Impurity Band Research Articles

Hopping Conduction in DX-Center-Related Impurity Bands in AlxGa1−xSb, AlxGa1−xAs, and GaAs1−xPx

Hopping Conduction in DX-Center-Related Impurity Bands in AlxGa1−xSb, AlxGa1−xAs, and GaAs1−xPx

P-Orbital Ferromagnetism Arising from Unconventional O- Ionic State in a New Semiconductor Sr2AlO4 with Insufficiently Bonded Oxygen.

Oxygen in solids usually exists in an O2- ionic state. As a result, it loses its magnetic nature of a single atom, wherein two unpaired electrons exist in its outer 2p orbitals. Here, it is shown that an unconventional stable ionic state of O- is realized in a new semiconductor material Sr2AlO4, leading to an intrinsic p-orbital ferromagnetism stable until ≈900K. Experimental and theoretical investigations have clarified that one-fourth of the oxygen atoms in Sr2AlO4 are insufficiently bonded in the crystal structure, resulting in a unique O--state and p-orbital ferromagnetism. To date, the O- state is reported to exist only in non-equilibrium conditions, and p-orbital magnetism is only suggested in impurity bands with small ferromagnetic moments. The present work provides a new route for creating ferromagnetism in semiconductors and exploring new p-orbital physics and chemistry. In addition, the material shows elastic-mechanoluminescence that may enable unprecedented mechano-photonic-spintronics.

Creating an intermediate energy band to boost the photoelectrochemical efficiency of TiO2 for solar-driven hydrogen production

The utilization of photoelectrochemical processes for hydrogen generation from water and solar energy offers a promising avenue to replace non-renewable energy sources. Nevertheless, achieving high-performance photoanode electrodes poses a significant challenge. In this study, we present the synthesis cerium and chromium-doped TiO2 (CeCrTiO2) is produced through a simple, scalable, and cost-effective hydrothermal method on a fluorine-doped tin oxide (FTO) substrate. the introduction of Ce and Cr impurities is highlighted for its role in creating an impurity band within CeCrTiO2. This impurity band is instrumental in enhancing the separation and movement of electrons and holes, contributing to the improved performance of CeCrTiO2 as a photoanode in photoelectrochemical applications. Hence, the photocurrent density value of CeCrTiO2 is 4.5 times higher than that of bare TiO2. This suggests that CeCrTiO2 exhibits improved efficiency in generating a photocurrent when exposed to light, indicating enhanced photoelectrochemical performance. The amount of hydrogen produced by CeCrTiO2 is noted to be 4.88 times greater than that of bare TiO2. This highlights the material's effectiveness in harnessing solar energy to drive the water-splitting reaction, leading to a higher yield of hydrogen gas. The synthesis of CeCrTiO2 through a hydrothermal method results in a photoanode with significantly enhanced performance, positioning it as a promising candidate for advancing photoelectrochemical processes aimed at replacing non-renewable energy sources.

Microscopic Insights into Metallization of Diamond with Transition Metals

AbstractMetallization of diamond with transition metals (TMs) has attracted much attention as they play an important role in the development of precision machining, high‐end thermal management, especially for electronic devices. However, it is found that the solid‐state reaction does not stop at the interface, potentially hampering interface engineering. To unleash the full potential of diamond devices, deeper reactions are investigated in monocrystalline intrinsic diamond metallized with Ti/Pt/Au ohmic contacts combing experiments and theoretical calculations. Apart from exhibiting TiC, graphite, TMs, and diamond nanocrystallites at the interface, a unique ion‐implanted‐like multilayered structure is observed at micrometer depth. TM atoms penetrate through diamond produce a buried amorphous carbon layer under the damaged diamond lattice. In addition, these theoretical calculations reveal that the structural amorphization is catalyzed by the incorporation of TM atoms through vacancy‐mediated diffusion, which induces the closure of the bandgap of the diamond. This facilitates carriers crossing the bandgap via impurity band conduction and promotes the formation of the ohmic contact. This study provides critical insight into the microscopic mechanisms of the metallization between diamonds and TMs and could facilitate the development of diamond‐based electronic devices with a tailored electronic property.

