Quantitative study on the photoemission of AlGaN nanoarrays based on the three-dimensional transportation within a four-step process.

Photocathodes play a crucial role in photoelectronic imaging and vacuum electronic devices. The quantum efficiency of photocathodes, which determines their performance, can be enhanced through materials engineering. However, the quantum efficiency of conventional planar photocathodes remains consistently low, at around 25%. In this paper, we propose what we believe is a novel structure of AlGaN nanowire array to address this issue. We investigate the photoemission characteristics of the nanowire array using the "four-step" process, which takes into account optical absorption, electron transportation, electron emission, and electron collection. We compare the quantum efficiency of nanowire arrays with different structure sizes and Al components. After studying the effect of incident light at various angles on the nanowire array photocathode, we identify the optimal dimensional parameters: a height of 400∼500 nm and a wire width of 200∼300 nm. Furthermore, we improved the collection efficiency of the photocathode by introducing a built-in/external electric field, and obtained a 104.4% enhancement of the collection current with the built-in electric field, meanwhile the photocurrent was increased by 87% compared to the case without the external electric field. These findings demonstrate the potential of optimizing photocathode performance through the development of a novel model and adjustment of parameters, offering a promising approach for photocathode applications.

Polariton Microfluidics for Nonreciprocal Dragging and Reconfigurable Shaping of Polaritons.

Dielectric environment engineering is an efficient and general approach to manipulating polaritons. Liquids serving as the surrounding media of polaritons have been used to shift polariton dispersions and tailor polariton wavefronts. However, those liquid-based methods have so far been limited to their static states, not fully unleashing the promise offered by the mobility of liquids. Here, we propose a microfluidic strategy for polariton manipulation by merging polaritonics with microfluidics. The diffusion of fluids causes gradient refractive indices over microchannels, which breaks the symmetry of polariton dispersions and realizes the microfluidic analogue to nonreciprocal polariton dragging. Based on polariton microfluidics, we also designed a set of on-chip polaritonic elements to actively shape polaritons, including planar lenses, off-axis lenses, Janus lenses, bends, and splitters. Our strategy expands the toolkit for the manipulation of polaritons at the subwavelength scale and possesses potential in the fields of polariton biochemistry and molecular sensing.

Direct observation of contact resistivity for monolayer TMD based junctions via PL spectroscopy.

Monolayer transition metal dichalcogenides (mTMDs) possess a direct band gap and strong PL emission that is highly sensitive to doping level and interfaces, laying the foundation for investigating the contact between mTMD and metal via PL spectroscopy. Currently, electrical methods have been utilized to measure the contact resistance (RC), but they are complicated, time-consuming, high-cost and suffer from inevitable chemical disorders and Fermi level pinning. In addition, previously reported contact resistances comprise both Schottky barrier and tunnel barrier components. Here, we report a simple, rapid and low-cost method to study the tunnel barrier dominated contact resistance of mTMD based junctions through PL spectroscopy. These junctions are free from chemical disorders and Fermi level pinning. Excluding the Schottky barrier component, solely tunnel barrier dominated contact resistances of 1 L MoSe2/Au and 1 L MoSe2/graphene junctions were estimated to be 147.8 Ω µm and 54.9 Ω µm, respectively. Density functional theory (DFT) simulations revealed that the larger RC of the former was possibly due to the existence of intrinsic effective potential difference (Φbarrier) between mTMD and metal. Both junctions exhibit an increasing tendency of RC as temperature decreases, which is probably attributed to the thermal expansion coefficient (TEC) mismatch-triggered interlayer spacing (d) increase and temperature-induced doping. Remarkably, a significant change of RC was observed in 1 L MoSe2/Au junctions, which is possibly ascribed to the changes of their orbital overlaps. Our results open new avenues for exploring fundamental metal-semiconductor contact principles and constructing high-performance devices.

Electronic and optical properties of two-dimensional GaN/ZnO heterojunction tuned by different stacking configurations.

Two-dimensional (2D) semiconductors show novel electronic and optoelectronic applications due to their excellent performance. The van der Waals (vdW) heterostructures are also a new method for the design of low dimensional optoelectronic devices. However, their fundamental electronic structure and optical properties are sensitive to stacking configurations. Herein, we perform systematic first-principle calculations for monolayer GaN and ZnO by six different stacking styles. The results suggest that the bonding type and stability vary with the stacking method. Chemical bonding and vdW interaction are respectively observed in different models. However, the carrier mobilities for different models are all enhanced after integration. Both type I and II band alignment can be generated from different stacking models. The optical properties suggest high absorptivity in the solar-blind region. This study is an early stage for the design and synthesis of photodetectors or solar cells based on 2D GaN/ZnO heterojunctions and also opens a far-ranging research interest in optoelectronic materials and devices with more advanced semiconductor materials.

Photocurrent enhancement of AlxGa1-xN nanowire arrays photodetector based on coupling effects of pn junction and gradient component.

