Electron Doping Research Articles

Resonance‐Assisted Self‐Doping in Robust Open‐Shell Ladder‐Type Oligoaniline Analogues

AbstractA novel resonance‐assisted self‐doping mechanism has been demonstrated in ladder‐type oligoaniline‐derived organic conductors. The new class of compounds has a unique structure incorporating acidic phenolic hydroxyl groups into the ladder‐type cyclohexadiene‐1,4‐diimine core, enabling efficient resonance‐assisted proton transfer and electronic doping without the need for external dopants. Mechanistic and computational studies confirm the open‐shell, zwitterionic nature of the self‐doped state and the significant role played by the dielectric environment. This new self‐doping mechanism allows for higher stability and durability in the material‘s electronic performance. The self‐doped form retains durability under harsh conditions and maintains its properties over extended periods of time.

Promoted room temperature NH3 gas sensitivity using interstitial Na dopant and structure distortion in Fe0.2Ni0.8WO4

The demand for gas-sensing operations with lower electrical power and guaranteed sensitivity has increased over the decades due to worsening indoor air pollution. In this report, we develop room-temperature operational NH3 gas-sensing materials, which are activated through electron doping and crystal structure distortion effect in Fe0.2Ni0.8WO4. The base material, synthesized through solid-state synthesis, involves Fe cations substitutionally located at the Ni sites of the NiWO4 crystal structure and shows no gas-sensing response at room temperature. However, doping Na into the interstitial sites of Fe0.2Ni0.8WO4 activates gas adsorption on the surface via electron donation to the cations. Additionally, the hydrothermal method used to achieve a more than 70-fold increase in the surface area of structure-distorted Na-doped Fe0.2Ni0.8WO4 powder significantly enhances gas sensitivity, resulting in a 4-times increase in NH3 gas response (Rg/Ra). Photoluminescence and XPS results indicate negligible oxygen vacancies, demonstrating that cation contributions are crucial for gas-sensing activities in Na-doped Fe0.2Ni0.8WO4. This suggests the potential for modulating gas sensitivity through carrier concentration and crystal structure distortion. These findings can be applied to the development of room-temperature operational gas-sensing materials based on the cations.

Geometric structure and electronic properties of bilayer graphene with a Moire superlattice by interlayer asymmetric tensile strain

Abstract Using the density functional theory combined with the effective screening medium method, we investigated the energetics and electronic properties of bilayer graphene, comprising graphene layers without and with tensile strain. An interlayer interaction does not affect the structural reconstruction of each graphene layer despite the bilayer structure possessing a Moire superlattice. The structural asymmetry between the graphene layers causes a potential difference across the layers, leading to electron and hole doping in the layers without and with the tensile strain. Accumulated carriers show unique lateral distribution in each graphene layer, which depends on the interlayer atomic arrangements.

Possible Superconductivity for Layered Metal Boride Carbide Compounds MB2C2 (M = Alkali, Alkaline-Earth, or Rare-Earth Metals).

The possible emergence of superconductivity in layered metal boride carbide compounds MB2C2 (M = Sc, Y, Be, Ca) was investigated using density functional theory calculations upon the topology of a boron-carbon network and the nature of the metal. ScB2C2 and YB2C2 show metallic and superconductive properties with low critical temperatures (Tcs). The semiconducting BeB2C2 compound may show superconductivity upon carrier doping with a high Tc of 47.8 K by hole doping─comparable to the structurally related MgB2 superconductor─but with a low Tc by electron doping. In contrast, the semiconducting CaB2C2 compound is predicted to be a superconductor by hole and electron doping but with low Tcs. These differences arise from the spatial distribution of electrons at the Fermi level. For compounds with low Tcs, electrons at the Fermi level are localized primarily on B and C π states perpendicular to the BC layers, experiencing minimal influence from atomic oscillations and resulting in weak electron-phonon interactions. Conversely, for a high Tc, electrons are found in σ-bonding states, leading to strong electron-phonon interactions. Electrons at the Fermi level in boron-carbon σ-bonding states seem to be a prerequisite to expect high Tc superconductivity in this kind of compound.

Electrical Control of Valley Polarized Charged Exciton Species in Monolayer WS2.

Excitons are key to the optoelectronic applications of van der Waals semiconductors, with the potential for versatile on-demand tuning of properties. Yet, their electrical manipulation remains challenging due to inherent charge neutrality and the additional loss channels induced by electrical doping. We demonstrate the dynamic electrical control of valley polarization in charged excitonic states of monolayer tungsten disulfide, achieving up to a 6-fold increase in the degree of circular polarization under off-resonant excitation. In contrast to the weak direct tuning of excitons typically observed using electrical gating, the charged exciton photoluminescence remains stable, even with increased scattering from electron doping. By exciting at the exciton resonances, we observed the reproducible nonmonotonic switching of the charged state population as the electron doping is varied under gate bias, indicating a resonant interplay between neutral and charged exciton states.

