Materials under the tip: Quantitative Imaging of Localized Functionalities at the Nanoscale

掃描探針顯微方法在先進功能材料的應用與挑戰

In the last two decades, nanostructured materials have revolutionized information technology, energy systems, and medicines. With the advances in microscopy and analytical tools, we can now image compositions, phases, and structures with atomic and sub-atomic resolution. The length scales upon which physical properties can be quantitatively measured, however, largely lag behind, making it difficult to directly connect compositions, structures and properties of nanostructured materials and systems. In this talk, I will briefly review our efforts using scanning probe microscopy to address this challenge in the past a few years, with particular emphasis on mechanistic understanding regarding competing microscopic mechanisms and data analytic approaches with machine learning and artificial intelligence. Cases studies on resistive switching in ferroelectrics, polar order in organic-inorganic solar cells, and low-dimensional heat conduction in nanostructured thermoelectrics will be given, and future challenges and opportunities will be highlighted.

Novel Dielectrics for Passive Integration and Energy Storage

面向無源集成和儲能的新型電介質材料

With the development of electronic and information system towards miniaturization and high density integration, especially the speedy applications of wireless network and internet of things, it is required that the electronic materials and components should have the matching properties such as high performance, multifunctional, high frequency enabling and low energy consumption. The highlights of our research on the novel LTCC dielectric ceramics for passive integration and dielectric nanocomposites for energy storage applications will be presented, while the remaining challenges and the promising opportunities of the development will be discussed as well.

Application of pair distribution function method for characterization of atomic structure in Pb-free dielectric and electrocaloric materials

偶分佈函數法在無鉛介電和電熱材料原子結構表徵中的應用

Development of new Pb-free relaxor ferroelectrics has received much attention in recent years for application as high-temperature dielectric and electrocaloric materials. Elucidation of composition-structure-property relationships is a prerequisite first step towards rational and efficient design of new materials. Nevertheless, relaxor ferroelectrics are challenging to understand from a scientific viewpoint because of the disordered nature of their atomic structure. For the new Pb-free relaxors, the polar atomic displacements are correlated over only nanometer length scales, although the details of such nanoscale correlations are yet to be fully resolved. In this talk, I will present our recent findings on the nanoscale orderings of polar atomic displacements in the newly designed Pb-free relaxors, which are characterized using pair distribution function analysis of X-ray and neutron total scattering patterns. In addition, I will demonstrate the application of the dynamic pair distribution function (DyPDF) method to elucidate the timescales over which such nanoscale atomic orderings become stable in some Pb-free relaxor systems. The implication of nanoscale atomic ordering and dynamics towards the dielectric, ferroelectric and electrocaloric properties of Pb-free relaxors will be discussed.

Engineering the Shape and Morphology of 2d Transition Metal Dichalcogenide Alloys

二維材料二硫屬化物合金的形狀和形態的工程化

The properties of 2D materials can be broadly tuned through alloying and phase and strain engineering. Shape programmable materials offer tremendous functionality, but sub-micron objects are typically unachievable with conventional thin films. Here we propose a new approach, combining phase/strain engineering with shape programming, to form 3D objects by patterned alloying of 2D transition metal dichalcogenide (TMD) monolayers. Conjugately, monolayers can be compositionally patterned using non-flat substrates. For concreteness, we focus on the TMD alloy MoSe2cS2(1−c); i.e., MoSeS. These 2D materials down-scale shape/composition programming to nanoscale objects/patterns, provide control of both bending and stretching deformations, are reversibly actuatable with electric fields, and possess the extraordinary and diverse properties of TMDs. Utilizing a first principles-informed continuum model, we demonstrate how a variety of shapes/composition patterns can be programmed and reversibly modulated across length scales. The vast space of possible designs and scales enables novel material properties and thus new applications spanning flexible electronics/optics, catalysis, responsive coatings, and soft robotics.

Materials Data Science

材料數據科學

Taking advantages of unprecedented computing power, material research is transforming into the 4th paradigm of scientific research featuring “intensive data and artificial intelligence (AI)”. AI can surpass human limitation in analyzing multi-dimensional, complex material data and quickly identify the relationships between application-oriented key properties and corresponding parameters.
In our recent work, advanced AI methods have been employed to study superconductors and metallic glass. Deep Neural Network has been successfully applied to predict the critical superconducting temperature of inorganic superconductors with high correlation (R2: 0.97), which is better than those reported in material-related literature. However, in most material fields, data is extremely scarce limiting the successful application of advanced AI.
Shenzhen Material Genome Scientific Infrastructure led by Southern University of Science and Technology is a "Material Data Foundry" to solve the issue of material data scarcity. In the infrastructure, we are developing far-field optical techniques capable of rapid characterization of composition, structure, and optical, electrical, thermal, magnetic, mechanical properties of materials, We will discuss the details of these techniques and their impact to material sciences.