The thermal performance model of a 105 mW@4.41 K 4He Joule-Thomson cryocooler designed for space applications

The 4He Joule-Thomson cryocooler (JTC) ensures the operation of space detectors working within the 4 K temperature range, such as the Block Impurity Band Infrared Detector (BIBID). In addition, the 4He JTC is one of the essential components of the 300 K to 50 mK space cryochain. It can precool sub-Kelvin refrigerators, such as the adiabatic demagnetization refrigerator (ADR) and the dilution refrigerator (DR). With the development of space science, the need for cooling power at the 4 K temperature range is growing. Thus, a thermal performance model of the 4He JTC precooled by a two-stage thermally coupled pulse tube cryocooler (PTC) is designed, manufactured, and tested with an eye toward space applications. A four-stage compression system with a high operating frequency and large pistons is designed and used to drive the JT cycle. This 4He JTC has a total mass of 55 kg and can provide a cooling capacity of 105 mW at 4.41 K with a total input electric power of 548 W.

Identification and Activity Study of an Impurity Band Observed in the nrSDS-PAGE of Aflibercept.

Aflibercept is a biopharmaceutical targeting vascular endothelial growth factor (VEGF) that has shown promise in the treatment of neovascular age-related macular degeneration (nAMD) and diabetic macular edema (DME) in adults. Quality control studies of aflibercept employing non-reduced SDS-PAGE (nrSDS-PAGE) have shown that a significant variant band (IM1) is consistently present below the main band. Considering the quality control strategy of biopharmaceuticals, structural elucidation and functional studies are required. In this study, the variant bands in nrSDS-PAGE were collected through electroelution and identified by peptide mass fingerprinting based on liquid chromatography-tandem MS (LC-MS/MS). This variant was expressed using knob-into-hole (KIH) design transient transfection for the detection of ligand affinity, binding activity and biological activity. The variant band was formed by C-terminal truncation at position N99 of one chain in the aflibercept homodimer. Then, this variant was successfully expressed using KIH design transient transfection. The ligand affinity of the IM1 truncated variant was reduced by 18-fold, and neither binding activity nor biological activity were detected. The efficacy of aflibercept is influenced by the loss of biological activity of the variant. Therefore, this study supports the development of a quality control strategy for aflibercept.

Design and control of topological Fano resonance in Kane-Mele nanoribbons for sensing applications

The concept of topological Fano resonance, characterized by an ultrasharp asymmetric line shape, is a promising candidate for robust sensing applications due to its sensitivity to external parameters and immunity to structural disorder. In this study, the vacancy-induced topological Fano resonance in a nanoribbon made up of a hexagonal lattice with armchair sides is examined by introducing several on-site vacancies, which are deliberately created at regular distances, along a zigzag chain that stretches across the width of the ribbon. The presence of the on-site vacancies can create localized energy states within the electronic band structure, leading to the formation of an impurity band, which can result in Fano resonance phenomena by forming a conductivity channel between the edge modes on both armchair sides of the ribbon. Consequently, an ultracompact tunable on-chip integrated topological Fano resonance derived from the graphene-based nanomechanical phononic crystals is proposed. The Fano resonance arises from the interference between topologically protected even and odd edge modes at the interface between trivial and nontrivial insulators in a nanoribbon structure governed by the Kane-Mele model describing the quantum spin Hall effect in hexagonal lattices. The simulation of the topological Fano resonance is performed analytically using the Lippmann-Schwinger scattering formulation. One of the advantages of the present study is that the related calculations are carried out analytically, and in addition to the simplicity and directness, it reproduces the results obtained from the Landauer-Büttiker formulation very well both quantitatively and qualitatively. The findings open up possibilities for the design of highly sensitive and accurate robust sensors for detecting extremely tiny forces, masses, and spatial positions.

Ion-Exchange Doping of Semiconducting Single-Walled Carbon Nanotubes.

Semiconducting single-walled carbon nanotubes (SWCNTs) are a promising thermoelectric material with high power factors after chemical p- or n-doping. Understanding the impact of dopant counterions on charge transport and thermoelectric properties of nanotube networks is essential to further optimize doping methods and to develop better dopants. This work utilizes ion-exchange doping to systematically vary the size of counterions in thin films of small and large diameter, polymer-sorted semiconducting SWCNTs with AuCl3 as the initial p-dopant and investigates the impact of ion size on conductivity, Seebeck coefficients, and power factors. Larger anions are found to correlate with higher electrical conductivities and improved doping stability, while no significant effect on the power factors is found. Importantly, the effect of counterion size on the thermoelectric properties of dense SWCNT networks is not obscured by morphological changes upon doping. The observed trends of carrier mobilities and Seebeck coefficients can be explained by a random resistor model for the nanotube network that accounts for overlapping Coulomb potentials leading to the formation of an impurity band whose depth depends on the carrier density and counterion size. These insights can be applied more broadly to understand the thermoelectric properties of doped percolating disordered systems, including semiconducting polymers.

