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Significant progress has been recently reported in solar water splitting, but two major challenges remain. First, although solar-to-hydrogen (STH) efficiency as high as 30% has been demonstrated, the use of expensive and non-scalable III-V semiconductors is required. On the other hand, low-cost metal-oxide based devices, using mainly BiVO4 as the absorber, have only achieved STH efficiency of &lt; 10%, due to the limited absorption of BiVO4. Novel metal oxide semiconductors with a bandgap of 1.7-1.9 eV, which are stable and efficient, are therefore desired. The second challenge is related to the scale-up of the devices, from laboratory experiments to large-area device demonstrations. Several efforts exist, but the performance of the large-area devices is still significantly smaller (up to a factor of 5) as compared to the small-area equivalent. In this talk, I will outline our efforts in overcoming the two challenges above. The first is on the development of  SnWO4 (Eg = 1.9 eV) as a novel photoanode, which suffers from poor stability. This could be overcome by depositing NiOx as a protection layer, but it is also accompanied by the reduction of the photovoltage. We performed a thorough α-SnWO4/NiOx interface investigation by means of synchrotron-based hard X-ray photoelectron spectroscopy,[1] which reveals the presence of an interfacial oxide layer (likely to be SnO2). Control experiments show that such a layer can indeed be responsible for the reduction of photovoltage; its formation must therefore be avoided to overcome the photovoltage limitation. Concerning the scale-up of devices, I will discuss our approach in quantifying the different loss mechanisms associated with scaling-up photoelectrodes using a combination of multiphase multiphysics simulations and in-situ quantitative analysis of the fluid dynamics as well as pH of the electrolyte.[2-4] Based on these insights, electrochemical engineering strategies to overcome the losses associated with scale-up are offered.

1. Schnell et al. Adv. Energy Mater. 11 2021 2003183
2. Abdi et al. Sust. Energy Fuels 4 2020 2734
3. Obata et al. Energy Environ. Sci. 13 2020 5104
4. Obata et al. Cell Rep. Phys. Sci. 2 2021 100358

MATERIALS FOR HUMANITY (MH 21)

Development and scale-up of complex oxide-based devices for solar water splitting

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Significant progress has been recently reported in solar water splitting, but two major challenges remain. First, although solar-to-hydrogen (STH) efficiency as high as 30% has been demonstrated, the use of expensive and non-scalable III-V semiconductors is required. On the other hand, low-cost metal-oxide based devices, using mainly BiVO4 as the absorber, have only achieved STH efficiency of < 10%, due to the limited absorption of BiVO4. Novel metal oxide semiconductors with a bandgap of 1.7-1.9 eV, which are stable and efficient, are therefore desired. The second challenge is related to the scale-up of the devices, from laboratory experiments to large-area device demonstrations. Several efforts exist, but the performance of the large-area devices is still significantly smaller (up to a factor of 5) as compared to the small-area equivalent. In this talk, I will outline our efforts in overcoming the two challenges above. The first is on the development of  SnWO4 (Eg = 1.9 eV) as a novel photoanode, which suffers from poor stability. This could be overcome by depositing NiOx as a protection layer, but it is also accompanied by the reduction of the photovoltage. We performed a thorough α-SnWO4/NiOx interface investigation by means of synchrotron-based hard X-ray photoelectron spectroscopy,[1] which reveals the presence of an interfacial oxide layer (likely to be SnO2). Control experiments show that such a layer can indeed be responsible for the reduction of photovoltage; its formation must therefore be avoided to overcome the photovoltage limitation. Concerning the scale-up of devices, I will discuss our approach in quantifying the different loss mechanisms associated with scaling-up photoelectrodes using a combination of multiphase multiphysics simulations and in-situ quantitative analysis of the fluid dynamics as well as pH of the electrolyte.[2-4] Based on these insights, electrochemical engineering strategies to overcome the losses associated with scale-up are offered.

1. Schnell et al. Adv. Energy Mater. 11 2021 2003183
2. Abdi et al. Sust. Energy Fuels 4 2020 2734
3. Obata et al. Energy Environ. Sci. 13 2020 5104
4. Obata et al. Cell Rep. Phys. Sci. 2 2021 100358

technical paper

The quality of the perovskite films has an important effect on the performance of the perovskite solar cells. In this talk, we demonstrate the nucleation process plays a crucial role in the formation of the perovskite films with various morphologies. The low supersaturation results in films with porous morphologies because of the secondary nucleation. The high supersaturation results in films with compact morphologies due to the homogeneous nucleation. By adjusting the proportion of homogeneous nucleation and secondary nucleation based on an interface precipitation process induced by the antisolvent of n-hexane, we achieve perovskite films from porous morphologies to compact morphologies. The perovskite film with the three layers of structures has high light absorbance and large area of interface between perovskite and the hole transport materials, which result in high efficiency of the corresponding solar cells. In particular, surface of the carbon electrode is rather rough and the porous perovskite film enhances the interfacial contact area between carbon electrode and perovskite film. With the optimized perovskite films and carbon electrodes, we accomplish the fabrication of carbon based large-area and stable perovskite solar modules with the efficiency of over 13%. In addition, the morphology of TiO2 electron transport layer has also important influence on the efficiency of the solar cells. Based on the stable carbon-based perovskite solar cells, a kW-scale demonstration application of perovskite solar cells has been established.