Migration from Low Dose Electron Microscopy to Quantum Electron Microscopy for Soft Materials Dynamics

​軟材料動力學中低劑量電子顯微鏡到量子電子顯微鏡的遷移

Advancements of aberration-corrected electron optics and data acquisition schemes have made TEM capable of delivering images with sub-Ångström resolution and single-atom sensitivity.  However, three bottlenecks, namely, radiation damage,  static picture and 2D projection are still waiting to overcome for soft materials imaging. For the first barrier the radiation damage, it has been shown that high energy electron beam may not only alter the shape and surface structure at atomic level, but also induce radiolysis artifact in in-situ chemical reactions at nanoscale, e.g., photocatalysis, metal-insulator transformation (MIT),  super-dissolution of oxides.  In our approach, to overcome three imaging bottlenecks simultaneously,  in stead of recording single high resolution image using high dose rate electron beam, the electron dose was evenly distributed onto many different focal planes at low dose rate mode.  Time-resolved in-line electron holograms with high signal/ noise ratio can then be reconstructed from focal series of atomic resolution images recorded at low dose rate mode. From the holograms, atomic structure in 3D, including the atomic position and atom content can be retrieved. In the quest to suppress electron-induced alterations and to enable dynamical observations, it therefore becomes mandatory to exercise control over the electron dose, dose-rate and energy. Here, we will demonstrate the dose rate can be precisely controlled via a gun monochromator and the pulse electron source TRACE (time-resolved aberration corrected environmental) TEM to be setup in city university via which we are able to record dynamics of soft materials with high space time resolution ~10-16 m.sec with preserving the sample in its pristine state.   However, for the single molecule case, we may need to take one step further to even avoid the electron-sample interaction.  I will discuss in detail  how we implement the recent development of  interaction-free measurement  in light optics to electron optics to achieve the concept of “Quantum Seeing in the Dark” in “quantum electron microscope.

Control the Defects and Doping of Wide-band-gap Semiconductors using External Voltage

通過外電壓調控寬禁帶半導體的缺陷和摻雜類型

Doping asymmetry is a long-standing issue with the wide-band-gap semiconductors and has been the major obstacle to their commercial applications. A well-known example is that the as-grown zinc oxide is an n-type semiconductor without intentional doping, but its stable and high-quality p-type doping is yet to be realized. I will show in this talk that it is possible to control the defects and doping in the wide-band-gap semiconductors using a proper external voltage, which can tune the defect thermodynamics and change the formation of defects and dopants. I will take the zinc oxide as an example to demonstrate the cause of the n-type doping in the hydrothermal growth method and show the way to realize the intrinsic and p-type zinc oxide based on first-principles calculations.

Atomic Scale Magnetic and Structural Imaging by Achromatic Transmission Electron Microscopy

​消色差透射電子顯微鏡的原子尺度磁和結構成像

The atomic-level knowledge of local spin configuration of the magnetic ma-terials is of great importance to predict and control their physical properties, in order to meet the challenges of ever-increasing demands on performance of func-tional materials. However, it is highly challenging to experimentally characterize magnetic properties of such materials with atomic scale spatial resolution. The best option to push the spatial resolution of the spectromicroscopies lies in the electron beam equivalent technique electron energy-loss magnetic chiral dichroism (EMCD), which is also called electron magnetic circular dichroism. Physically, XMCD and EMCD shares the same underlying physics in which the angular momentum transferred during X-ray absorption or inelastic electron scattering can selectively excite magnetic sublevels in atoms. The structured electron beams generated through interference of suitably phased plane waves can produce beams with orbital angular momentum. Electron beams can be easily focused compared with X-rays, allowing for atomic scale magnetism to be probed. Previously, we have found a strong EMCD signal in transition metal oxides allowing them to use standing wave methods to identify the different spin states of Fe atoms with site specificity.
In principle EMCD can offer higher spatial resolution and greater depth sen-sitivity due to the short de Broglie wavelength and penetration of high-energy electrons compared to XMCD. Recently by using EMCD and achromatic electron microscopy, we are able to access the magnetic circular dichroism with atomic plane resolution. Combining with advanced capability of structural and chemical imaging by using aberration-corrected transmission electron microscopy, all the information including magnetic polarization, atomic configurations and chemical states can be simultaneously accessed from the very same sample region. In the examples of complex oxides e.g. Sr2FeMoO6, we would like to show how to achieve local atomic-scale magnetic, chemical and structural information and un-derstand the structure-property relationship of these magnetic materials at the atomic level.