Electrode-Dependent and Tunable Sub-to-Super-Linear Responsivity in Mott Material-Enabled Near-Infrared Photodetectors for Advanced Near-Sensor Image Processing.

Brain-like intelligence is ushering humanity into an era of the Internet of Perceptions (IoP), where the vast amounts of data generated by numerous sensing nodes pose significant challenges to transmission bandwidth and computing hardware. A recently proposed near-sensor computing architecture offers an effective solution to reduce data processing delays and energy consumption. However, a pressing need remains for innovative hardware with multifunctional near-sensor image processing capabilities. In this work, Mott material (vanadium dioxide)-based photothermoelectric near-infrared photodetectors are developed that exhibit electrode-dependent and tunable super-linear photoresponse (exponent α > 33) with ultralow modulation bias. These devices demonstrate an opto-thermo-electro-coupled phase transition, resulting in a large photocurrent on/off ratio (>105), high responsivity (≈500 A W-1), and well detectivity (≈3.9× 1012 Jones), all while maintaining rapid response speeds (τr = 2 µs and τd = 5 µs) under the bias of 1V. This electrode-dependent super-linear response is found to arise from the electron doping effect determined by the polarity of the Seebeck coefficient. Furthermore, the work showcases intensity-selective near-sensor processing and night vision pattern reorganization, even with noisy inputs. This work paves the way for developing near-sensor devices with potential applications in medical image preprocessing, flexible electronics, and intelligent edge sensing.

Correlated topological flat bands in rhombohedral graphite

Flat bands and nontrivial topological physics are two important topics of condensed matter physics. With a unique stacking configuration analogous to the Su-Schrieffer-Heeger model, rhombohedral graphite (RG) is a potential candidate for realizing both flat bands and nontrivial topological physics. Here, we report experimental evidence of topological flat bands (TFBs) on the surface of bulk RG, which are topologically protected by bulk helical Dirac nodal lines via the bulk-boundary correspondence. Moreover, upon in situ electron doping, the surface TFBs show a splitting with exotic doping evolution, with an order-of-magnitude increase in the bandwidth of the lower split band, and pinning of the upper band near the Fermi level. These experimental observations together with Hartree-Fock calculations suggest that correlation effects are important in this system. Our results demonstrate RG as a platform for investigating the rich interplay between nontrivial band topology, correlation effects, and interaction-driven symmetry-broken states.

Indirect-To-Direct Bandgap Crossover and Room-Temperature Valley Polarization of Multilayer MoS2 Achieved by Electrochemical Intercalation.

Monolayer (1L) group VI transition metal dichalcogenides (TMDs) exhibit broken inversion symmetry and strong spin-orbit coupling, offering promising applications in optoelectronics and valleytronics. Despite their direct bandgap, high absorption coefficient, and spin-valley locking in K or K' valleys, the ultra-short valley lifetime limits their room-temperature applications. In contrast, multilayer TMDs, with more absorptive layers, sacrifice the direct bandgap and valley polarization upon gaining inversion symmetry from the bilayer structure. It is demonstrated that multilayer molybdenum disulfide (MoS2) can maintain 1) a structure with broken inversion symmetry and strong spin-orbit coupling, 2) a direct bandgap with high photoluminescence (PL) intensity, and 3) stable valley polarization up to room temperature. Through the intercalation of organic 1-ethyl-3-methylimidazolium (EMIM+) ions, multilayer MoS2 not only exhibits layer decoupling but also benefits from an electron doping effect. This results in a hundredfold increase in PL intensity and stable valley polarization, achieving 55% and 16% degrees of valley polarization at 3 K and room temperature, respectively. The persistent valley polarization at room temperature, due to interlayer decoupling and trion dominance facilitated by a gate-free method, opens up potential applications in valley-selective optoelectronics and valley transistors.

