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Energy Band Alignment of a Monolayer MoS <sub/>2</sub> with SiO <sub/>2</sub> and Al <sub/>2</sub> O <sub/>3</sub> Insulators from Internal Photoemission
摘要： Internal photoemission of electrons (IPE) from large area one monolayer 2H-MoS2 films synthesized on top of amorphous (a-) SiO2 or Al2O3 is used to determine the energy of the semiconductor valence band (VB) relative to the reference level of the insulator conduction band (CB). This allows us to compare the VB top energy in MoS2 to that of the (100)Si substrate crystal at the interface with the same insulator. Despite the CB in a–Al2O3 is found to be ~1 eV below that in SiO2 as measured relative to the Si VB edge, the authors observe nearly no shift of the spectral threshold in the case of IPE from the MoS2 VB. This observation indicates violation of electroneutrality at the MoS2/a–Al2O3 interface causing an increase in barrier by ~1 eV. This conclusion is supported by the much weaker field dependence of the IPE threshold at the MoS2/a–Al2O3 interface compared to the MoS2/a–SiO2 one, suggesting the presence of negative charges and/or interface dipoles. Therefore, the commonly accepted electron affinity rule (EAR) appears to be not appropriate to describe the band alignment at 2D/insulator interfaces.
关键词： internal photoemission spectroscopy,MoS2,electron affinity,electron barrier,band alignment
更新于2025-09-19 17:15:36
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Femtosecond Laser-Induced Electron Emission from Nanodiamond-Coated Tungsten Needle Tips
摘要： We present femtosecond laser-induced electron emission from nanodiamond-coated tungsten tips. Based on the shortness of the femtosecond laser pulses, electrons can be photoexcited for wavelengths from the infrared (1932 nm) to the ultraviolet (235 nm) because multiphoton excitation becomes efficient over the entire spectral range. Depending on the laser wavelength, we find different dominant emission channels identified by the number of photons needed to emit electrons. Based on the band alignment between tungsten and nanodiamond, the relevant emission channels can be identified as specific transitions in diamond and its graphitic boundaries. It is the combination of the character of initial and final states (i.e., bulk or surface-near, direct or indirect excitation in the diamond band structure), the number of photons providing the excitation energy, and the peak intensity of the laser pulses that determines the dominant excitation channel for photoemission. A specific feature of the hydrogen-terminated nanodiamond coating is its negative electron affinity that significantly lowers the work function and enables efficient emission from the conduction band minimum into vacuum without an energy barrier. Emission is stable for bunch charges of up to 400 electrons per laser pulse. We infer a normalized emittance of <0.20 nm rad and a normalized peak brightness of >1.2 × 1012 A m?2 sr?1. The properties of these tips are encouraging for their use as laser-triggered electron sources in applications such as ultrafast electron microscopy as well as diffraction and novel photonics-based laser accelerators.
关键词： nanodiamond,ultrafast electron microscopy,tungsten tips,multiphoton excitation,electron emission,negative electron affinity,femtosecond laser
更新于2025-09-16 10:30:52
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Candidate for Laser Cooling of a Negative Ion: High-Resolution Photoelectron Imaging of
摘要： Laser cooling is a well-established technique for the creation of ensembles of ultracold neutral atoms or positive ions. This ability has opened many exciting new research fields over the past 40 years. However, no negatively charged ions have been directly laser cooled because a cycling transition is very rare in atomic anions. Efforts of more than a decade currently have La? as the most promising candidate. We report on experimental and theoretical studies supporting Th? as a new promising candidate for laser cooling. The measured and calculated electron affinities of Th are, respectively, 4901.35e48T cm?1 and 4832 cm?1, or 0.607 690(60) and 0.599 eV, almost a factor of 2 larger than the previous theoretical value of 0.368 eV. The ground state of Th? is determined to be 6d37s2 4Fe 5=2. The consequence of this there are several strong electric dipole transitions between the bound levels arising from configurations 6d37s2 and 6d27s27p in Th?. The potential laser-cooling transition is 2So 3=2 with a wavelength of 2.6 μm. The zero nuclear spin and hence lack of hyperfine structure in Th? reduces the potential complications in laser cooling as encountered in La?, making Th? a new and exciting candidate for laser cooling.
关键词： Laser cooling,photoelectron imaging,electron affinity,Th?,negative ion
更新于2025-09-12 10:27:22
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Acceptor Gradient Polymer Donors for Non-Fullerene Organic Solar Cells
摘要： In organic solar cells, maximizing the open-circuit voltage (VOC) via minimization of the ionization energy or electron a?nity o?sets of the blended donor and acceptor often comes at the expense of achieving a considerable amount of short-circuit current (JSC). To explore a hypothesis for the design of materials that may circumvent this tradeo?, eight structurally similar polymers were synthesized consisting of a ?uorinated/non-?uorinated benzothiadiazole (BTDF/BTD) strong acceptor moiety, a thiophene ester (TE) weak acceptor, and various donor units composed of bithiophene (T2), biEDOT, and benzodithiophene (BDT) to form six acceptor gradient and two nongradient polymers. The acceptor gradient motif was designed and theorized to induce more facile exciton dissociation in low driving force solar cells by creating a further separated intramolecular charge-transfer state between the strong BTD acceptor and various donor units through a bridging TE component. Solar cells were fabricated using the eight polymers blended with phenyl-C71-butyric-acid methyl ester (PC71BM) to reveal two top performing isomeric polymers, T2-BTDF-(TE2) and TE2-BTDF-(T2), which were further tested with several non-fullerene acceptors (NFAs): EH-IDTBR, ITIC, and ITIC-4F. In order to fabricate optimally performing solar cells, a 0.2 eV ionization energy o?set was found to be essential or the short-circuit current of the NFA cells diminished dramatically. Ultimately, optimized NFA solar cells were fabricated using ITIC-4F paired with each of the top performing polymers to produce an average PCE of 7.3% for TE2-BTDF-(T2) (nongradient) and 3.6% for T2-BTDF-(TE2) (gradient). The acceptor gradient e?ect was not shown to reduce the amount of charge recombination in NFA solar cells mainly due to the inability to fabricate solar cells, with minimal ionization energy or electron a?nity o?sets along with morphological complications. This work stresses the importance of acquiring accurate ionization energies and electron a?nities when characterizing solar cell energetics, as di?erences as small as 0.1 eV in the o?sets can make a signi?cant impact on overall charge collection.