Surface Engineering for Water-energy Nexus

水能關系的表面工程

The global demand for clean water and green energy has been in increasing rise. Currently, most of conventional industrial technologies are accompanied with large water and energy consumption. During 3.8 billion years of evolution, biological systems have orchestrated a variety of principles that allow them to process and manage information and energy using the least materials while high efficiency. One of the secrets lies in the ingenious utility of interfacial topographical structures, which effectively mediates the flow and transport of mass, momentum, and energy.
In this talk, I will discuss our efforts in the fundamental understanding of surface/interfacial phenomena as well as the design, fabrication of artificial surfaces/materials for a wide range of engineering implementations, ranging from water collection from air, water energy harvesting and phase change heat transfer.

Electrolyte Solutions for High Energy Density xEV LIBs

高能量密度動力鋰電池電解液策略

Solid-electrolyte-interphase (SEI) on anode and cathode-electrolyte-interphase (CEI) on cathode are two passivation components on electrode surfaces. They have long been regarded to serves as effective electrochemical barrier that prevents further side reactions between electrolyte and electrode but allows facile Li+-transport. The formation of SEI & CEI introduces additional impedance to Li+-transport, which affects the kinetics of cell reactions and eventually power density of LIBs. The adoption of additives into electrolytes are the most cost-effective way to modify the CEI and SEI, and we need to develop additives with low impedance for high power density LIBs. Nevertheless, the additive practice remains a “dark art” in electrolyte research, and the screening and selection of new additives are often conducted on trial-and-error basis, with almost no rationale correlation established between the structure and chemical/electrochemical behaviors of additives in Li-ion battery environment.
We attempt to explore the fundamental science behind the empirical use of additives with low impedance. It was found that the effectiveness of these additives lies not only in how they participate the interphasial chemistries, but also in how they suppress the major side reactions between the bulk electrolyte solvents, the trans-esterification. The correlation between the additive chemistries and electrochemical performances provide valuable guidelines to rational selection, design and synthesis of future additives for higher effectiveness and for new battery chemistries.

Flexible Zinc based Batteries

柔性鋅基電池

While the widely reported Zn chemistries operate in and benefit from the adopted aqueous electrolytes, distressingly, Zn anode also persistently suffers from several deep-seated issues resulting from the aqueous electrolyte. One is parasitic hydrogen evolution reaction (HER) through water splitting, eventually leading to cell volume expansion and relatively low coulombic efficiency. The other one is the dendrite growth during Zn anode plating/stripping process. A derived issue is that the current hydrogels-based Zn based batteries are only quasi-solid-state systems, benefiting from the flexibility of hydrogels and meanwhile suffering from their poor mechanical properties and dehydration. In this work, we firstly demonstrate that an ionic liquid-based Zn salt electrolyte is an effective route to solve both side-reaction of HER and Zn dendrite growth. The developed electrolyte enables hydrogen-free, dendrite-free Zn plating/stripping over 1500 h cycle (3000 cycles) at 2mA×cm-2 with nearly 100% coulombic efficiency. Meantime, the oxygen induced corrosion and passivation are also be effectively suppressed. These features bring Zn-ion batteries unprecedented long lifespan over 40000 cycles at 4 A×g-1 and high voltage of 2.05 V with a cobalt hexacyanoferrate cathode. Furthermore, a 28.6 mm-thick solid polymer electrolyte of a poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) film filled with polyethylene oxide (PEO)/ionic liquid-based Zn salt is constructed to build an all-solid-state Zn-ion battery. The all-solid-state Zn-ion batteries shows excellent cycling performance of 30000 cycles at 2 A×g-1 at room temperature and withstand high temperature up to 70 oC, low temperature to -20 oC, as well as abuse test of bending deformation up to 150o for 100 cycles and 8 times cutting. This is the first demonstration of an all-solid-state Zn-ion battery based on newly developed electrolyte, which meanwhile solves the deep-seated hydrogen evolution and dendrite growth problem in traditional Zn-ion batteries. 

Molecular Engineering on Hole Transporting Materials towards Efficient and Stable Perovskite Solar Cells

高效高穩定鈣鈦礦太陽電池空穴傳輸材料的分子設計與制備

Organic-inorganic hybrid perovskite solar cells (PSCs) have received great attention due to their excellent photoelectric performance, especially the rapid increase of their power conversion efficiencies. However, the hole transporting materials (HTMs) of the most efficient PSCs is primarily based on Li-salt doped spiro-OMeTAD, which has a big negative impact on the cost and device long-term stability. In this talk I will introduce our work on how to design new, dopant-free HTMs for PSCs from the point view of molecular engineering. This includes small molecule and polymer HTMs for regular PSCs, and conjugated polyelectrolytes HTMs for inverted PSCs. The PSC devices based on these dopant-free HTMs can achieve more than 22% efficiency with excellent light, humidity and temperature stabilities.

