosheets by using different twisted angles, and provides a theoretical basis for its application in the field of micro-nano optoelectronics.

Optical nonlinearities in the excited carrier density of atomically thin transition metal dichalcogenides

In atomically thin semiconductors based on transition metal dichalcogenides, photoexcitation can be used to generate high densities of electron-hole pairs. Due to optical nonlinearities, which originate from Pauli blocking and many-body effects of the excited carriers, the generated carrier density will deviate from a linear increase in pump fluence. In this paper, we describe nonlinear absorption properties and excited carrier dynamics using a theoretical approach that combines results from ab initio electronic-state calculations with a many-body treatment of optical excitation. We determine the validity range of a linear approximation for the excited carrier density vs pump power and identify the role and magnitude of optical nonlinearities at elevated excitation carrier densities for ${\mathrm{MoS}}_{2}, {\mathrm{MoSe}}_{2}, {\mathrm{WS}}_{2}$, and ${\mathrm{WSe}}_{2}$ considering various excitation conditions. For photoexcitation at the bandgap, we find that the use of a linear absorption coefficient of the unexcited system can strongly underestimate the achievable carrier density in ${\mathrm{MoS}}_{2}$ due to many-body renormalizations of the two-particle density of states. The same holds for excitation of the high-energy band continuum in W-based materials.

Angle-resolved one and Two-Photon absorption spectrum in twisted bilayer graphene quantum dots

In this paper, one-photon absorption (OPA) spectrum and two-photon absorption (TPA) spectrum and its electronic excitation, charge transfer, partial density of states (PDOS) and magnetic induction current density of zigzag edge twisted bilayer graphene quantum dots (Z-TwBLG-QDs) with different twisted angles which were divide into two pattern of Moiré superlatrate (MS) are qualitativly and quantitativly analyed by using density functional theory (DFT), excited state wave function, single electron approximation, and gauge-invariant atomic orbital (GIAO) method. The inter-layer local excitation in the electronic transition and intra-layer charge transfer dominated by MS and anti-symmetric electron-hole pair density induced by PDOS are discussed. The twisted angle is decisive to the absorption spectrum, large twisted angle will lead to the red shift of the absorption spectrum. The two-photon transition shows that the twisted angle can regulate the TPA cross section and the boundary effect of Z-TwBLG-QDs in a specific wavelength range. The edge effect of two-step transition can be enhanced by MS with specific twisted angles. The physical mechanism of twisted angle regulating electron transition behavior is revealed by magnetic induction current density analysis.

Coherent transfer matrix analysis of the transmission spectra of Rydberg excitons in cuprous oxide

In this study we analyze the transmission spectrum of a thin plate of Cuprous Oxide in the range of the absorption of the yellow exciton states with the coherent transfer matrix method. We demonstrate that, in contrast to the usual analysis using Beer's law, which turns out to be a rather good approximation only in the spectral region of high principal quantum numbers, a consistent quantitative description over the whole spectral range under consideration is possible. This leads to new and more accurate parameters not only for the Rydberg exciton states, but also for the strengths of indirect transitions. Furthermore, the results have consequences on the determination of the density of electron-hole pairs after optical excitation.

Mechanism for Synchronization of Charge Oscillations in Dimer Lattices

We discuss the mechanism and the conditions for the appearance of synchronized charge oscillations which have been observed experimentally and theoretically after strong photoexcitation of dimerized systems. In the Hubbard model with on-site repulsion, the Bloch equations for a wave-number-dependent pseudospin -- whose components describe the charge-density difference, current density, and bond density between the two sublattices -- involve an alternatingly tilted pseudomagnetic field, which assists the synchronization of pseudospins with different wave numbers, irrespective of the initial condition. This fact is numerically confirmed by the dynamics in finite lattices based on the exact diagonalization method. In the presence of nearest-neighbor repulsion, however, the synchronization can be hindered by excitons. Therefore, the excitation of a sufficiently large density of free electron-hole pairs, but low density of excitons, is needed to achieve synchronization.

Stability of electron–hole liquid in quantum wells

Density functional theory is used to calculate the energy of electron–hole liquid and the equilibrium density of electron-hole pairs in quantum wells. Nonlinear Kohn–Sham equations for electrons and holes are solved numerically. The influence of the depth and width of the quantum well, the ratio of the hole and electron masses, and the spin splitting of the hole band on the properties of electron–hole liquid is studied. The critical temperature of electron–hole liquid in quantum wells is estimated. Good agreement between the calculations and experimental results is obtained.

