An algorithm is presented to first derive the average kinetic energy of the injected electrons during emission from flat metallic cathodes as a function of temperature and external electrostatic field. The average injection velocity is then calculated at a de Broglie wavelength beyond the classical turning point of the potential energy barrier in front of the cathode. This approach is then used to obtain the average injection velocity distribution across all emission regimes, including thermionic emission, thermal-field emission, and pure field emission for the case of non-planar metallic cathodes. An analytical expression for the average kinetic energy on injected electrons is proposed which is in good agreement with numerical simulations with a 3% accuracy over a wide range of temperature and external electrostatic field. We illustrate the strong spatial and electric field dependence of the average electron injection velocity distribution for the case of a nanoscale vacuum diode with a spheroidal tungsten cathode facing a planar anode.

The average kinetic energy and injection velocity of electrons emitted from metallic cathodes are obtained as a function of temperature and external electrostatic field using a recently developed algorithm that evaluates the injection velocity at a de Broglie wavelength beyond the classical turning point of the potential energy barrier. This approach yields a unified treatment across all emission regimes (thermal, thermal-field, and pure field emission), for both planar and non-planar cathode geometries. These physically accurate injection velocity distributions are then incorporated into a semiclassical framework that self-consistently solves Poisson's equation to determine the space charge limited current. Substantial quantitative differences are found in the computed emission characteristics and Miram curves when using realistic initial velocity distributions compared to those obtained under the conventional fixed average velocity assumption, demonstrating the importance of a physically accurate treatment of electron injection in space charge limited emission systems.

We study the field emission (FE) properties of centimeter-long carbon nanotube (CNT) arrays grown by a thermal chemical vapor deposition process under both DC and pulsed modes of operation. The FE properties of CNT arrays 0.5cm and 1.5cm long are examined as a function of the distance between the tip of the arrays and the anode. A study of the FE properties of the arrays when operated in a pulse mode with pulses up to 50ms when varying the separation between pulses will also be reported. We will also discuss the importance self-heating effects in these long CNT arrays when operated in both FE DC and pulsed modes.

Photoluminescence (PL) emission in two-dimensional (2D) materials is of great interest for nanophotonics applications. While excitonic emission has been observed in numerous 2D materials, tunable multi-band luminescence is rare. Here, we present single-crystalline AgErP2Se6, a 2D material that exhibits bright, multi-band PL emission from Er3+ ions within the lattice. The emission bands cover a wide range (350- 1,550 nm), with ultra-narrow (as low as 0.5 nm at room temperature) emission peaks and room temperature lifetimes up to 4 ms. The intensities of the PL emission bands from the single crystals depend strongly on temperature and pressure, enabling sensing over a wide temperature and pressure range. Furthermore, the PL persists in exfoliated flakes down to at least 11 nm thick and demonstrates thickness-dependent Purcell enhancement. This work establishes 2D AgErP2Se6 as a multi-band luminescent emitter and sensor, poised to enable integration into a number of optoelectronic and nanophotonic applications.

We report field emission (FE) properties of cold cathodes fabricated through scalable chemical vapor deposition synthesis of three-dimensional graphene (3DG) from cast catalyst followed by cold rolling, a process that significantly enhances mechanical strength and electrical conductivity. Three distinct cathode configurations were systematically evaluated: face emission, edge emission, and cylindrical structures. A maximum FE current of 5 mA was measured from a 3 mm diameter cold-rolled 3DG cylinder with 6 turns at a bias of 2400 V with a 0.5 mm separation between the cylindrical cathode tip and the anode. This geometric optimization approach demonstrates a practical pathway to developing high-current field emitters while maintaining material consistency.

Strict control of both crystallographic orientation and band structure is crucial in realizing future high performing semiconducting microelectronic devices based on 2D covalent organic frameworks (COFs). Due to the insoluble nature from extensive aromaticity, processing of these materials into well-ordered, highly crystalline thin films presents a great challenge. In this work, a strategy to enable controlled covalent doping of imine COF thin films with thiophene linkers is presented. By incorporating different aldehyde ratios of terephthalaldehyde (PDA) and 2,5-thiophenedicarboxaldehyde (TDA) with 1,3,5-tris(4-aminophenyl)benzene (TAPB) in a liquid-solid synthesis approach, a series of highly crystalline and uniformly oriented TAPB-PDA-TDA COF thin films with varying percentages of TDA linkers incorporated into the framework were obtained. In this case, incorporation of thiophene linkers up to 20% resulted in minimal disruption of the long-range crystallographic ordering. Moreover, a small amount of thiophene molecules covalently doped into the highly ordered structure results in a small reduction in the band gap and a corresponding increase in the work function and decrease in the valence band maximum, effectively behaving like a p-type dopant in conventional semiconductors. The covalently doped thiophene unit is shown to increase the π-conjugation through enhanced crystallinity in the framework, improving the electron delocalization in the structure.