Recent advance of inverted planar perovskite solar cells

反式鈣鈦礦光伏電池研究進展

This talk will summarize the research progress of inverted planar perovskite solar cells, especially in a view of interface engineering, inckuding both HTL/PVK and ETL/PVK interface.

Smart Behavior of Spider silks, their Directional Water Collectionand An Artificial Substitute

蜘蛛絲智慧的行為 - 它們有方向性水的收集和人工替代品

Spider silks are one of the extensively studied natural protein materials due to their outstanding mechanical and smart behavior. A spider web collects water by its capture-silk for recovering the daytime-distorted shape during night through water-sensitive shape memory effect. This unique smart function and geometrical structure of spider-silkhasinspired the development of artificial fibers with periodic knots for directional-water-collection with vast potential applications in water scarce regions. Existing such fibers were mainly based on nylon filaments coated with petroleum-originated synthetic polymer solutions, which proves to have limited capability in water collection. Different from using synthetic materials, an all silk-protein fiber (ASPF) with periodic knots to endow extremely high volume-to-mass water collection capability. This fiber has a main bodyof B. mori degummed silkcoated with recombinant eMaSp2 of spider dragline silk. It is 252 times lighter than synthetic polymer coated nylon fibers that once was reported to have the highest water collection performance. The ASPF collected a volume of 6.6 μL of water and has 100 times higher water collection efficiency compared to existing best water collection artificial fibers in terms of volume-to-mass index (VTMI) at the shortestlength (0.8 mm) of three phase contact line (TCL). Since silkworm silks are available abundantly, effective use of recombinant spidroins tandemly shows great potential for scalability.

Imide-Functionalized Polymer Semiconductors for High-Performance Organic Thin-Film Transistors and Solar Cells

酰亞胺功能化聚合物半導體在高性能有機薄膜晶體管和太陽能電池中的應用

Imide-functionalized π-conjugated polymers are highly promising semiconductors in the field of organic electronics. Semiconducting polymers derived from naphthalene diimide (NDI) and perylene diimide (PDI) are the “benchmark materials” in organic thin-film transistor and all-polymer solar cell field. We report here a series of novel imide-functionalized ladder-type bithiophene imide derivatives (BTIn, n = 1-5) with up to 5 imide groups and 15 rings. The homopolymers show tunable frontier molecular orbital energy levels and film morphologies. Novel imide-functionalized thiazoles were also synthesized, enabling the access of all-acceptor homopolymers, which exhibit unipolar n-type transport in organic thin-film transistors with the highest electron mobilities (μes) > 3 cm2 V−1 s−1. Notably, these polymers do not show undesirable kink in transistor curves, thus avoiding mobility overestimation. The deep-lying polymer frontier molecular orbital levels led to suppressed Ioffs of 10−10-10−11 A, thus remarkable Ion/Ioffs of 107-108 achieved, while maintaining a large μe of 1.6 cm2 V−1 s−1. Besides all-acceptor homopolymers, these novel imide-functionalized arenes were incorporated to donor-acceptor and donor-acceptor-acceptor copolymers, which also showed unipolar n-type transport with substantial μes > 1 cm2 V−1 s−1. When applied as acceptor materials, the all-polymer solar cells exhibited power conversion efficiencies of >14%.

Development of potassium ion sensors

鉀離子探針研制

HPotassium ions which make up about 0.4% of the mass in the human body and are the most abundant intracellular cation, play diverse roles in biological processes including muscle contraction, heartbeat, nerve transmissions, and kidney functions. Abnormal K+ fluctuations are early indicators of diseases such as alcoholism, anorexia, bulimia, heart disease, diabetes, AIDS, and cancer. Therefore the detection of K+ in physiological environment is of great significance.
We will describe our results for developing highly selective potassium ion sensors. We used triazacryptand (TAC) as a high selective potassium ion ligand and various fluorophores for preparing highly selective potassium molecular and planar polymeric probes. We constructed a potassium ion sensor using a 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) as a strong electron withdrawing group and the TAC as the electron donating group for the first intracellular potassium ion sensor. Later we incorporated a triphenylphosphonium (TPP) unit into a BODIPY fluorophore with TAC as the ligand for the first mitochondrial targeting potassium ion probe. These two molecular probes show high selectivity for potassium ions and capable for monitoring intracellular potassium fluxes. Especially the probe with TPP moiety showed high co-localization efficiency for mitochondria. We also prepared a polymerizable potassium ion probe using naphthalimide as the fluorophore for generation of planar thin film-based potassium ion sensors. Very recently, we developed new mitochondria-targeted sensors and ratiometric/colorimetric sensors also. Thus in this presentation, we will give detailed results about our potassium ion sensors