Effect of film morphologies on performance of perovskite solar cells and modules

While numerous research groups have contributed to significant advancement of three-dimensional (3D) halide perovskites for use in photovoltaic applications, the number of publications reporting the development and fundamental properties of the materials of lower dimensionalities remains relatively small. Two-dimensional (2D) hybrid metal halide perovskites possessing the general chemical formulas (RNH3)2MX4 and (NH3RNH3)MX4 (R corresponds to an organic functional group, M a group 14 metal dication, and X a halide) are derived from their 3D congeners by slicing the inorganic lattice along specific crystallographic axes (e.g., the (100)- or (110)-planes). This can be induced through incorporation of moderately sized organic cations, such as 2-phenylethylammonium and 1-butylammonium, and it yields alternating 2D layers of organic cations and single Pb-atom-thick inorganic lattices. These are referred to as n = 1 2D hybrid metal halide perovskites, where n refers to the number of contiguous 2D inorganic layers, i.e., not separated by organic cations. The resulting 2D materials possess synthetic versatility and moisture stability greater than that of pure 3D perovskites. However, they are, generally, considered ill-suited for photovoltaic applications because their reduced dimensionality manifests in poor charge transport and compromised visible light absorption. In addition, they typically possess high excitonic binding energy values as a result of the dielectric mismatch between the organic and inorganic constituents. In this talk, I will present and review the factors that control the performance of n = 1 2D halide perovskite photovoltaics, including the judicious choice of organic and group 14 metal cations and selection of charge-transport materials that are compatible with perovskite layers. Key challenges and potential pathways for achieving efficient solar cells will then be identified and discussed.

Progresses and Challenges with Two-Dimensional Halide Perovskite Photovoltaics

This presentations may interest you if you work on:
- Upscaling perovskite solar cells (it discusses reactions that may occur in perovskite inks for blade-coating of perovskite)
- Sn-perovskites (it shows how a conventional solvent can oxidize Sn)
- Record-breaking perovskite solar cells (it demonstrates how multi-cation/anion perovskites are crystallized that lead to better efficiencies and stubilities)

Why Mixing is Good for Perovskites?

There has been growing interests in perovskite-silicon tandem solar cells to reach ultrahigh efficiency beyond the Shockley-Queisser limit. The ideal band gap for the tandem configuration is ~1.67 to 1.75 eV for the top cell and 1.12 eV for the bottom cell. The band gap of perovskites can be tuned by (partial) replacement of iodine anions with bromine or chlorine. However, the replacement of I with Br by more than 20%, which is necessary to enlarge the band gap to ~1.7 eV, leads to stability issues under illumination through phase separation. One approach to stabilize the perovskite is to introduce a two-dimensional (2D) phase in which sheets of [PbX6]−2 octahedra are separated by an excess number of long-chain molecules. Most of the recent studies have focused on the cation components of the 2D additives rather than the anions. We developed a 2D-3D mixed wide band gap (1.68 eV) perovskite using a mixture of thiocyanate with the more conventional choice, iodine. Through a careful application of atomic resolution transmission electron microscopy, we demonstrated that electrical and charge transport properties as well as the physical location of 2D passivation layers can be controlled with anion engineering of the 2D additives. Moreover, we can use this approach to extend light stability and to improve device performance. For a perovskite device, we achieved a PCE of 20.7% that retained > 80% of its initial efficiency after 1000 hours of continuous illumination in working conditions. For a monolithic 2T perovskite/Si tandem solar cells, the champion 2T tandem device achieved a PCE of 26.7%.

Efficient, stable silicon tandem cells enabled by anion-engineered wide-bandgap perovskites

Thermochromic material responds to solar spectrum differently at the stimulus of heat which makes it attractive in the energy saving smart windows application due to its rational response, simple set up and passive response. The most studied inorganic VO2 has the intrinsic problems of low luminous transmission (Tlum) and low solar modulation (ΔTsol the transmission difference between high and low temperatures).Numerous efforts such as employing dopings, nanoparticle-based composites, and nano-porous structuring have been widely studied. Our group have developed five new approaches to tackle this veritable challenge, namely, biomimetic nanostructuring including photonic structure6 and moth eye gridded structures tunable plasmonic structuresorganic and hybrid structures. In addition, new active controls has also been applied to thermochromic material to generate a new electro-thermochromics or mechanical-thermochromic materials. A very recent new liquid window developed will be presented.