On‐Chip Room‐Temperature Operated Short‐Wavelength‐Infrared Si:S Photodetector with a Vertical Junction

AbstractSilicon (Si)‐based photodetectors are cost‐effective, eco‐friendly, and compatible with on‐chip complementary metal‐oxide‐semiconductor (CMOS) technology. However, expanding their photoresponse into the short‐wavelength‐infrared region beyond 1.1 µm remains challenging because of the intrinsic bandgap of Si. In this study, an ion implantation sulfur‐doped Si‐based infrared photodetector featuring a blocked impurity band (BIB) structure is investigated. The detector achieves an extended sub‐bandgap infrared response up to 2 µm through impurity band transitions, facilitated by the artificial creation of a deep‐level impurity band within the Si bandgap. The device exhibited a competitive photodetection effect with a high responsivity of 107.3 mA W−1 and an external quantum efficiency of 10.2%. Notably, the detector yielded an enhanced response speed with a raising time of 46 µs and a decay time of 135 µs at 1310 nm under room temperature. Finally, the versatile applications of this detector is presented in a multitude of scenarios encompassing material composition identification, multi‐band imaging, and optical communication. The proposed investigation not only proposed a method for Si‐based infrared detectors but also constituted a contribution to the advancement of silicon photonics.

Enhanced Quantum Metric due to Vacancies in Graphene.

Random vacancies in a graphene monolayer induce defect states that are known to form a narrow impurity band centered around zero energy at half filling. We use a space-resolved formulation of the quantum metric and establish a strong enhancement of the electronic correlations in this impurity band. The enhancement is primarily due to strong correlations between pairs of vacancies situated on different sublattices at anomalously large spatial distances. We trace the strong enhancement to both the multifractal vacancy wave functions, which ties the system exactly at the Anderson insulator transition for all defect concentrations, and preserving the chiral symmetry.

Electric field control of the energy gap in ZnO and BaSnO3 films grown on PMN-PT

ZnO and BaSnO3 (BSO) thin films grown on Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) substrates have been studied using electrical resistance and photoconductivity (PC) spectra measurements under different applied electric fields on the substrate. The behavior of the resistance and the energy gap (EG) extracted from the PC spectra are modified by the polarization state of the substrate in the case of the ZnO film, while for BSO, these physical parameters depend on the strain imposed by the substrate when a voltage is applied on the PMN-PT. In the latter case, an in-plane tensile (compressive) strain leads to a reduction (increase) in the resistance and the energy gap when an external electric field is applied on the substrate. The behavior of ZnO and BSO can be explained by the different crystalline structure in both films and by the fact that ZnO is also a piezoelectric material. In ZnO, a change in the polarization state of the substrate is associated with an imposed strain and an induced polarization on the film that leads to a modification of the band bending and hence of the energy gap. In the case of BSO, a shift of the impurity and conduction band generates a modification of the energy gap for the different types of strain.

First-principles calculation study on the mechanism of p-type ZnO through Mi-nNO (M= Mg and Cu) complexes

First principles calculation based on density functional theory is applied to study the possible approach to realize p-type ZnO through Mi-nNO (Mi is an interstitial defect; n = 2 or 3) complexes. In codoping method, the substitutional defects usually serve as the donors in donor-acceptor complexes, which are seen as the source of p-type ZnO. However, our research shows that the interstitial defects can replace the substitutional defects to become the donors. In Mg + N and Cu + N codoped ZnO, the binding, formation and ionization energies of possible complexes are investigated. Mgi-2NO and Cui-NO are two neutral complexes which can introduce a fully occupied impurity band, 0.66 and 0.87 eV above the VBM of the pure ZnO, respectively. It is found that Mgi-3NO and Cui-nNO (n = 2,3) can become the stable source of p-type conductivity under Zn-rich condition. In addition, the requirements of “M” in Mi-nNO complex are discussed.