Phase Engineering of Nanomaterials (PEN)

納米材料的相工程（PEN）

In this talk, I will summarize the recent researches on phase engineering of nanomaterials (PEN) in my group. In particular, we focus on the rational design and synthesis of novel nanomaterials with unconventional phases for various promising applications. For example, by using wet-chemical methods, for the first time, we have successfully prepared novel Au nanostructures (e.g., the hexagonal-close packed (hcp) Au nanosheets, 4H hexagonal Au nanoribbons, and crystal-phase heterostructured 4H/fcc and fcc/2H/fcc Au nanorods), epitaxially grown metal nanostructures with unconventional crystal phases on the aforementioned Au nanostructures, and amorphous/crystalline hetero-phase Pd and PdCu nanosheets. In addition, by using gas-solid reactions, metastable 1T'-phase group VI transition metal dichalcogenides (TMDs), e.g., WS2, WSe2, MoS2, MoSe2, WS2xSe2(1-x) and MoS2xSe2(1-x), have been prepared. Moreover, the phase transformation of TMDs during our developed electrochemical Li-intercalation process has also be observed. Impressively, the lithiation-induced amorphization of Pd3P2S8 has been achieved. Currently, my group focuses on the investigation of phase-dependent physicochemical properties and applications in catalysis, (opto-)electronic devices, clean energy, chemical and biosensing, surface enhanced Raman scattering, waveguide, photothermal therapy, etc., which we believe is quite unique and very important not only in fundamental studies, but also in future practical applications. Importantly, the concepts of phase engineering of nanomaterials (PEN), crystal-phase heterostructures, and heterophase nanomaterials are proposed.

Nonlinear and plasmonic spectroscopy of excitonic dark states in 2D semiconductors

利用非線性和等離激元光譜探測二維半導體激子暗態

Low-dimensional transition metal dichalcogenides (TMDs) have rich parity- and spin-forbidden excitonic dark states, which have no spectrosopic signatures in one-photon based absorption, reflection and photoluminescence spectra. In this talk, I will show how to combine one- and two-photon photoluminescence excitation spectroscopy to determine the energy splitting between parity-allowed and -forbidden exciton states in MoS2 nanodots, and also demonstrate C-exciton resonance enhnaced second-harmonic generation in MoS2 nanosheets [1]. I will then discuss how to ultilize plasmon-bright-exciton coupling spectroscopy to determine the energy of dark excitons in monolayer MoS2 and WS2 coupled with plasmonic nanocavities [2]. Following this, I will present an elegant tight-binding model to interpret measured layer-dependent exciton resonance energies of WS2 [3], showing good agreement with density-functional theory calculations; the temperature dependence of A-excion emission energy in monolayer WS2 can be well described with the Varshni formula and that of its emission intensity is associated with thermal redistribution of bright and dark exciton populations at elevated temperatures. Based the results obtained in Refs. 2 and 3, we recently demonstrate that sandwiching monolayer MoS2 in a plasmonic particle-on-film nanocavity can significantly enhance the bright exciton emission [4]. 

​
 

Large Elastic Deformation in Nanoscale Covalent Crystals and Strain Engineering

納米共價晶體的大彈性變形及應變工程

Covalent crystals refer a class of crystalline solids in which the atoms are bonded by covalent bonds in a continuous network extending throughout the entire material. They are usually very hard and brittle at bulk scale, with many mechanical applications; on the other hand, many of them are important semiconductor materials with wide applications in electronics and optoelectronics. However, to achieve their full potential in MEMS and functional devices requires in-depth understanding on their mechanical behavior at the small scales. In this talk, we firstly report our recent nanomechanical studies of 1D and 2D covalent crystals including silicon nanowires, diamond nanoneedles and monolayer graphene, and demonstrate their unexpected large elastic deformation (Science Advances 2016; Science 2018; Nature Communications 2020). Then, we will illustrate that, in addition to the promising flexible electronics and MEMS/NEMS applications, the discovered large elasticity in nanoscale covalent crystals could open up new avenues to design and modulate the performance of nanoelectronics and photonics devices through the emerging concept of “elastic strain engineering”.