Relationship between ion track characteristics and single event transients in nanometer inverter chain

Ions with the same linear energy transfer (LET) value, but different energies and species have various ion track characteristics, and thus induce different single event transient (SET) responses in combinational logics. As the technology feature size shrinks, this issue continues to be serious. The research of the relationship between ion track characteristics and SETs in the nanometer combinational logic circuits is of great significance for accurately predicting the soft error rates of nanometer devices used in spacecraft. The combinational logic circuit investigated in this paper is a 65 nm bulk silicon complementary metal oxide semiconductor (CMOS) technology inverter chain. The three-dimensional TCAD model of the inverter chain is established, and the particle transport program for the ion track in bulk silicon is developed by Geant4. The track characteristics of high- and low-energy ions (the energy of low-energy ions is less than 10 MeV·n<sup>–1</sup>, the energy of high-energy ions ranges from tens of MeV·n<sup>–1</sup> to hundreds of MeV·n<sup>–1</sup>) which have the same LET value are compared with each other. Based on the coupled simulation of TCAD and Geant4, the difference in SET pulse widths, induced by ions with the same LET value but different energies, are investigated. It is found that when the energy per nucleon ratio of high-energy ion to low-energy ion increases, the difference in electron-hole pair densities near the center axis of the ion track is more significant, and thus the difference in SET pulse widths is larger. If the energy per nucleon ratio is similar, raising the LET value of ions can increase the difference in electron-hole pair densities on the same radial scale of the ion tracks, and make the SET pulse width difference more obvious. For the high- and low-energy ions with the same LET value, the single event charge generations are different. On the other hand, the change of the ion incident angle results in different charge collections. The coupling of these two factors makes the difference in SET pulse widths dependent on the incident angle of ions. In addition, the influence of ion track characteristics on SET has a weak correlation with the bias voltage of the inverter chain.

Electrically controlled two-dimensional electron-hole fluids

We study the electronic properties of dual-gated electron-hole bilayers in which the two layers are separated by a perfectly opaque tunnel barrier. Combining an electrostatic and thermodynamic analysis with mean-field theory estimates of interacting system chemical potentials, we explain the dependence of the electron and hole densities on the two gate voltages. Because chemical potential jumps occur for both electrons and holes at neutrality, there is a finite area in gate voltage parameter space over which electron and hole densities are equal. In that regime the electron-hole pair density depends only on the sum of the two gate voltages. We are able to explain a recent experimental study of electrically controlled bilayers by allowing for interlayer tunneling and using a non-equilibrium steady-state transport picture.

Optically Driven Magnetic Phase Transition of Monolayer RuCl3.

Strong light-matter interactions within nanoscale structures offer the possibility of optically controlling material properties. Motivated by the recent discovery of intrinsic long-range magnetic order in two-dimensional materials, which allow for the creation of novel magnetic devices of unprecedented small size, we predict that light can couple with magnetism and efficiently tune magnetic orders of monolayer ruthenium trichloride (RuCl3). First-principles calculations show that both free carriers and optically excited electron-hole pairs can switch monolayer RuCl3 from a proximate spin-liquid phase to a stable ferromagnetic phase. Specifically, a moderate electron-hole pair density (on the order of 1 × 1013 cm-2) can significantly stabilize the ferromagnetic phase by 10 meV/f.u. in comparison to the competing zigzag phase, so that the predicted ferromagnetism can be driven by optical pumping experiments. Analysis shows that this magnetic phase transition is driven by a combined effect of doping-induced lattice strain and itinerant ferromagnetism. According to Ising-model calculations, we find that the Curie temperature of the ferromagnetic phase can be increased significantly by raising carrier or electron-hole pair density. This enhanced optomagnetic effect opens new opportunities to manipulate two-dimensional magnetism through noncontact, optical approaches.

High-temperature electron-hole liquid in diamond films

The ground state of the electron-hole liquid in diamond films is studied by density functional theory. The energy and equilibrium density of the liquid in films with thicknesses of a few nanometers are calculated. It is shown that the electron-hole liquid is multicomponent and consists of electrons and heavy, light, and spin-orbit split holes. The equilibrium density of electron-hole pairs in (111)-oriented diamond films is much higher than that in films with (100) orientation. The critical temperature of the electron-hole liquid in (111)-oriented diamond films is close to room temperature.