Ultraviolet photodetector has a variety of applications in medical diagnosis, civilian testing and military security. The enhancement of photo response has far been a hot topic regrading to the performance improvement of the devices. In this study, we proposed a self-powered photodetector based on AlxGa1-xN nanowire arrays (NWAs) utilizing axial pn junction integrating with gradient Al component. The merit of the coupling structure is demonstrated by theoretical model and simulations. The photoelectric conversion model is built based on a continuity equation derived by its corresponding boundary conditions. The photocurrent for a single nanowire and NWAs are respectively obtained. According to the simulation results of a single nanowire, the optimal nanowire height is obtained with a photocurrent enhancement up to 330%. For NWAs, the aspect ratio of NWAs and incident angle of light synergistically determine the output photocurrent. The optimal aspect ratio for NWAs is 1:1 with an optimal incident angle of 57°. This study provides a reliable method for the design of photodetectors with micro-nano structures.

Impact of gas adsorption on the stability and electronic properties of negative electron affinity GaAs nanowire photocathodes.

The influence of CO, CO2, H2O, H2 and CH4 adsorption on the stability and electronic properties of negative electron affinity (NEA) GaAs nanowire surfaces activated by Cs/O and Cs/NF3 are systematically investigated via first-principles. The calculations indicated that GaAs nanowires activated with 3Cs/O are more susceptible to the surface contamination. After residual gas molecule adsorption, 3Cs/O activated surfaces exhibit direct bandgap character, while 3Cs/NF3 activated surfaces are inversely indirect bandgap. In addition, residual gas adsorption results in a notable increase of band gap, work function and electron affinity of GaAs nanowire surfaces. The incoporation of residual gas molecules also induces a new electric dipole [Cs-gas] with a direction from Cs to gas molecule. From the perspective of theoretical calculation, it is predicted that GaAs nanowires activated through Cs/NF3 has a stronger stability compared with Cs/O in the aspect of gas exposure.

Enhancement of photoemission capability and electron collection efficiency of field-assisted GaN nanowire array photocathode.

GaN has interesting prospects in applications for spectrum-tunable solid-state devices with photoelectric conversion function. Similarly, single nanowires or nanowire arrays (NWAs) proceed to exhibit good photon absorbance and photoemission characteristics as vacuum devices based on the external photoelectric effect. However, the collection of photoelectrons emitted from a nanowire surface has become the greatest impediment to the progress of GaN NWAs photocathodes. In this study, a field-assisted GaN NWA photocathode is proposed. The photoemission efficiency and electron collection efficiency of the field-assisted GaN NWA photocathode are derived. The results suggest that the external field can effectively enhance the photoemission capacity and electron collection efficiency of the photocathode. Based on the theoretical model, the structural parameters of NWAs and the field intensity are optimized. When the field intensity is 1 V µm-1, the collected photocurrent of the GaN NWA photocathode reaches a maximum. For NWAs with an aspect ratio of 1:1, the optimal incident angle of light is 70°. This study provides a theoretical guide for the incorporation of an external field in a GaN NWA photocathode with the purpose of enhancing photoemission and electron collection capacity.

Adsorption mechanism of Pt, Ag, Al, Au on GaAs nanowire surfaces from first-principles.

The electronic and optical properties of metal (M) atoms adsorbed GaAs nanowires are systemically investigated utilizing first-principles calculations based on density functional theory. Different materials (M = Pt, Ag, Al and Au) and different coverages (1M, 2M, 3M and 4M) are considered to construct surface adsorption models. The calculations show that all metal-adsorbed GaAs nanowire surfaces are stable, and the difficulty of metal atom adsorption on nanowire surfaces follows the rule of Ag > Au > Al > Pt. In addition, the layer distance variation of nanowire surfaces after metal atom adsorption mainly take place near the outmost layer region. In 1M coverage case, the work function is reduced by Pt, Ag, Al adsorption, while increased by Au adsorption. Specially, Pt- and Al-adsorbed GaAs nanowire surfaces are direct band gap semiconductors, but Ag- and Au-adsorbed surfaces are indirect band gap. The adsorption of metals on GaAs nanowire surfaces are via chemisorption. Moreover, metal atom adsorption can enlarger the absorption coefficient of GaAs nanowires, which are gradually enhanced with increasing the coverage of metal atoms.

Automatic projection image registration for nanoscale X-ray tomographic reconstruction.