Progress in Energy Saving Thermochromic Smart Windows

In many industries such as Aerospace, Defense, Marine, Transportation, Off-shore high performance materials are required. For such applications, laminated carbon fiber reinforced polymeric (CFRP) composites are preferred due to their high specific strength and stiffness compared to other advanced materials. While CFRP composites provide excellent inplane properties, their performance when subjected to transverse loadings, both static and dynamic, needs further improvements. In recent years, addition of nanoparticles such as carbon nanotubes, carbon nanofibers, graphene and nanoclay, when suitably modified with chemical functional groups on their surfaces, in the polymers used for the fabrication of fiber reinforced composites has resulted in significant enhancement in their static and dynamic properties as well as durability. Over last few years, systematic investigations were carried out by our group to optimize the processing of epoxy resin systems with combinations of nanoclay and multiwalled carbon nanotubes through a multi-step process involving combinations sonication, magnetic mixing, three-roll mill, and high shear mixing. Epoxy resin modified with the nanoparticles was then used to fabricate carbon fiber composites. Both modified epoxy and nanophased fiber reinforced composites were characterized for properties such as fracture toughness, storage modulus, loss modulus, tensile, flexure, low-velocity impact and flexural fatigue. Results of the study indicate significant enhancement is all the properties and reduced damages. These enhancements were attributed to increased interfacial interactions between the three phases: nanoparticles, polymers and fibers. Details of these studies will be presented. Financial support through funding from the Department of Defense, as well as the research work carried out by the graduate students are hereby acknowledged.

Performance Enhancements in Carbon Fiber Reinforced Polymer Composites through Nanoparticles Inclusion

The formation and control of surface creases in soft materials under compression have intrigued the mechanics community for decades and recently found many applications in tissue biomechanics, soft robotics and tunable devices. In spite of a rapidly growing literature in this field, existing methods of analysis often rely on a presumed crease configuration and consequently there is still a lack of profound theoretical understanding on crease nucleation. In this study, we propose a force based perturbation approach to predicting the occurrence of crease nucleation without assuming a post-instability configuration. In a set of carefully controlled FEM simulations, by considering the relative magnitudes among the element size, perturbation displacement and sample size, we find that beyond a critical strain around , a flat surface under uniform deformation becomes metastable, while the creased configuration becomes stable, with energy barrier for creasing proportional to the square of the FEM element size and therefore vanishing in the continuum limit. Beyond the Biot critical strain of , the uniformly deformed configuration of a flat surface becomes unstable. Our force-based instability criterion also enabled us to determine the critical conditions of crease formation for different materials under general loading conditions, leading to a set of crease diagrams. Interestingly, it is shown theoretically and validated experimentally that some highly compressible soft materials do not undergo creasing under loading conditions close to equibiaxial compression.

A perturbation force based approach to creasing instability in soft materials under general loading conditions

Light is a ubiquitous renewable energy resource that nature has implemented as the foundational power supply for biomass generation on Earth. Human society is also built upon an intense reliance on chemical demands for our food, materials, and pharmaceuticals; yet, the chemical industry has evolved to power those industrial processes with energy derived from the combustion of fossil fuels. While the chemical industry is a cornerstone of the modern world, it also exists as one of the most polluting sectors on the planet. Is it possible to develop novel nanophotonic materials that effectively harness optical energy to power chemical processes and transition to a sustainable energy supply of the global chemical industry? In this talk, I will detail recent efforts in the development of plasmonic photocatalysts. These nanophotonic materials have demonstrated unique pathways to activate and engage chemical bond formation when illuminated with visible light, primarily due to non-thermal contributions to chemical processes at interfaces. The talk will introduce fundamental concepts in the rational design of plasmonic catalysts, highlight several important demonstrations of plasmonic photocatalysts in reactions key for global sustainability (e.g., H2 generation and greenhouse gas mitigation), and describe recent developments in optically coupled transmission electron microscopy to reveal the influence of light on atomic-scale phase transformations. Important attention will be paid to describe work where the influence of light results in unique mechanistic pathways for chemical reactions, particularly hydrogenation chemistry and nanoscale phase transformations. Finally, this talk will conclude with an outlook on the future of nanophotonic chemical processes in the next decade.