Ge lateral overgrowth on patterned Si (001)

鍺在硅襯底上的橫向外延生長

Epitaxial lateral overgrowth (ELO) over a free-standing dielectric mask is an unexplored territory in selective epitaxy growth (SEG) of semiconductors. By shrinking the dielectric mask dimension to the micron scale, the growth fronts from ELO are able to converge and coalesce, thus providing the freedom to engineer the interfacial structure between the epi-layer and dielectric mask. We demonstrate, herein, anomalous adatom diffusion and migration at the Ge/SiO2 interface upon SEG on a Si (100) wafer. We find, depending on the oxide strip length, a polyhedral cavity or tunnel can form on the oxide layer. More importantly, we observe a thermally induced substantial internal surface reconfiguration process of Ge atoms that connects two tunnels and one cavity in order to form a single tunnel. Defect-free Ge above the oxide strips is obtained after coalescence. Our findings yield new insight into adatom migration in an enclosed space, and the cavity and tunnel show the first known three-dimensional geometric configuration in selective heteroepitaxial structures.

Ionic thermoelectric materials

離子型熱電材料

Harvesting heat from the environment into electricity has the potential to power Internet-of-Things (IoT) sensors, freeing them from cables or batteries especially for use as wearable devices. We demonstrate a giant positive thermopower of 17.0 mV K-1 in a flexible, quasi-solid state, ionic thermoelectric material using synergistic thermodiffusion and thermogalvanic effects. The ionic thermoelectric material is a gelatin matrix modulated with ions providers (KCl, NaCl, and KNO3) for thermodiffusion effect and redox couple (Fe(CN)64-/Fe(CN)63-) for thermogalvanic effect. A proof-of-concept wearable device consisting of 25 unipolar elements generated over 2 V and a peak power of 5 μW using body heat. This ionic gelatin shows promises for environmental heat-to-electric energy conversion utilizing ions as energy carriers

Electrochemical Intermediates Modulation in Organic Electrode Materials

有機電極材料的電化學反應中間體調控

Radicals are inevitable intermediates during the charging and discharging of organic redox electrodes. The increase of the reactivity of the radical intermediates is desirable to maximize the capacity and enhance the rate capability but is detrimental to cycling stability. Therefore it is a great challenge to controllably balance the redox reactivity and stability of radical intermediates to optimize the electrochemical properties with a good combination of high specific capacity, excellent rate capability and long-term cycle life.

We have shown that conjugation interactions could contribute to delocalize the energy density of the unpaired electrons and the steric effect could restrict intermolecular electron self-exchange behavior1. However, this simple introduction of the inactive aromatic units to provide conjugation interactions unavoidably results in reduction of specific capacities of the electrode materials. Hence, stabilizing the radical intermediates without the compromise of capacities decrease remains a great challenge.

Two-dimensional covalent organic frameworks (2D COFs) have attracted increasing attention in energy storage due to their crystalline polymeric frameworks for restraining dissolution in electrolyte, π-conjugated skeletons for charge transportation, and abundant nanopores for ionic transportation. Based on our previous research, we further designed and synthesized 2D COFs of varying thicknesses. The redox reactivity and stability of the intermediates are highly dependent on the thickness of the 2D COFS2. It is significant to regulate the reactivity and stability of the radical intermediates to modify the electrochemical performance of organic electrode materials.

References：
[1] S.F. Wu, W.X. Wang, M.C. Li, et al. Nat. Commun. 2016, 7: 13318.
[2] S. Gu, S.F. Wu, M.C. Li, et al. J. Am. Chem. Soc. 2019, 141: 9623.
 

Toward Highly Efficient Solar Water Splitting: A Concurrent Electrical, Optical, and Catalytic Design

邁向高效的電、光和催化並存的太陽能水分解設計

Energy crisis is a broad and complex global topic. Natural resources such as gas and oil are in limited in supply. Development towards renewable resources is one of the most important technologies in the world. Currently, photocatalytic and photoelectrochemical (PEC) water splitting devices under the irradiation of sunlight have received much attention for the production of renewable hydrogen from water. Solar energy conversion and storage through photoelectrolysis of water using semiconductors as both light absorber and energy converter to store solar energy in simple chemical bond, H2, become highly desirable approaches to solving the energy shortage challenge.
We focus on the effort to develop an efficient Si-based PEC water splitting device. We introduce the surface textured Si heterojunction PEC cell consisting of ultrathin amorphous Si/crystalline Si as efficient and robust photoelectrodes. The solar to hydrogen conversion efficiency has been improved to 13.26%, which is the highest ever reported for Si-based photocathodes. Later on, we design the cascading energy band structure in Si via doping for facilitating carrier separation and novel electrode structures for 360° light harvesting for hydrogen generation with ultrahigh current densities of 61.2 mAcm-2. The cells have been further demonstrated with excellent hydrogen production rate. In addition, our method can significantly improve the stability of Si-based solar cells in water to sustain up to 300 hr. These multifunctional designs provide the potential for the future development in the renewable energy market.