Investigation of TM (TM=Mg, Cu) doping effect on the luminescence performance of CsPbCl3 from a first-principles investigation

The inorganic perovskite CsPbCl3 has raised great concern in recent years due to its great tunability of luminescence properties via impurity doping. However, the blue-emitting mechanism of the impurity-doped CsPbCl3 is unexplored. In this work, we focus on the structural, electronic, and optical properties of CsPb1-x TM x Cl3 (TM=Mg, Cu; x = 0, 0.037, 0.074) based on the first-principles calculations. It is indicated that TM doping decreases the lattice parameter, deforms octahedral structure, and improves the stability of CsPbCl3. The increased direct bandgap values and unique TM energy levels occupation show that the doped systems behave only blue-emitting well. The Mg-s and Cu-3d (eg) states out the bandgaps are close to the valence band edge and conduction band edge respectively, both promoting the carrier radiation recombination. Furthermore, the density of states analyses demonstrates that the enhanced emission of TM-doped CsPbCl3 benefits from the TM different electronic configurations and the different hybridization ways (Mg 3s/Cl 3p, Cu eg/Cl 3p), producing more carriers with increasing x respectively. The obtained optical properties imply that the TM-doped systems exhibit significant optical absorption and high carrier mobilities, promoting excellent luminescence efficiency. Our work explains the blue-emitting mechanism of the TM-doped CsPbCl3, providing a prospective strategy for designing highly efficient blue-emitting devices for optoelectronic applications based on the available parent materials by modulating the bandgap, synergistic relation of impurity energy level and band edge, and optical property.

Room-temperature telecom Si:Te PIN planar photodiodes: A study on optimizing device dimensions

The lack of efficient, cost-effective, room-temperature, and silicon-based photodetectors operating at the telecom bands has posed a persistent challenge in the realm of silicon photonics. One potential solution lies in introducing an impurity band within the silicon bandgap, offering a pathway to create a silicon-based photodetector that not only meets the requirements but also benefits from seamless integration with mature CMOS technology. Here, we report on enhancing device performance by introducing geometric modifications while retaining the doping concentration, device area, and operating conditions. The modified devices showcase improved responsivities, reaching approximately 3 × 10−2 A/W at 1300 nm (O band) and 1 × 10−2 A/W at 1500 nm (C band). These values significantly surpass prior work on Si:Te planar photodetectors, demonstrating an improvement of over 30 times. The noise equivalent power however was found to unavoidably increase by one order of magnitude to 10−6 (W /Hz).

Orchestrating phase transition in GeTe thermoelectrics: An investigation into the role of electronegativity

In thermoelectrics, mastering solid-state phase transition is paramount for enhancing the figure of merit and potentially designing novel thermoelectric materials. Focusing on GeTe, its reversible rhombohedral-to-cubic phase transition poses challenge for practical application. The determinant governing this phase transition remains obscure. Here, we introduce a simple, intuitive, and broadly applicable strategy to facilitate this phase transition by exploiting the large electronegativity disparity between cations and anions. Leveraging this concept, we adeptly identify AgInTe2 as a potent alloying agent and incorporate Sb to enhance the dissolution of AgInTe2 within the matrix, hastening the formation of cubic GeTe thanks to the increased electron transfer. These leads to the enhanced band degeneracy, formed multiscale microstructures, enhanced lattice anharmonicity, and reduced sound velocity, substantially increasing the density-of-states effective mass and decreasing the lattice thermal conductivity. Notably, AgInTe2 alloying engenders impurity bands, significantly narrowing the bandgap and rendering it ideal for low-temperature power generation. Consequently, the cubic (Ge0.94Sb0.06Te)0.9(AgInTe2)0.1 achieves a peak zT ∼ 1.01 at 473 K, resulting in a maximum energy-conversion efficiency ∼ 5.3% (9.2%) in a single-leg device under a 300 K (500 K) temperature differential. These results underscore the critical role of electronegativity difference in managing the rhombohedral-cubic phase transition in chalcogenides.

