ACS Applied Electronic Materials最新文献

Electrical contacts to layered semiconductors often struggle with current crowding at the contact edge, which is intensified in materials, such as layered graphene and transition-metal dichalcogenides (TMDs) with highly anisotropic conductivity. Conventional models for calculating spreading (constriction) resistance typically assume isotropic transport, which may result in an inaccurate determination of spreading resistance, as well as intrinsic transport parameters. Here, we present an exact analytical solution that generalizes the classical two-layer vertical thin film model to anisotropic systems, incorporating in-plane and out-of-plane resistivities, interfacial resistivity, and both planar and cylindrical (disk-shaped) contact geometries. This framework allows for exact quantitative modeling of the spreading resistance over a wide range of contact sizes, film thicknesses, and anisotropy ratios. Our model uncovers high current crowding at the contact edge driven by the high in-plane conductivity of two-dimensional (2D) materials. In materials such as MoS₂ and WSe₂, this high in-plane conductivity causes potential distributions to extend laterally along the contact plane, rather than vertically into the material. The model’s predictions are further validated by finite-element method (FEM) simulations. When applied to experiments on highly oriented pyrolytic graphite (HOPG) and MoS₂ thin films, it matched the spreading resistance data without any fitting parameters and recovered the classical diffusive limit for wide, thick substrates. Notably, we also find that the standard diffusive model approximations for spreading resistance fail for ultrathin films or short channels. This makes it essential to use our full anisotropic model to reliably interpret experimental data and obtain accurate transport parameters. This unified framework provides a robust physics-based tool for designing and optimizing electrical contacts in 2D materials and van der Waals heterostructures.

As an emerging and promising type of electronic devices, optoelectronic synaptic devices emulate the synaptic plasticity. Moreover, by the coordinated modulation of electrical and optical signals, this device can efficiently store and process information. Based on poly(vinylpyrrolidone): nitrogen-doped graphene oxide quantum dots (PVP:N-GO QD) nanocomposites, we fabricated an organic optoelectronic synaptic device and deeply explored their synaptic properties during optoelectronic modulation. Introducing nitrogen (N) into GO QDs through the hydrothermal method effectively enhances the n-π* electronic transition, thereby achieving additional photoinduced conductance and providing an important physical basis for optoelectronic modulation. In addition, exposing the device to light at 365 nm significantly enhanced synaptic characteristics and achieved light-assisted regulation. In the Ag/PVP:N-GO-QD/ITO device structure, the top Ag electrode is used as the source of Ag ions, where Ag atoms are oxidized and migrated to the active layer under positive bias. By promoting the reduction of silver ions and optimizing the growth of conductive filaments, the device can stably simulate various biological synaptic behaviors. Finally, the pattern recognition accuracies of 90.62% (dark) and 91.11% (light) in learning and inference tests further demonstrate its broad prospects for applications in neuromorphic computing and artificial intelligence.

Understanding the atomic-scale interfaces in advanced semiconductor heterostructures is essential for controlling defects, optimizing material properties, and ensuring device reliability. In this work, we present a comprehensive study of the atomic structure at Al(Ga)N interfaces in N-polar GaN high-electron-mobility transistor structures (HEMTs) via aberration-corrected annular dark field scanning transmission electron microscopy (ADF STEM). We investigate heterostructures grown on 4H-SiC (0001̅) with different offcut angles toward the m-plane, a crucial platform for high-quality N-polar layers that have not been thoroughly characterized. Here, we demonstrate periodic vertical inversion domain boundaries (IDBs) on step-flow-grown N-polar GaN via a multistep hot-wall metal–organic chemical vapor deposition (MOCVD) process on 4° m-plane offcut 4H-SiC (0001̅). We directly confirm the polarity by ADF STEM and conclude that two terraces with opposing polarity coexist on 4° m-plane offcut surfaces. In contrast, an offcut angle of 1° does not lead to the formation of periodic vertical IDBs but results in hexagonal hillocks and surface pits of nanometer size. The vertical IDBs originate from the interface between the reconstructed 4° m-plane–plane offcut of 4H-SiC and the AlN nucleation layer. The ratio between the N-polar and metal-polar surface area is proportional to the step density at the 4H-SiC surface, which can be controlled by the offcut angle. The presence of these periodic vertical IDBs were undetected by conventional X-ray diffraction measurements and may affect the long-term stability of N-polar GaN-based high-power devices.