Titanium suboxide (TiOx) thin films are incredibly versatile materials for applications in energy, sensing, and electronics, yet precise control of structure and chemistry in highly uniform thin films remains challenging. This work demonstrates a versatile, high-throughput method for the synthesis of titanium suboxide thin films using local, continuous wave (CW) laser induced oxidation. By systematically varying the laser power and exposure time, over 1200 unique temperature-time processing conditions were generated, converting a titanium metal film into an array of different TiOx phases and compositions. Synchrotron micro-XRD, Raman spectroscopy, and UV–Vis–NIR characterization data were included in multi-modal analysis of the synthesized materials, and 10 unique phases were detected across the laser processing phase space: Ti, Ti2O, TiO, Ti2O3, γ-Ti3O5, α/β-Ti3O5, Ti4O7, Ti5O9, rutile TiO2, and black TiO2. The collective datasets resulted in the construction of a transient phase diagram for oxidation of titanium which can serve as a guide for achieving desired properties of interest. This work illustrates the transformative potential of laser processing for the controlled synthesis of metastable suboxide materials, which can be patterned on a single substrate for advanced device fabrication.

The perovskite ceramic strontium titanate (SrTiO3 or STO) presents remarkable properties that make it very promising for catalytic, photocatalytic, electronic, magnetic, and spintronic applications. The bulk SrTiO3 crystal consists of alternating TiO2 and SrO layers along the (001) direction, with surfaces terminated by either layer. The deposition of Fe on SrTiO3 (001) can provide Fe nanoparticles of interest for various applications. Adequate characterization of the properties and structures of these nanoparticles and substrate is essential. In this work, the formation and structure of Fe nanoparticles grown on SrTiO3 (001) (/5 x /5)R26.6 degrees surface were characterized by low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD). The comparison between experimental and theoretical XPD results involving multiple scattering calculations indicates a coexistence of Fe nanoparticles in both bcc and fcc structures.

It is known that field electron emission (FE) characteristics of emitters are influenced by their flexibility, as bending under an applied electrostatic field, EP, can significantly increase their characteristic field enhancement factor and emission current. However, this effect has not been thoroughly studied. In this Letter, a numerical algorithm is developed to investigate the impact of electrostatic forces on the FE properties of a flexible metallic carbon nanotube (CNT), classically modeled as a cylinder with a hemi-spherical cap, as a function of EP, its sharpness aspect ratio, and its initial (assumed to be fixed) angle θ0 of inclination with the substrate, i.e., the horizontal emitter's plate. Our numerical simulations are based on an iterative process to include the mechanical stress problem and resulting bending of the CNT, taking into account its finite Young's modulus and the electrostatic problem due to the redistribution of surface charges on the emitter until a convergent solution is found. The process is repeated as a function of EP. Importantly, it is found that the emitted current of the bendable CNT can be several orders of magnitude larger than those for the case when the emitter is modeled as a rigid body. Moreover, the curvatures of the Murphy–Good plots predicted theoretically align with those observed experimentally in FE from carbon-based materials under the high EP limit, where protrusion deflection effects are likely to influence emission. Therefore, our findings highlight the necessity of considering the flexibility of nanoscale field emitters to accurately interpret their FE characteristics.

Vacuum field effect transistors (VacFETs) are a promising alternative to traditional semiconductor-based transistors, offering a superior frequency response, radiation hardness, and high-temperature operation. In this work, we illustrate the use of a multi-objective optimization (MOO) technique to systematically explore the design space of all-metallic VacFETs. The approach is based on a MATLAB implementation of an elitist genetic algorithm to identify the Pareto front of optimal configurations resulting from trade-offs between key VacFET performance metrics, such as a large ON–OFF field emission (FE) current ratio and a low leakage current. The analysis is illustrated for the case of an all-metallic VacFET with either one or two in-plane side gates based on a two-dimensional analysis of the effects of electrostatics. A comparison of the FE characteristics demonstrates that the optimized dual-gate VacFET achieves an approximately three orders of magnitude improvement in both the ON/OFF current ratio and the ON/Leakage current ratio compared to the similarly optimized single-gate configuration. Based on these results, a first-order small signal high-frequency model is developed, which predicts a unity current gain cutoff frequency of several hundred GHz for the optimized dual-gate VacFET. The versatile MOO technique proposed here can be readily extended to three-dimensional simulations incorporating various gate configurations (top, bottom, or lateral gates) and material combinations with increasing number of constraints and an implementation of their corresponding cost functions. The insights gained from MOO techniques offer valuable guidelines for designing next-generation vacuum nanoscale devices with substantially improved figures of merit.