Enhanced photocatalytic activity of oxygen vacancy modulation interfacial electric field in S-scheme heterojunction VO/BiVO4-TiO2 and its mechanism

Constructing heterojunction photocatalysts is a promising way to efficiently degrade organic pollutants, while its application is constrained by slow interfacial dynamics. Reported herein is an easy strategy to construct S-scheme heterojunction between BiVO4 with oxygen vacancies (VO) and TiO2. The Vo/BiVO4-TiO2 heterojunction exhibited excellent efficiency in degrading Levofloxacin (LVFX) (99.21 % degradation within 90 min) and significantly enhanced degradation kinetic (4.03 × 10-2min−1), which was 2.6 times than BiVO4-TiO2 (1.53 × 10-2min−1). The mechanism lies in the fact that the introduction of VO into BiVO4 produces impurity bands that reduce its work function, which increases the difference in Fermi energy levels of the heterojunction and, in turn, enhances the Interfacial Electric Field. Based on the experimental and theoretical simulations results, the charge transfer path at the interface of Vo/BiVO4-TiO2 was elucidate. This work reveals the microscopic mechanism of electric field enhancement at heterojunction interface and provides new insights for designing efficient heterojunction photocatalysts.

Enhancing the electrochemical and mechanical performance of LiFePO4 cathode material by ternary doping of Mn, Ti, and N

This study employs a first-principles approach based on density functional theory to analyze the electrochemical and mechanical properties of LiFePO4 system with Mn, Ti, and N ternary doping. Computational findings reveal a formation energy of −1.98 eV, affirming excellent stability. Doping generates impurity bands, reducing the bandgap to 1.21 eV and enhancing conductivity. Doping significantly improves Li-ion diffusion, increasing the coefficient by 6 orders of magnitude and reducing the energy barrier by 0.35 eV. Moreover, Ti doping induces favorable local structural changes and enhances Li-ion diffusion. Mechanical performance calculations indicate improved stability, toughness, and isotropy. Ternary doping raises de-lithiation voltage, optimizing charge–discharge curves without compromising the capacity of LiFePO4, enhancing electrochemical performance.

First-Principles Investigations of the Electronic Structure and Mechanical Characteristics of Nd3+-Doped YAlO3 Crystals

Near-infrared laser radiation based on Nd3+-doped yttrium ortho-aluminate (Nd:YAlO3, Nd:YAP) has garnered significant interest regarding solid-state lasers. Nevertheless, the crystal microstructures and electronic characteristics of Nd:YAP are still unclear, and the unique physical properties underlying its enormous applications require clarification. In this study, we conducted first-principles calculations at the atomic level to explore the electronic properties and mechanical characteristics of both pure YAP and Nd3+-doped YAP. The results suggest that the substitution of the Y3+ ion site with the Nd3+ impurity ion induces slight structural distortion in the YAP crystal lattice. An impurity band emerges between the original conduction band and the valence band, attributed to the 4f orbital of the Nd3+ ion, exerting a substantial influence on the narrowing of the band gap. Through an analysis of the mechanical characteristics of both pure YAP and Nd:YAP, we conclude that the incorporation of Nd3+ atoms leads to a reduction in the mechanical properties of YAP to a certain extent. Our study can serve as a foundational data source for investigations into material performance, especially for the application of Nd:YAP in solid-state laser systems.

Evidence of indium impurity band in superconducting (Sn,In)Te thin films

Evidence of indium impurity band in superconducting (Sn,In)Te thin films

Indium defect complexes in (In x Ga1−x )2O3: a combined experimental and hybrid density functional theory study

Indium defects in small concentration (In x Ga1−x )2O3 were studied using a combination of spectroscopic and magnetic measurements on thin films varying the indium concentration, coupled with hybrid density functional theory simulations using the supercell method. X-ray diffraction spectra along with Tauc plots and density of states plots reveal a decrease (increase) in the electronic band gap (interlayer lattice spacing) due to the inclusion of indium in monoclinic Ga2O3, while room-temperature Hall measurements show an increase in n-type conductivity. Formation energy calculations reveal that the defect complex of substitutional indium at the octahedrally coordinated cation site (InGa) coupled with an indium interstitial (Ini) in the largest Ga–O cavity in the bulk (ia ), where the two impurities are a maximal distance away in the unit cell, results in the lowest formation energy across much of the electronic band gap; near the conduction band edge the single InGa defect becomes the lowest energy defect, though. These calculations help shed light on the impurity band enhanced, n-type conductivity increase due to small concentration indium doping in Ga2O3 as seen in the spectroscopic/magnetic measurements.