The misaligned energy level between metallic electrodes and the organic semiconductor (OSC) causes inefficient charge injection in organic thin-film transistors (OTFTs), leading to an increased contact resistance (R_c) at the source (R_s). To date, we demonstrate an innovative strategy by depositing an additional different organic semiconductor (OSC) layer onto the initial OSC surface to effectively reduce R_c. Herein, we investigate top-contact bottom-gate (TCBG) S-shaped dinaphtho[2,1-b:2′,1′-f]thieno[3,2-b]thiophene-10 (S-DNTT-10)-based OTFTs and found that the linear-mode field-effect mobility (μ_lin) increased 4-fold─from 0.44 to 1.67 cm²·V^–1·s^–1─after thermally depositing a pentacene thin film on S-DNTT-10, resulting in TCBG S-DNTT-10/pentacene-based OTFTs that possessed a decreased measured R_s. Meanwhile, the configuration of TCBG S-DNTT-10/pentacene-based OTFTs was optimized by restricting pentacene deposition to the source area only and effectively improved the reduced saturation-mode field-effect mobility (μ_sat) of initial TCBG S-DNTT-10/pentacene-based OTFTs, which was likely caused by increased drain contact resistance. Through this study of S-DNTT-10-based OTFTs, we present an effective approach to address the crucial challenge of high R_c in TC-OTFTs with poor charge injection and low μ_lin. We confirm that this approach holds promise for practical applications in industrial and commercial OTFT development.

We present a comprehensive DFT study of the structural, magnetic, optical absorption, electronic, water redox properties, and thermodynamic stability of Cr-doped bulk anatase TiO₂ for solar-driven photocatalytic hydrogen production. Calculations employed a modified hybrid PBE0 functional (15% HF + 85% PBE exchange), providing accurate electronic structure predictions. A wide range of Cr doping configurations, including substitutional and interstitial geometries with various spatial distributions such as isolated, separated, gathered, dissociated, and clustered dopant arrangements (not previously reported), are systematically explored. Among all investigated systems, the ferromagnetic TiO_(2+3x)Cr_2x emerged as the most promising candidate for visible-light-driven photocatalytic water splitting. Its structure corresponds to interstitial Cr₂O₃ species, with one O atom bridging two neighboring Cr³⁺ ions in distorted octahedral sites. This material can be thermodynamically stabilized under standard and O-rich growth conditions using Cr-containing precursors with a weak reducing character. Its optical absorption spectrum exhibits visible-light-responsive bands beyond 700 nm, originating from hybridized Cr 3d-Ti 3d-O 2p states, promoting hole mobility and suppressing electron–hole recombination. These findings provide valuable atomic-scale insights for the rational design of clustered Cr-doped TiO₂ photocatalysts for efficient solar-driven hydrogen production.

High-κ dielectrics that offer high permittivity with minimal leakage are essential for advanced electronic devices; however, achieving such performance within porous hybrid materials remains challenging. In this study, we report the Mn-based metal–organic framework, [Mn₂(SBA)₂(DMSO)₂].DMSO (Mn-MOF-D), synthesized via a solvothermal method using Mn(OAc)₂ 4H₂O salt, 4,4ˈ-sulfonyldibenzoic acid (H₂SBA), and 5-methyl-2-pyrazinecarboxylic acid (mpyzc) in a mixture of DMSO-H₂O. Single-crystal analysis reveals a unique dinuclear {Mn₂O₁₀} secondary building unit (SBU) comprising corner-shared distorted octahedral and square pyramidal Mn centers. Dielectric measurements show that Mn-MOF-D is a high-κ material (κ = 40.5 at 1 kHz, 303 K) with a low 6.7 × 10^–9 S·cm^–1 AC conductivity (5 kHz, 303 K). The metal–insulator–metal (MIM) device fabricated from the material showed minimal leakage with a current density of 2.08 × 10^–12 A·cm^–2. The high dielectric response arises from dipolar contributions of coordinated and lattice-confined DMSO molecules along with the intrinsic polarizability of the framework. After removal of polar solvents from Mn-MOF-D, the dielectric constant ε_rˈ(ω) dropped to κ = 16.8 (at 1 kHz, 303 K), underscoring their crucial role in overall dielectric polarization. Impedance and AC conductivity analyses confirm Maxwell–Wagner–type relaxation and thermally activated hopping conduction. The combination of a large dielectric constant, low loss, and excellent framework stability makes this material promising for solvent-tunable high-κ dielectrics in gate dielectric applications.