We have studied the surface structure of a single crystal (beta-Ga2O3(010) using quantitative Low Energy Electron Diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). The XPS measurements show spectra typical of stoichiometricGa(2)O(3) with a clean surface. LEED consistently shows a p(1x1) pattern, free of surface reconstruction. Quantitative LEED I(V) curves are acquired for 41 distinct diffraction spots. The experimental I(V) curves are compared to simulations over the first five layers. The best fits to the experimental LEED I(V) curves acquired at all diffraction spots are then used to calculate the interplanar relaxation and atomic rumpling. Significant atomic rumpling and interplanar relaxation are found over the first 5 atomic layers. As a result of rumpling a polarization of similar to 2 mu C/cm(2) develops in the topmost surface layer. The structural results are in good agreement with previous density functional theory calculations and experimental X-ray photoelectron diffraction.

This work demonstrates a multi-objective optimization (MO) technique using an elitist genetic algorithm to systematically explore the design space of all-metallic VacFETs and identify optimal configurations. Our analysis compares single-gate and dual-gate VacFET configurations, showing that the optimized dual-gate design achieves approximately three orders of magnitude improvement in both ON/OFF current ratio and ON/Leakage current ratio. A first-order high-frequency model predicts unity current gain cutoff frequencies of several hundred GHz for the optimized design. This optimization framework can be extended to three-dimensional simulations with various gate configurations and material combinations, providing valuable guidelines for designing next-generation vacuum nanoelectronic devices.

The ferroelectric (FE) properties of hafnium zirconium oxide (Hf1-xZrxO2), a material of importance for many electronic applications such as low-power computing and non-volatile memories, are heavily dependent on the Zr content (x). Here we report that in some cases, samples with the same x display different FE character and these variations can be related to changes in the location of the Zr 3d and O 1s peaks in X-ray photoelectron spectroscopy (XPS) spectra. This suggests that lattice strain and Zr-O bonding variations play a significant role in the FE character of Hf1-xZrxO2 thin films.

A scattering matrix technique is used to study the transmission probability through a flat semiconductor / vacuum interface considering the effective mass difference between the two materials. A comparison is made between the longitudinal and transverse energy dependence of the transmission probability calculated using the BenDaniel-Duke (BD) and ZhuKroemer (ZK) boundary conditions at the semiconductor / vacuum interface. For longitudinal energy between the minimum of the conduction band and a few kappa T-B above the Fermi level in the semiconductor, the transmission probability can be an order of magnitude smaller when using the ZhuKroemer boundary conditions compared to the BenDanielDuke boundary conditions. The Zhu-Kroemer boundary conditions should be used to calculate the field emission current through a semiconductor/vacuum interface.

The electrical conduction mechanism in 2D van der Waals (vdW) thin film made of graphene flakes can be characterized by the variable range hopping (VRH) over a wide range of temperature, followed by a transition into an Arrhenius type conduction at the higher temperature. Regarding the nature of the subsequent Arrhenius type conduction, a lack of consensus exists between the nearest neighbor hopping (NNH) and the band conduction (BC). We discuss how to assess the transition in conduction mechanism based on the transient behavior of the reduced activation energy (W) and the structure of electronic density of states (DOS) of the sample. With a multi-channel conduction model, we show that the transition from VRH to BC always accompany an increase in W, which is absent in the transition from VRH to NNH. Also discussed are the peculiarities of VRH in vdW thin film and what they imply towards the properties of defects. In most vdW flake thin films, the spread of defect band in DOS is governed by the intrinsic defect energy not by the Coulomb interaction between defects. It can further be deduced that the gap in DOS near Fermi level associated with Efros-Shklovskii VRH observed in vdW film is not a Coulomb gap as found in weakly doped crystalline semiconductor, but inherent in the distribution of defect intrinsic energy. For the graphene flake thin film we synthesized, Mott VRH and BC are identified.

