Differences and Similarities of Photocatalysis and Electrocatalysis in Two-Dimensional Nanomaterials:Strategies,Traps,Applications and Challenges

Photocatalysis and electrocatalysis have been essential parts of electrochemical processes for over half a century.Recent progress in the controllable synthesis of 2D nanomaterials has exhibited enhanced catalytic performance compared to bulk materials.This has led to significant interest in the exploitation of 2D nanomaterials for catalysis.There have been a variety of excellent reviews on 2D nanomaterials for catalysis,but related issues of differences and similarities between photocatalysis and electrocatalysis in 2D nanomaterials are still vacant.Here,we provide a comprehensive overview on the differences and similarities of photocatalysis and electrocatalysis in the latest 2D nanomaterials.Strategies and traps for performance enhancement of 2D nanocatalysts are highlighted,which point out the differences and similarities of series issues for photocatalysis and electrocatalysis.In addition,2D nanocatalysts and their catalytic applications are discussed.Finally,opportunities,challenges and development directions for 2D nanocatalysts are described.The intention of this review is to inspire and direct interest in this research realm for the creation of future 2D nanomaterials for photocatalysis and electrocatalysis.

2D metamaterials coherently steer nonlinear valley photons of 2D semiconductor

Monolayer transition-metal dichacolgenides (TMDCs) present a direct optical bandgap at the Brillouin zones, socalled valleys. Those energetically degenerate valleys (K and K’) present different valley pseudospins, emitting the valley photons with opposite spin angular momentums due to nonlinear optical selection rules. Furthermore, although atomically thin, two-dimensional (2D) TMDCs have giant nonlinearity, which can be enhanced by the valley-excitons.

Growth and characterization ofβ-Ga_(2)O_(3)thin films grown on off-angled Al_(2)O_(3)substrates by metal-organic chemical vapor deposition

Beta-gallium oxide(β-Ga_(2)O_(3))thin films were deposited on c-plane(0001)sapphire substrates with different mis-cut angles along<>by metal-organic chemical vapor deposition(MOCVD).The structural properties and surface morphology of as-grownβ-Ga_(2)O_(3)thin films were investigated in detail.It was found that by using thin buffer layer and mis-cut substrate technology,the full width at half maximum(FWHM)of the()diffraction peak of theβ-Ga_(2)O_(3)film is decreased from 2°on c-plane(0001)Al_(2)O_(3)substrate to 0.64°on an 8°off-angled c-plane(0001)Al_(2)O_(3)substrate.The surface root-mean-square(RMS)roughness can also be improved greatly and the value is 1.27 nm for 8°off-angled c-plane(0001)Al_(2)O_(3)substrate.Room temper-ature photoluminescence(PL)was observed,which was attributed to the self-trapped excitons formed by oxygen and gallium vacancies in the film.The ultraviolet-blue PL intensity related with oxygen and gallium vacancies is decreased with the increas-ing mis-cut angle,which is in agreement with the improved crystal quality measured by high resolution X-ray diffraction(HR-XRD).The present results provide a route for growing high qualityβ-Ga_(2)O_(3)film on Al_(2)O_(3)substrate.

Organic electro-optic polymer materials and organic-based hybrid electro-optic modulators

High performance electro-optic modulator,as the key device of integrated ultra-wideband optical systems,have be-come the focus of research.Meanwhile,the organic-based hybrid electro-optic modulators,which make full use of the advant-ages of organic electro-optic(OEO)materials(e.g.high electro-optic coefficient,fast response speed,high bandwidth,easy pro-cessing/integration and low cost)have attracted considerable attention.In this paper,we introduce a series of high-perform-ance OEO materials that exhibit good properties in electro-optic activity and thermal stability.In addition,the recent progress of organic-based hybrid electro-optic devices is reviewed,including photonic crystal-organic hybrid(PCOH),silicon-organic hy-brid(SOH)and plasmonic-organic hybrid(POH)modulators.A high-performance integrated optical platform based on OEO ma-terials is a promising solution for growing high speeds and low power consumption in compact sizes.

A new 3-dB bandwidth record of Ge photodiode on Si

Si photonics is a promising technological approach to realize a photonic integrated circuits on Si substrate with small footprint,high performance,low cost,and being highly compatible with Si complementary metal oxide semiconductor(CMOS)technology[1].Because of good compatibility of Si and the relatively high absorption coefficient in the near-infrared region,Ge waveguide photodiode on Si is almost the only option for optical receiving in Si photonic integrated circuits.For a high performance Ge photodiode,the critical parameters are optical responsivity,3-dB bandwidth,and dark current.