Templated guided synthesis of nonprecious metal, single atom electrocatlyst

非貴金屬單原子催化劑的模板導向合成

We synthesize a highly efficient electrocatalyst with single Fe atoms anchored by N-doped short-range ordered carbon loading on 2 D reduced graphene oxide (RGO). Unlike the highly graphitized carbon materials in previous ORR catalysts, in which the diffusion of oxygen molecules (~3.46 Å) are blocked by long carbon chains and small interlayer spacing (~3.4 Å), it is found that the Fe/N-doped nanographene possesses large interlayer spacing (> 4 Å) and short carbon fragments in one layer. The unique nanographene structure in nanoscale can facilitate the transport of oxygen molecules to the active sites of atomically dispersed FeN4 and FeN5. In acidic media for ORR, as-prepared Fe1-N-NG/RGO catalyst exhibited half-wave potential (E1/2) of 0.84 V versus the reversible hydrogen electrode, and the loss of E1/2 is less than 5 mV during 15,000 potential cycles.

Photonic Metasurface and Applications

光學超構表面及應用

By designing an ultrathin metasurface, which consists of spatially variant plasmonic structures with engineered geometric Berry phase, it is shown that spin-orbit coupling of light can be utilized to manipulate both the spin and orbital angular momentum of light. It was demonstrated that the spin dependent metasurface can be applied to design highly efficient optical holograms, which have important applications in the areas including holographic displays, beam shaping, data storage, optical trapping, optical tweezers and so on.
In nonlinear optical regime, we applied the concept of nonlinear geometrical Berry phase for designing nonlinear photonic metasurfaces. For example, in a second- and third- harmonic generation processes, the plasmonic meta-atoms, with certain rotational symmetries, can acquire a nonlinear geometric Berry phase. This nonlinear phase can be continuously tuned by from zero to 2π by simply varying theta. Several interesting applications such as nonlinear image encryption, nonlinear spin-orbit interaction and so on will be discussed.

Constructing Luminescent Materials by Lanthanide Doping

鑭系摻雜制備發光材料

Lanthanides are a family of elements that feature partially filled 4f orbitals. Electronic transition within the 4f configuration can result in a wealth of absorption and emission spectra across the full spectrum from ultraviolet to infrared. As such, lanthanide ions are frequently used to construct luminescent materials with high designability and tunability. In this talk, I focus on our recent efforts on the incorporation of lanthanide ions into various host materials with different compositions and structures. We demonstrate rational control over luminescence processes and properties by using different combinations of host/dopant, coupled with precise control of dopant concentration and spatial distribution of dopants in the host. The lanthanide doping renders highly stable emission with tunable wavelength, sharp bandwidth, and long excited-state lifetime by various modes of excitations including ultraviolet and near-infrared light, mechanical action, and electric field. We show that the lanthanide-doped luminescent materials hold promise in many exciting technological applications such as short-wavelength lasing, anti-counterfeiting, as well as biomedical imaging and therapy.

Near Infrared Pt(II) Phosphors and Associated OLEDs

近紅外磷光材料與有機二極體器件

Near-infrared organic light-emitting diodes (NIR-OLEDs) enables many unique applications ranging from invisible displays, oximeter and photoplethysmography (PPG). However, development of NIR-OLEDs with both high efficiency and low efficiency roll-off remains highly challenging. Here, a series of new heteroleptic Pt(II) complexes (1 – 4) flanked by functional pyridyl pyrimidinate and azolate chelates were synthesized. The much reduced ππ* energy gap of the pyridyl pyrimidinate chelate, and strong intermolecular interaction and high crystallinity in vacuum deposited thin films endowed extensive metal-metal-to-ligand charge transfer (MMLCT) interaction; thereby, giving highly efficient NIR emission within the range 776 – 832 nm and beyond, together with shortened radiative lifetimes (0.52 – 0.79 µs). Consequently, non-doped NIR emitting OLEDs based on these Pt(II) complexes are fabricated, to which Pt(II) complexes 2 and 4 give record high maximum external quantum efficiency (EQE) of 10.61% at 794 nm and 9.58% at 803 nm, respectively. Low efficiency roll-off was also observed, among which the device efficiencies of 2 and 4 are at least four times higher than that of the best NIR-emitting OLEDs recorded at current density of 100 mA cm-2, in comparison to those documented in literature. Finally, a new result showing NIR emission peak max. over 950 nm and EQE exceeding 3-4 % will be presented.