Band alignments, conduction band edges and intralayer bandgap renormalisation in MoSe2/WSe2 heterobilayers

Abstract Stacking two semiconducting transition metal dichalcogenide (MX2) monolayers to form a heterobilayer creates a new variety of semiconductor junction with unique optoelectronic features, such as hosting long-lived dipolar interlayer excitons. Despite many optical, transport, and theoretical studies, there have been few direct electronic structure measurements of these junctions. Here, we apply angle-resolved photoemission spectroscopy with micron-scale spatial resolution (µARPES) to determine the band alignments in MoSe2/WSe2 heterobilayers, using in-situ electrostatic gating to electron-dope and thus probe the conduction band edges. By comparing spectra from heterobilayers with opposite stacking orders, that is, with either MoSe2 or WSe2 on top, we confirm that the band alignment is type II, with the valence band maximum in the WSe2 and the conduction band minimum in the MoSe2. The overall band gap is EG = 1.43 ± 0.03 eV, and to within experimental uncertainty it is unaffected by electron doping. However, the offset between the WSe2 and MoSe2 valence bands clearly decreases with increasing electron doping, implying band renormalisation only in the MoSe2, the layer in which the electrons accumulate. In contrast, µARPES spectra from a WS2/MoSe2 heterobilayer indicate type I band alignment, with both band edges in the MoSe2. These insights into the doping-dependent band alignments and gaps of MX2 heterobilayers will be useful for properly understanding and ultimately utilizing their optoelectronic properties.

Spin-dependent electronic phenomena in heavily-doped monolayer graphene

The controlled manipulation of doping levels in graphene is crucial for advancing next-generation low-power electronics. Owing to its peculiar band structure, graphene's electronic functionalities can be easily tuned by electron doping, resulting in the bandgap openings, orbital hybridization, and induced spin-polarization. Here, we unveil the substantial doping of the graphene layer and the emergence of a significant bandgap by sandwiching a monolayer graphene, supported on ferromagnetic cobalt, between two europium layers. Our angle- and spin-resolved photoemission spectroscopy experiments, accompanied by dedicated density functional theory calculations, demonstrate that single-spin electron pockets form as a result of the hybridization of the topmost europium majority bands with graphene, while hybridization gaps are induced in the graphene π∗ bands by the Eu minority bands. Spin-resolved measurements further emphasize a single-spin dispersionless contribution proximal to the Fermi level, possibly arising from the coupling of single-spin polarized graphene bands to optical phonons. This work provides crucial insights into the phase diagram of heavily doped graphene, with significant potential for the design and development of novel 2D systems.

Analysis of the electronic nature and transport properties of Co2CrGe, Co2FeGe, and Co2NiGa by computational electronic structure calculations

AbstractElectronic and thermoelectric properties of the full‐Heusler materials Co2CrGe, Co2FeGe, and Co2NiGa are studied, using first‐principles full potential linearized augmented plane waves method. To treat the exchange correlation, Generalized Gradient Approximation parameterized by PBE‐sol with the inclusion of spin polarization is used. The frequency fluctuation order is continual at 300 K after heating to 3 ps with very small distraction in the atomic structure conforming the structure stability of the studied structures. Also inclusion of spin‐orbit coupling effect on density of states undermines significant change in peaks specially d‐states in the materials and caused a degeneracy. Band structure analysis clarified that states crossing the Fermi level have the importance for enhancing the thermoelectric properties of the materials. The topology of the states indicates a gap at the bulk level, which is the unique character of the heavy elements in the materials Co2CrGe, Co2FeGe, and Co2NiGa. Thermoelectric properties are calculated for a temperature of 300 K using semi‐classical Boltzmann's theory implemented in BoltzTraP code. In these calculations, it is found that these materials favor electron doping with having maximum values of ZT in the n‐type region.

Prediction of s^{±}-Wave Superconductivity Enhanced by Electronic Doping in Trilayer Nickelates La_{4}Ni_{3}O_{10} under Pressure.

Motivated by the recently reported signatures of superconductivity in trilayer La_{4}Ni_{3}O_{10} under pressure, we comprehensively study this system using abinitio and random-phase approximation techniques. Without electronic interactions, the Ni d_{3z^{2}-r^{2}} orbitals show a bonding-antibonding and nonbonding splitting behavior via the O p_{z} orbitals inducing a "trimer" lattice in La_{4}Ni_{3}O_{10}, analogous to the dimers of La_{3}Ni_{2}O_{7}. The Fermi surface consists of three electron sheets with mixed e_{g} orbitals, and a hole and an electron pocket made up of the d_{3z^{2}-r^{2}} orbital, suggesting a Ni two-orbital minimum model. In addition, we find that superconducting pairing is induced in the s^{±}-wave channel due to partial nesting between the M=(π,π) centered pockets and portions of the Fermi surface centered at the Γ=(0,0) point. With changing electronic density n, the s^{±} instability remains leading and its pairing strength shows a domelike behavior with a maximum around n=4.2 (∼6.7% electron doping). The superconducting instability disappears at the same electronic density as that in the new 1313 stacking La_{3}Ni_{2}O_{7}, correlated with the vanishing of the hole pocket that arises from the trilayer sublattice, suggesting that the high-T_{c} superconductivity of La_{3}Ni_{2}O_{7} does not originate from a trilayer and monolayer structure. Furthermore, we confirm the experimentally proposed spin state in La_{4}Ni_{3}O_{10} with an in-plane (π, π) order and antiferromagnetic coupling between the top and bottom Ni layers, and spin zero in the middle layer.