Making and Breaking Chemical Bonds with Light

Meta-lenses consist of a large number of optical nano-antennas which are capable of focusing the incoming wavefront of light [1-6]. We use a 60 × 60 dielectric achromatic meta-lens array to capture multidimensional optical information. The highest efficiency can be up to 74% at a wavelength of 420 nm, while the average efficiency is approximately 39% over the whole working bandwidth. The light field images and the depth information of objects can be determined by reorganizing the patches of sub-images and calculating the disparity of neighbor sub-images, respectively. The depth information can be used to optimize the patch sizes to render the all-in-focus images without artifacts. The smallest feature of objects that could be resolved in our system is 1.95 μm under the incoherent white light. Our work provides several advantages associated with light field imaging: elimination of chromatic aberration, polarization selectivity and compatibility of the semiconductor process. Considering the flexibility, the achromatic multiplexed meta-lens array with integrated functionalities may be promising for multi-focusing microscopy, high-dimension quantum technology, hyperspectral microscopy, micro robotic vision, autonomous sensing, virtual and augmented reality (VR and AR), drones, and miniature personal security systems [7-10].
References: 
[1] S. M. Wang, et al., Nature Comm. 8, 187 (2017). 
[2] B. H. Chen, et al., Nature Nanotechnology 13, 227 (2018). 
[3] V.-C. Su, et al., Adv. Optical Mater. 6, 1800554 (2018). 
[4] H.‐H. Hsiao, et al., Adv. Optical Mater. 6, 1800031 (2018).
[5] R. J. Lin, et al., Nature Nanotechnology, 14, 227 (2019). 
[6] L. Li, et al., Science 368, 1487–1490 (2020).
[7] G. Qu, et al., Nature Communications 11, 5484 (2020).
[8] M. K. Chen, et al., Advanced Optical Materials 4, 202001414 (2021). 
Acknowledgment: This work was supported by RGC Areas of Excellence Scheme 2020/21 (AoE/P-502/20).


Meta-optics: Meta-lens for imaging, sensing, and quantum chip

Coherent optical excitations in van der Waals materials (2D polaritons) span a large range of optical phenomena which arise from their extraordinary dispersion relations [1]. Among the different polaritons, it is particularly interesting to examine polaritons in hyperbolic materials (hyperbolic phonon-polaritons, hPhP) which show rich physical behavior, ranging from negative refraction and subdiffraction imaging. The phononic resonance creates a dispersion relation that contains multiple branches, which were shown to be tunable by the 2D material geometry, thickness, and surrounding environment [2], reaching relatively low losses in room temperature [3].
Among the various experimental techniques that are used in the field of 2D polaritons, scanning nearfield optical microscopy (SNOM) and its variants dominates the direct nearfield imaging of 2D polaritons. Specifically, recent advances in time-resolved SNOM allowed adressing the polariton’s dynamical properties as group velocity from the changes of interference of scattered polaritons with different time delays [4]. However, this interferometric technique cannot image the wavepacket dynamics, as it excludes the phase difference between different wavelengths. Different imaging techniques such as electron energy loss spectroscopy (EELS), where the 2D polariton dispersion was recently extracted [5], obtains only time-independent information on the polaritonic modes. 
Here we present how an ultrafast transmission electron microscope (UTEM), driven by femtosecond mid-infrared pulses, can probe the spatiotemporal dynamics of 2D wavepackets.
We record how the mid-infrared pulses excites hPhP wavepacket and measure the wavepacket's trajectory inside the sample, revealing unforseen physical behaviors: multi-branch wavepacket splitting and wavepacket acceleration and deceleration. These phenomena are especially surprising since conventional wavepackets propagate in a fixed group velocity. We show that it is the unique dispersion relations of the hyperbolic PhPs that facilitates these phenomena, while recording group velocities between c/45 to c/850. These results examplify the wide physical spatiotemporal dynamical phenomena that can be probed through a non-destructive measurement using the unique combination of femtosecond temporal and nanometer spatial resolution of the UTEM.

Refs:
[1] D. N. Basov, A. Asenjo-Garcia, P. J. Schuck, X. Zhu, A. Rubio, Polariton panorama. Nanophotonics 10, 549–577 (2020).
[2] S. Dai et al. Tunable Phonon Polaritons in Atomically Thin van der Waals Crystals of Boron Nitride. Science. 343, 1125–1129 (2014).
[3] A. J. Giles et al. Ultralow-loss polaritons in isotopically pure boron nitride. Nat. Mater. 17, 134–139 (2018).
[4] E. Yoxall et al. Direct observation of ultraslow hyperbolic polariton propagation with negative phase velocity. Nat. Photonics. 9, 674–678 (2015).
[5] N. Li et al. Direct observation of highly confined phonon polaritons in suspended monolayer hexagonal boron nitride. Nat. Mater. 20, 43–48 (2021).

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