In recent years, flexible piezoelectric polymers and devices have attracted significant attention due to the booming development of artificial intelligence and the internet of Things. Piezoelectric polymers, typically poly(vinylidene fluoride) (PVDF) and its copolymer with trifluoroethylene (P(VDF-TrFE)), offer several distinct advantages such as intrinsic flexibility, ease of processing, chemical inertness, and biocompatibility. However, their relatively lower piezoelectric coefficients compared with their inorganic counterparts greatly limit practical applications in high-performance sensors and energy harvesters. Both modulation of microstructure and construction of three-dimensional (3D) structured devices have been demonstrated to be effective measures to enhance the piezoelectric performance. This work focused on the fabrication of highly sensitive copolymer sensors by coordination of both polytetrafluoroethylene template-induced crystallization and construction of wave-shaped 3D devices. Template-induced crystallized P(VDF-TrFE) devices demonstrated an average d₃₃ coefficient of −40.9 pC/N within a frequency range of 100–1000 Hz. Further construction of wave-shaped 3D piezoelectric devices promoted their sensitivity to weak mechanical excitation. This wave-shaped device detected diversities of physiological and action signals of the human body. With the help of a pulse wave velocity model, wave-shaped devices were utilized for blood pressure measurements along with pulse detection. The device was further extended for audible sound detection with a frequency resolution better than 1 Hz and the capability of frequency spectrum analysis of varieties of acoustic sources. This work provides a convenient and effective strategy to construct high-performance and flexible piezoelectric devices for weak signal detection.

Diamond’s exceptional properties make it a promising material for electronics, optoelectronics, and quantum technologies. The development of high-quality and smooth diamond epilayers has emerged as a critical advancement for achieving superior performance and reliability of diamond-based devices. Here, we report the growth of high-quality diamond epilayers with subnanometer surface roughness on two types of commercial substrates, high-pressure high-temperature (HPHT) and chemical vapor deposition (CVD) substrates, via an optimized microwave plasma CVD process. By combining hydrogen plasma etching with elevated growth pressure (∼180 Torr), moderate microwave power (1–1.2 kW), and a practical CH₄/H₂ ratio (1%), we achieve epilayers exhibiting surface roughness as low as 0.2 nm and improved crystalline quality. Temperature-dependent Hall measurements reveal a clear correlation between surface quality and carrier mobility: epilayers grown on HPHT substrates exhibit four times higher mobility than those grown on CVD substrates due to reduced scattering from surface defects. Diamond field-effect transistors (FETs) fabricated on the epilayer exhibited enhanced performance, with a 7-fold increase in maximum drain current, and 2 orders of magnitude higher on/off ratios compared to those fabricated directly on substrates. This improvement is attributed to higher carrier mobility, resulting from reduced scattering caused by surface defects and surface roughness. These findings not only highlight the transformative potential of ultrasmooth diamond epilayers in advancing diamond electronics but also provide a robust framework for future developments in high-performance photonics and quantum technologies.

The nonstoichiometric titanium oxide TiO_x is a promising material for memristive devices, where its resistive switching properties are governed by structural and chemical state variations, particularly the proportion between the Ti(IV), Ti(III), and Ti(II) components. While X-ray photoelectron spectroscopy (XPS) is widely employed for chemical state analysis, conventional XPS depth profiling is limited by ion beam-induced reduction artifacts. The double-angle XPS depth profiling proposed in this paper allows one to determine the elemental composition and chemical state of titanium atoms of TiO_x thin films as a function of sample depth while excluding the contribution from the damaged surface. Using this approach, we investigate the films of nonstoichiometric titanium oxides obtained by magnetron deposition in a medium with different oxygen contents (1, 3, and 5%), which exhibit different resistive switching behaviors. Based on the current–voltage characteristics measured by means of the conducting probe of an atomic-force microscope (AFM) in the contact mode, it is shown that, as the oxidation level of the film increases, so does its resistance and switching voltage. Due to atmospheric humidity, the anodic polarization of the sample leads to the oxidation of the metallic component and the Ti(II) and Ti(III) components of the film.

A fundamental obstacle in developing high-performance ionic conductive hydrogels (ICHs) is the inherent trade-off between ionic conductivity and long-term hydration stability. Herein, we demonstrate ion–dipole anchoring as a powerful strategy to overcome this limitation. This strategy is realized in a semi-interpenetrating network hydrogel composed of polyquaternium-10 (PQ-10) and polymerized ionic liquid (PIL) via electron beam irradiation (PQ-10/PIL ICH). The ion–dipole anchoring effect arises from the strong specific interactions between the imidazolium cation (VEIM⁺) and bromide anions (Br^–) from the PIL and water molecules, which drastically reduce water activity and suppress evaporation. Crucially, this anchoring mechanism operates independently within the continuous PIL phase, preserving high ionic conductivity (5.02 S m^–1 at 25 °C) while enabling exceptional water retention (83.04% after 60 days). The PQ-10/PIL ICH demonstrates outstanding performance in flexible sensing and solid-state supercapacitors, retaining 85.00% capacitance after 10,000 cycles. This work establishes ion–dipole anchoring as a foundational design principle for creating durable and high-performance flexible electronic devices.