Iron selenide intercalated by uncharged molecules: Superconductivity in FeSe(C2H7NO)0.3

The charge‐neutral intercalated iron selenide compound FeSe(C2H7NO)0.3 is formed from FeSe(C2H8N2)0.3 through amine exchange under solvothermal conditions. Although the exchanged molecules are of similar size, the distance of the FeSe‐layers increases from 10.68 Å to 13.58 Å. The combination of Xray diffraction and DFT calculations suggests a chemically reasonable, albeit somewhat simplified, model for the orientation of the molecules between the FeSe layers, which interact via weak hydrogen bonds. The neutral guest species is unable to transfer charge to the FeSe layers, which remain electronically undoped. While the starting compound is non‐superconducting, the resulting product exhibits a transition to superconductivity at Tc = 14 K. The topology of the calculated Fermi surface undergoes only minimal alteration with increased layer spacing, a change that is considerably less pronounced than that observed with electron doping. Our findings indicate that undoped FeSe layers become superconducting when the interlayer distance is sufficiently large, while the highest critical temperatures necessitate additional electron doping.

Roles of Impurity Levels in 3d Transition Metal-Doped Two-Dimensional Ga2O3.

Doping engineering is crucial for both fundamental science and emerging applications. While transition metal (TM) dopants exhibit considerable advantages in the tuning of magnetism and conductivity in bulk Ga2O3, investigations on TM-doped two-dimensional (2D) Ga2O3 are scarce, both theoretically and experimentally. In this study, the detailed variations in impurity levels within 3d TM-doped 2D Ga2O3 systems have been explored via first-principles calculations using the generalized gradient approximation (GGA) +U method. Our results show that the Co impurity tends to incorporate on the tetrahedral GaII site, while the other dopants favor square pyramidal GaI sites in 2D Ga2O3. Moreover, Sc3+, Ti4+, V4+, Cr3+, Mn3+, Fe3+, Co3+, Ni3+, Cu2+, and Zn2+ are the energetically favorable charge states. Importantly, a transition from n-type to p-type conductivity occurs at the threshold Cu element as determined by the defect formation energies and partial density of states (PDOS), which can be ascribed to the shift from electron doping to hole doping with respect to the increase in the atomic number in the 3d TM group. Moreover, the spin configurations in the presence of the square pyramidal and tetrahedral coordinated crystal field effects are investigated in detail, and a transition from high-spin to low-spin arrangement is observed. As the atomic number of the 3d TM dopant increases, the percentage contribution of O ions to the total magnetic moment significantly increases due to the electronegativity effect. Additionally, the formed 3d bands for most TM dopants are located near the Fermi level, which can be of significant benefit to the transformation of the absorbing region from ultraviolet to visible/infrared light. Our results provide theoretical guidance for designing 2D Ga2O3 towards optoelectronic and spintronic applications.

Multiferroicity in a two-dimensional vanadium dioxide

Two-dimensional (2D) multiferroic materials have attracted great interest owing to the integration of ferroelastic and ferromagnetic properties. We identify a novel 2D multiferroic vanadium dioxide (VO₂) monolayer exhibiting a monoclinic phase with a C2/m space group using density functional theory (DFT) calculations. The energetic, dynamic, thermodynamic and mechanical analyses indicate that the monolayer exhibits excellent stability and can be prepared experimentally. The arrangement of the electronic energy bands is analogous to that of a type I heterostructure. The electron doping at a concentration of 0.2 electrons per V atom results in a significant increase in the Curie temperature (TC) from 11.2 to 184 K estimated by Monte Carlo simulations, and a transition from semiconductor to half-metallicity. In addition, the VO₂ monolayer exhibits 120° ferroelastic switching with a moderate switching energy barrier of 32 meV per atom, subsequently allowing 120° rotation of the easy magnetisation axis. Our work reveals the intrinsic multiferroicity of VO₂, which may provide a guidance on the design of next-generation mechanical/spintronic devices.

Surface Doping and Dual Nature of the Band Gap in Excitonic Insulator Ta2NiSe5.