Total ionizing dose effects of <b>γ</b> and X-rays on 55 nm silicon-oxide-nitride-oxide-silicon single flash memory cell

The total ionizing dose (TID) effects on 55 nm SONOS flash cell, caused by <sup>60</sup>Co-γ ray and 10 keV X-ray radiation source, are systematically investigated in this paper. The degradation of electrical characteristics is discussed while the underlying physical mechanism is analyzed. The drift of <i>I-V</i> characteristic curve, the degradation of memory window, and the increase of stand-by current are observed after TID irradiation separately by the two radiation sources. The data retention capability is also affected by the TID irradiation. The <i>I</i>-<i>V</i><sub>g</sub> curve of the programmed single flash cell significantly drifts towards the negative direction after TID irradiation, while the negative drift of erased state is much slower. Referring to the erased state, the drift directions of <i>I</i><sub>d</sub>-<i>V</i><sub>g</sub> curves for γ- and X-ray radiation source are obviously different. The physical mechanism of irradiation damage in a 55 nm SONOS single flash cell is discussed in detail by the energy band theory and TCAD simulations. The storage charge loss in silicon nitride layer, the charge accumulation, and the generation of interface states all together lead to the degradation of threshold voltage and stand-by current after TID irradiation. Another cause for the increase of stand-by current is the positive trapped charges in the isolated oxide induced by irradiation, which leads to the generation of leakage paths. Significant dose enhancement effect of X-ray irradiation is observed in this paper. Device model of memory transistor c is established while the dose enhancement effect of X-rays is investigated by Geant 4 tool. The high-<i>Z </i>materials above the poly-silicon gate lead to the dose enhancement effect of X-rays’ irradiation, which results in the higher degradation. The density of electron-hole pairs produced by irradiation in W layer is much higher than in Cu layer. In particular, W layer is a critical factor regardless of the thickness, which can be obviously observed in the simulation.

Quasi-two-dimensional electron-hole liquid with two type of holes in SiGe/Si quantum wells

The density functional theory was used to calculate the energy of a quasi-two-dimensional electron-hole liquid (EHL) in Si/Si1-xGex/Si quantum wells. We have performed a numerical solution of the three one-dimensional nonlinear Schrödinger equations for electrons, heavy and light holes. It is shown that EHL contains heavy and light holes for small x. With an increase in x, a transition to a state with one type of hole occurs, and the density of electron-hole pairs decreases by more than three times. The results of the calculations are in good agreement with the experimental results.

Quasi-Two-Dimensional Electron-Hole Liquid in a Magnetic Field

An analytical expression is derived for the energy of a three-component electron-hole liquid (EHL) in a magnetic field. Results of calculations of the EHL properties in Si/Si1–xGex/Si quantum wells are presented. The influence of the magnetic field on the EHL properties and stability depending on the germanium concentration in the SiGe layer is revealed. It is found that the equilibrium electron-hole pair density strongly increases with the magnetic field. It is shown that the dependences of the filling factors of the Landau levels on the magnetic field are shaped as plateaus.

Quasi-Two-Dimensional Electron–Hole Liquid in Si/SiO2 Quantum Wells

To calculate the energy and equilibrium density of electron-hole pairs in SiO2/Si/SiO2 quantum wells (QWs), nonlinear Schrodinger equations for electrons and holes were numerically solved. Calculations were carried out for (100) and (111) silicon surfaces and various values of the QW width. It is shown that the binding energy of electron-hole pairs in a quasi-two-dimensional electron-hole liquid (EHL) is much higher than the binding energy in a three-dimensional EHL. The results of calculations are compared with the experimental results for a wide range of QW widths.

Quasi-Two-Dimensional Electron–Hole Liquid in Shallow SiGe/Si Quantum Wells

An analytical expression is obtained for the energy of a quasi-two-dimensional electron-hole liquid (EHL) in shallow quantum wells. It is shown that in the Si/Si1–xGex/Si structures with small x, the EHL contains light and heavy holes. With increasing x, the transition of EHL to a state with heavy holes occurs, and the equilibrium density of electron-hole pairs strongly decreases. The effect of an external electric field on the EHL properties is studied.