We present a study of Ge segregation at the surface of highly germanium-doped gallium oxide (2.5 x 10(20) cm(-3) nominal doping level) grown by molecular beam epitaxy. We probed the dopant concentration as a function of depth by hard x-ray photoelectron spectroscopy and standard laboratory photoemission spectroscopy. We notably found that there is germanium segregation within the top 2 nm where its concentration is 3 times the nominal doping level. This increased dopant concentration leads to a threefold enhancement of surface conductivity. The results suggest a reliable method for delta doping for power electronics applications.

We report the design, fabrication, and measurement of the field emission (FE) characteristics of AlGaN/GaN nanoscale lateral vacuum diodes with triangular cathodes and cathode to anode spacings from 50 to 600 nm. The FE characteristics of the AlGaN/GaN diodes with metallic or AlGaN/GaN anodes show successful rectification with forward bias FE current in the range of microamperes or milliamperes, respectively, when biased within a maximum range varying from 10 to 30 V. In the forward bias mode, the measured current Im vs applied anode to cathode bias Vm are well fitted to Murphy–Good profiles associated with FE at higher biases, and an Ohmic leakage profile below the threshold for FE. Our results are the first successful demonstration of FE of electrons between the two two-dimensional electron gases (2DEGs) present on both sides of a nanogap formed by electron lithography through an AlGaN/GaN heterojunction. A qualitative explanation of the loop-type FE characteristics of both AlGaN/GaN vacuum diodes, with either metallic or AlGaN/GaN anodes, is presented.

Ferroelectricity in hafnium zirconium oxide (Hf1−xZrxO2) and the factors that impact it have been a popular research topic since its discovery in 2011. Although the general trends are known, the interactions between fabrication parameters and their effect on the ferroelectricity of Hf1−xZrxO2 require further investigation. In this paper, we present a statistical study and a model that relates Zr concentration (x), film thickness (tf), and annealing temperature (Ta) with the remanent polarization (Pr) in tungsten (W)-capped Hf1−xZrxO2. This work involved the fabrication and characterization of 36 samples containing multiple sets of metal-ferroelectric-metal capacitors while varying x (0.26, 0.48, and 0.57), tf (10 and 19 nm), and Ta (300, 400, 500, and 600 °C). In addition to the well-understood effects of x and Ta on the ferroelectricity of Hf1−xZrxO2, the statistical analysis showed that thicker Hf1−xZrxO2 films or films with higher x require lower Ta to crystallize and demonstrated that there is no statistical difference between samples annealed to 500 and 600 °C, thus suggesting that most films fully crystallize with Ta ∼ 500 °C for 60 s. Our model explains 95% of the variability in the Pr data for the films fabricated, presents the estimates of the phase composition of the film, and provides a starting point for selecting fabrication parameters when a specific Pr is desired.

We investigate the temperature dependence of field emission (FE) characteristics of AlGaN/GaN heterojunction-based lateral nanoscale vacuum diodes over a temperature range varying from 77 to 373 kelvin. The vacuum diodes have a sharp (trochoidal-like) cathode and blunted anode with the shortest distance between the tip of the cathode and the anode of 210 nanometers. The DC FE characteristics were recorded using a compliance current of 100 nanoamperes. Under forward mode of operation, the vacuum diodes exhibit three regimes of operation: one associated with FE from adsorbates on the tip of the cathode at low applied bias, followed by FE from the bare cathode at larger forward bias, and finally onset of self-heating effects. The latter leads to a smaller FE current under reverse sweep. In the reverse sweep, the current-voltage characteristics suggest the onset of conduction through the substrate.

The Nb2CTx MXene has garnered interest for its potential applications in energy storage, catalysis, and sensors. Recent reports of Raman spectra of Nb2CTx reveal stark differences of the MXene incorporated into devices, which may affect the devices' performance and reproducibility. These differences illuminate a need for fundamental characterization of the Nb2CTx Raman modes to better understand the material-specific properties of the MXene integrated into devices. We conducted a comprehensive multiexcitation Raman study and observed nine Raman-active modes in Nb2CTx that are consistent with our density functional theory calculations. Additionally, we investigated laser-induced surface modification of Nb2CTx through changes in the peak intensities and frequencies, along with the generation of amorphous carbon with increasing incident laser power. This fundamental study provides a framework to spectroscopically assess the integrity of Nb2CTx and a tool for creating a stable, tunable, and localized surface modification.