Excitons in semiconductors and molecules are widely utilized in photovoltaics and optoelectronics, and high-temperature coherent quantum states of excitons can be realized in artificial electron-hole bilayers and an exotic material of an excitonic insulator (EI). Here, we investigate the band gap evolution of a putative high-temperature EI Ta2NiSe5 upon surface doing by alkali adsorbates with angle-resolved photoemission and density functional theory (DFT) calculations. The conduction band of Ta2NiSe5 is filled by the charge transfer from alkali adsorbates, and the band gap decreases drastically upon the increase of metallic electron density. Our DFT calculation, however, reveals that there exist both structural and excitonic contributions to the band gap tuned. While electron doping reduces the band gap substantially, it alone is not enough to close the band gap. In contrast, the structural distortion induced by the alkali adsorbate plays a critical role in the gap closure. This work indicates a combined electronic and structural nature for the EI phase of the present system and the complexity of surface doping beyond charge transfer.

Influence of Mg doping on structure and optical properties of red-emitting phosphor Li2TiO3:Mn4+

Li2TiO3:Mn4+ has been previously identified as a potential candidate to replace commercial red-emitting Eu2+-activated phosphors. Its luminescent efficiency is limited by the presence of defects and the sensitivity of Mn4+ to redox processes. A series of co-doped Li2TiO3: x Mg2+, 0.01 Mn4+ samples were prepared to gain insight on the effect of electron doping on the structure and photoluminescent properties, through characterization by powder X-ray diffraction, elemental analysis, and optical spectroscopy. As Mg2+ substitutes for Li+, the ordered arrangement of cations within the honeycomb layers in the parent structure (Li2SnO3-type, an ordered derivative of rocksalt) gradually transforms to a more disordered arrangement. The increased cation site disorder leads to less intense and broader emission peaks as well as lower quantum yields.

Propagation properties of plasma-filled parallel plate waveguide (PPWG) surrounded by black phosphorus (BP) layers

Propagation properties of plasma-filled PPWG surrounded by BP layers are explored in this research. Field equations are derived with the help of Maxwell’s equations. Boundary conditions x = d / 2 and x = − d / 2 are applied to obtain the characteristic equation for transverse magnetic (TM) mode. The influence of plasma frequency, electron doping, number of BP layers and core width on effective mode index are analyzed for armchair and zigzag conductivity of BP in the THz frequency range. It is concluded that armchair conductivity has a higher effective mode index compared to the zigzag conductivity of BP for the proposed waveguide structure. The presented results may have potential applications to fabricate black phosphorus-based nanophotonic devices in the THz frequency range.

FenB2+2n (n = 1, 2), an intrinsic room temperature ferromagnetic MBene with significant magnetic performance enhancement achievable by carbon substitution

From spintronics to data storage technology, two-dimensional (2D) ferromagnetic materials show great promise for various applications. This work reports a series of stable ferromagnetic transition metal boride FenB2+2n (n = 1, 2), where robust long-range ferromagnetic exchange coupling and large perpendicular magnetic anisotropy energy (MAE) allow the ferromagnetic transition temperature (Tc) of the FenB2+2n monolayer to reach above room temperature. The metallic FenB2+2n exhibits n- and p-type Dirac transport in both spin channels with a high Fermi velocity. Furthermore, the application of biaxial compressive strain and electron doping can greatly increase the ferromagnetic coupling and MAE of FeB4 monolayers. On this basis, the FeB2C2 alloy with a high concentration of carbon substitution has been designed, which allows the nonvolatile integration of in-plane compressive strain and electron doping. As expected, this substitution doping resulted in a significant increase in the Tc and MAE of the system. Our findings provide perspectives for the study of 2D magnetic materials.

(Invited) Electronic Doping in Two-Dimensional Semiconductors

Quantum-confined semiconductors provide highly tunable optical and electrical properties for a wide variety of emerging applications. Two-dimensional semiconductors possess tunable bandgaps, high charge carrier mobilities, and strong light-matter interactions that can enable applications in photovoltaics, quantum information processing, and spintronics. Electronic doping, the intentional introduction of controlled charge carrier density and type (electrons or holes) is a fundamental enabler for many emerging applications of two-dimensional semiconductors. While electrostatic doping in transistor geometries is useful for certain applications, it is important to develop additional methods based on e.g. molecular redox doping and substitutional atomic doping that can provide tunable Fermi level control in the absence of an applied bias. In this presentation, I will discuss our efforts to develop such doping strategies for a variety of two-dimensional semiconductors. I will discuss how electronic doping modulates opto-electronic properties, including ground-state absorbance, charge transport, and excited-state dynamics in these 2D semiconductors. These studies provide fundamental insights into the degree to which molecular and substitutional doping strategies can modulate Fermi level, charge transport, and excitonic optical transitions in 2D semiconductors.