Design of Perovskite Nanostructures for High-Performance Electronics and Optoelectronics

高性能電子和光電電子的鈣鈦礦納米結構設計

Due to the advent of nanotechnology, materials can be readily fabricated into nanoscale configurations with different dimensionalities and widely tunable properties for technological applications. In this presentation, we will summarize and discuss the recent progress in our research group, emphasizing the design and development of various perovskite nanostructures, spanning from one-dimensional all-inorganic perovskite nanowires, quasi-two-dimensional perovskite layers, all the way to perovskite microplates, etc. For example, we developed a direct vapor-liquid-solid growth for single-crystalline all-inorganic lead halide perovskite (i.e., CsPbX3; X = Cl, Br, or I) nanowires (NWs). These NWs exhibited high-performance photodetection with the responsivity exceeding 4489 A/W and detectivity over 7.9 × 1012 Jones toward the visible light regime. Field-effect transistors based on individual CsPbX3 NWs were also fabricated, where they showed the impressive carrier mobility, being higher than other all-inorganic perovskite devices. All these results provide important guidelines for the further improvement of these perovskite nanostructures for practical utilization. 

Novel photocathode materials for high brightness electron source

用於高亮度電子源的新型光電陰極材料

Photocathode materials based on Einstein’s famous photoelectric theory links the two most important natural resources in our world: light and materials together.  When the materials are struck by a quantum photon with enough energy, electrons will be emitted. The photocathode materials are the key element in opto-electronic devices, sensors, electron sources, etc. In my presentation, I will discuss novel photocathode materials and new mechanism to create high brightness electron sources.

Light-Emitting Devices with Perovskite Nanocrystals

鈣鈦礦納米晶發光器件

High emission quantum yield, easily tuned emission colors, and high color purity of chemically synthesized lead halide perovskite nanocrystals make this class of materials particular attractive for applications in light-emitting devices [1,2]. I will introduce some of our recent synthetic strategies leading to highly luminescent perovskite nanorods with a strong polarized emission [3-5]. I will then provide some examples of use of perovskite nanocrystals in efficient charge injection LEDs [6,7], and for temperature imaging in microfluidics [8].

Hydrophobic Interaction: A Promising Driving Force for the Biomedical Applications of Nucleic Acids

基於疏水相互作用的核酸納米藥物

The comprehensive understanding and proper use of supramolecular interactions have become critical for the development of functional materials, and so is the biomedical application of nucleic acids (NAs). Relatively rare attention has been paid to hydrophobic interaction compared with hydrogen bonding and electrostatic interaction of NAs. However, hydrophobic interaction shows some unique properties, such as highly tuneable for application interest, minimal effect to NA functionality, and very sensitive to external stimuli. Therefore, the widespread use of hydrophobic interaction has promoted the evolution of NA-based biomaterials in higher-order self-assembly, drug/gene-delivery system, and stimuli-responsive system. Herein, we will summarize our recent progress of NA-based biomaterials whose fabrications or properties are highly determined by hydrophobic interactions.

Micro/Nano-fabrication and applications in microfluidics

微納加工在微流體上的應用

Our group at SUSTech engages in the development of micro/nano-fabrication techniques, and their applications in nanophotonics and bioMEMS. In this talk, I will provide a brief overview of our research interests, and then focus on our recent projects in microfluidic devices. First, we discuss the development of a novel all-purpose programmable and scalable digital microfluidic platform and its peripherals. The instrument can potentially serve as the fluid central processing unit (F-CPU) for automating a wide range of bio/chem-analytical and synthetic tasks. Second, we present a large-scale single cell trapping and pairing microfluidic device, which can potentially be used for cell fusion and for studying cell-cell interactions.

Solution-based comproportionation reaction for facile synthesis of black TiO2

TiO2溶液法歸中反應快速合成黑色二氧化鈦

Titanium dioxide (TiO2) is an intensively studied photoanode material for energy- and environmental-related applications. Nevertheless, the wide bandgap of TiO2 is only capable of absorbing the UV light which accounts for only ~ 5% of the solar energy, greatly limiting its photocatalytic efficiencies. To tackle this problem, black TiO2 capable of harnessing the UV and visible light,is particularly desirable. The current methods for producing black TiO2 generally involves complex fabrication process, expensive facilities, high temperature, vacuum, or high pressures. Here we report a convenient and efficient strategy for blackening TiO2 through an easy solution-soaking treatment at mild conditions (e.g., 60 oC) in polyol (e.g., glycerol, ethylene glycol) solutions. The F- ions, acidic polyol solvents, and contact between the TiO2 and Ti components are discovered to be the prerequisites for the water-promoted comproportionation between TiO2 and Ti, which generate Ti (III) species that effectively blacken different TiO2 nanomaterials--including the anodic nanotube arrays and P25 nanoparticles. The black TiO2 thus obtained from this convenient soaking method displays improved photoelectrochemical performance.