Novel developments in X-ray sources, optics and detectors have significantly advanced the capability of X-ray microscopy at the nanoscale. Depending on the imaging modality and the photon energy, state-of-the-art X-ray microscopes are routinely operated at a spatial resolution of tens of nanometres for hard X-rays or ∼10 nm for soft X-rays. The improvement in spatial resolution, however, has led to challenges in the tomographic reconstruction due to the fact that the imperfections of the mechanical system become clearly detectable in the projection images. Without proper registration of the projection images, a severe point spread function will be introduced into the tomographic reconstructions, causing the reduction of the three-dimensional (3D) spatial resolution as well as the enhancement of image artifacts. Here the development of a method that iteratively performs registration of the experimentally measured projection images to those that are numerically calculated by reprojecting the 3D matrix in the corresponding viewing angles is shown. Multiple algorithms are implemented to conduct the registration, which corrects the translational and/or the rotational errors. A sequence that offers a superior performance is presented and discussed. Going beyond the visual assessment of the reconstruction results, the morphological quantification of a battery electrode particle that has gone through substantial cycling is investigated. The results show that the presented method has led to a better quality tomographic reconstruction, which, subsequently, promotes the fidelity in the quantification of the sample morphology.

Mesoscale Battery Science: The Behavior of Electrode Particles Caught on a Multispectral X-ray Camera.

Functional materials and devices are usually morphologically complex and chemically heterogeneous. Their structures are often designed to be hierarchical because of the desired functionalities, which usually require many different components to work together in a coherent manner. The lithium ion battery, as an energy storage device, is a very typical example of this kind of structure. In a lithium ion battery, the cathode, anode, and separator are soaked in a liquid electrolyte, facilitating the back and forward shuttling of the lithium ions for energy storage and release. The desired performance of a lithium ion battery has many different aspects that need to be engineered and balanced depending on the targeted applications. In most cases, the cathode material has become the limiting factor for further improvements and, thus, has attracted intense attention from the research community. While the improvement in the overall performance of the lithium ion battery is the ultimate goal of the research in this field, understanding the relationship between the microscopic properties and the macroscopic behaviors of the materials/devices can inform the design of better battery chemistries for practical applications. As a result, it is of great fundamental and practical importance to investigate the electrode materials using experimental probes that can provide good chemical sensitivity and sufficient spatial resolution, ideally, under operating conditions. With this motivation, our group has been focusing on the development of the nanoscale full-field X-ray spectro-microscopy, which has now become a well-recognized tool for imaging battery electrode materials at the particle level. With nanoscale spatial resolution, this technique can effectively and efficiently tackle the intrinsically complicated mesoscale chemistry. It allows us to monitor the particles' morphological and chemical evolution upon battery operation, providing valuable insights that can be incorporated into the design of new battery chemistries. In this Account, we review a series of our recent studies of battery electrode materials using nanoscale full-field X-ray spectro-microscopy. The materials that are the subjects of our studies, including layer-structured and spinel-structured oxide cathodes, are technically very important as they not only play an important role in today's devices but also possess promising potential for future developments. We discuss how the subparticle level compositional and state-of-charge heterogeneity can be visualized and linked to the bulk performance through systematic quantification of the imaging data. Subsequently, we highlight recent ex situ and in situ observations of the cathode particles' response to different reaction conditions, including the spontaneously adjusted reaction pathways and the morphological changes for the mechanical strain release. The important role of surface chemistry in the system is also discussed. While the microscopic investigation at the particle level provides useful insights, the degree to which this represents the overall properties of the battery is always a question for further generalizing the conclusions. In order to address this concern, we finally discuss a high throughput experimental approach, in which a large number of cathode particles are scanned. We discuss a case study that demonstrates the identification and analysis of functionally important minority phases in an operating battery cell through big data mining methods. With an emphasis on the data/information mining aspect of the nanoscale X-ray spectro-microscopic study of battery cathode particles, we anticipate that this Account will attract more research to this field.

Oxygen Release Induced Chemomechanical Breakdown of Layered Cathode Materials.

Chemical and mechanical properties interplay on the nanometric scale and collectively govern the functionalities of battery materials. Understanding the relationship between the two can inform the design of battery materials with optimal chemomechanical properties for long-life lithium batteries. Herein, we report a mechanism of nanoscale mechanical breakdown in layered oxide cathode materials, originating from oxygen release at high states of charge under thermal abuse conditions. We observe that the mechanical breakdown of charged Li1- xNi0.4Mn0.4Co0.2O2 materials proceeds via a two-step pathway involving intergranular and intragranular crack formation. Owing to the oxygen release, sporadic phase transformations from the layered structure to the spinel and/or rocksalt structures introduce local stress, which initiates microcracks along grain boundaries and ultimately leads to the detachment of primary particles, i.e., intergranular crack formation. Furthermore, intragranular cracks (pores and exfoliations) form, likely due to the accumulation of oxygen vacancies and continuous phase transformations at the surfaces of primary particles. Finally, finite element modeling confirms our experimental observation that the crack formation is attributable to the formation of oxygen vacancies, oxygen release, and phase transformations. This study is designed to directly observe the chemomechanical behavior of layered oxide cathode materials and provides a chemical basis for strengthening primary and secondary particles by stabilizing the oxygen anions in the lattice.