关键词： acceptor gradient polymers,open-circuit voltage,electron affinity offset,charge recombination,organic solar cells,non-fullerene acceptors,ionization energy offset,short-circuit current
更新于2025-09-11 14:15:04
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High Resolution Photoelectron Spectroscopy of Cryogenically-Cooled NO <sub/>3</sub> ˉ
摘要： High-resolution anion photoelectron spectra of cryogenically cooled NO3ˉ anions obtained using slow photoelectron velocity-map imaging are presented and provide new insight into the vibronic structure of the corresponding neutral radical. A combination of improved spectral resolution, measurement of energy-dependent intensity effects, temperature control, and comparison to theory allows for full assignment of the vibronic features observed in this spectrum. We obtain a refined electron affinity of 3.9289(14) eV for NO3. Further, the appearance of Franck-Condon forbidden transitions from vibrationally cold anions to neutral states with excitation along the NO3 v4 mode confirms that these features arise from vibronic coupling with the ? 2 B E excited state of NO3 and are not hot bands as has been suggested. Together, the suite of experimental and simulated results provides clear evidence that the v3 fundamental of NO3 resides near 1050 cm?1, addressing a long-standing controversy surrounding this vibrational assignment.
关键词： vibronic structure,High-resolution photoelectron spectroscopy,NO3ˉ anions,electron affinity,vibronic coupling
更新于2025-09-11 14:15:04
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Introducing Four 1,1-Dicyanomethylene-3-Indanone (IC) End-capped Groups as an Alternative Strategy for the Design of Small Molecular Nonfullerene Acceptors
摘要： Linear A-D-A or A-π-D-π-A architectures are predominant in the design of promising non-fullerene acceptors (NFAs), which promoted the rapid progress of organic solar cells (OSCs). However, utilization of four electron-accepting units (A) to construct four-armed NFAs is rarely reported and the relationship of structure-properties-performance is unclear. In this study, we designed and synthesized a novel acceptor (A401) with (AA)-π-D-π-(AA) configuration, where four 1,1-dicyanomethylene-3-indanone (IC) groups were used as the end-capped segments. When A401 was paired with a classic p-type polymer PBDB-T, a power conversion efficiency (PCE) of 7.54% could be achieved, which was much higher than that of the reported two-armed analogue of DC-IDT2T (3.93%). The improved photovoltaic performance of A401 should be ascribed to the high electron-affinity. Our results indicate introducing more end-capped electron-accepting units is a simple and effective alternative strategy for the design of promising NFAs. Our conceptualized molecular architecture will encourage further research of high-performance multi-armed NFAs.
关键词： photovoltaic performance,electron-affinity,electron-accepting units,organic solar cells,non-fullerene acceptors
更新于2025-09-10 09:29:36
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Improved design of a photon enhanced thermionic energy converter
摘要： In this work, the model of photon enhanced thermionic energy converter (PETEC) is improved to design a new vacuum thermionic energy converter (VTEC) consisting of a cathode and an anode is established, in which the two electrode materials are made of p-type silicon semiconductor coated negative electron affinity material and boron doped diamond. Formulas for the power output density and efficiency of the VTEC are derived. The electron affinity is optimized to obtain the maximum power output density and efficiency of the VTEC. Making a trade-off between the power output density and efficiency, the parametric optimum design criteria are determined. The results obtained in this work are helpful for developing novel VTECs.
关键词： Electron affinity,Work function,Optimum design,Finite time thermodynamics,Thermionic emission
更新于2025-09-09 09:28:46
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A Thiazole-fused Antiaromatic Compound Containing an <i>s</i> -Indacene Chromophore with a High Electron Affinity
摘要： We report on the preparation and characterization of a thiazole-fused polycyclic compound containing an s-indacene chromophore, which has the characteristics of an antiaromatic the fused π-conjugated system. X-ray analysis showed that the backbone has a planar structure. Theoretical investigations indicate that the introduction of thiazole rings contributes to decreasing the frontier orbital energy level while permitting the antiaromatic character of the compound to be retained. Physical measurements indicate that the characteristic properties and structures originates from the antiaromatic framework. This study provides an effective method for the chemical modification of s-indacene chromophores with a high electron affinity.
关键词： Antiaromatic compound,Electron affinity,s-Indacene
更新于2025-09-09 09:28:46