Brightness and Lifetime Improved Light-Emitting Diodes from Sr-Doped Quasi-Two-Dimensional Perovskite Layers

Green Perovskite Light-Emitting Diodes(PeLEDs)have attracted wide attention for full spectrum displays.However,the inferior film morphology and luminescence property of quasi-two-dimensional(quasi-2D)perovskite layers limit the photoelectric property of the PeLEDs.In this paper,the effect of strontium(Sr)doped in quasi-2D perovskite layers is investigated to obtain a high-quality active layer.The morphologies and optical properties of Sr-doped quasi-2D perovskite films with different concentrations are studied.With the addition of strontium,more low-dimensional-layer perovskite phases(n D 2 and n D 3)appear in quasi-2D perovskite films,providing efficient intraband carrier funneling pathway and facilitating radiative recombination.The photoluminescence(PL)peak intensity of optimized Sr-doped quasi-2D perovskite layers increases 50%compared with the non-strontium counterpart.Moreover,green PeLEDs based on a Sr-doped quasi-2D perovskite layer reach a maximum luminance(Lmax)of 2943.77 cd/m^(2),which is three times of the control device.The electroluminescence(EL)peaks of Maximum External Quantum Efficiency(MEQE)and Lmax of Sr-doped PeLEDs exhibite a slight shift,indicating the excellent stability and performance of Sr-doped devices.The optimized device can continuously operate for 360 s at MEQE driving voltage,resulting in a half-lifetime of60 s,which is 3-fold greater than that of the control PeLEDs.

Remembering John Bardeen: inventor of transistor

Transistor's invention revolutionized global society by spawning electronics industry. John Bardeen is among one of the inventors of transistor. He was a genius and one of the most influential semiconductor Physicist of 20 th century who won two Nobel prizes in Physics.

SEMICONDUCTOR LASERS Supersymmetric laser arrays

Supersymmetry (SUSY) emerged within particle physics as a means to relate two fun dame ntally different classes of elementary particles: bosons (integer spin, Bose-Einstein statistics) and fermi on sfhalfintegerspin, Fermi-Dirac statistics). Exte nsions to the Standard Model have also been proposed based on SUSY theory in order to resolve Iong-standing issues in quantum field theory, including the nature of vacuum energy, origin of mass scales and dark matter. Even though the experimental validation of SUSY is still an ongoing issue, supersymmetric/ isospectral techniques have already found applications in low-energy physics, nonrelativistic quantum mechanics, and nonlinear dynamics, to name a few. On the other hand, waveguide laser arrays have been a subject of intense investigations for the purpose of building high-power phaselocked lasers, which are immune to the detrimental effects of nonlinearities orfilamentation. Nevertheless, such systems suffer from multimode operation, which in turn leads to a chaotic emission.

Preface to the Special Topic on Compound Semiconductor Materials and Devices on Si

The research in silicon photonics has been booming due to its potential for lowcost,reliable,energy-efficient and high-density chip-wise integration using widely available CMOS technology,featuring the tremendous success in modulator,detector and other passive waveguide components in industry.However,the absence of efficient and reliable electrical to optical converter on Si platform has been considered as“the last piece of the puzzle”,hindered by the in-direct bandgap property of Si bulk materials.CompoundⅢ–Ⅴsemiconductor devices offer highly efficient optical light emitting sources and optical amplifiers,hence the compound semiconductor materials and devices on Si platform are drawing more and more attention nowadays as it could make possible the long-dreamed light sources on Si substrates by combining their advantages with silicon ICs,enabling the fabrication of full functional optoelectronic circuits,chip-to-chip and even system-to-system optical chips.

Ammonia sensing using arrays of silicon nanowires and graphene

Ammonia （NH3） is a toxic gas released in different industrial, agricultural and natural processes. It is also a biomarker for some diseases. These require NH3 sensors for health and safety reasons. To boost the sensitiv- ity of solid-state sensors, the effective sensing area should be increased. Two methods are explored and compared using an evaporating pool of 0.5 mL NH4OH （28% NH3）. In the first method an array of Si nanowires （Si NWA） is obtained via metal-assisted-electrochemical etching to increase the effective surface area. In the second method CVD graphene is suspended on top of the Si nanowires to act as a sensing layer. Both the effective surface area as well as the density of surface traps influences the amplitude of the response. The effective surface area of Si NWAs is 100 × larger than that of suspended graphene for the same top surface area, leading to a larger response in amp- litude by a factor of -7 notwithstanding a higher trap density in suspended graphene. The use of Si NWAs in- creases the response rate for both Si NWAs as well as the suspended graphene due to more effective NH3 diffu- sion processes.

High gain-bandwidth product Ge/Si tunneling avalanche photodiode with high-frequency tunneling effect

This study presents a theoretical investigation of a novel Ge/Si tunneling avalanche photodiode（TAPD）with an ultra-thin barrier layer between the absorption and p＋ contact layer. A high-frequency tunneling effect is introduced into the structure of the barrier layer to increase the high-frequency response when frequency is larger than 0.1 GHz, and the-3 dB bandwidth of the device increases evidently. The results demonstrate that the avalanche gain and-3 dB bandwidth of the TAPD can be influenced by the thickness and bandgap of the barrier layer.When the barrier thickness is 2 nm and the bandgap is 4.5 eV, the avalanche gain loss is negligible and the gainbandwidth product of the TAPD is 286 GHz, which is 18% higher than that of an avalanche photodiode without a barrier layer. The total noise in the TAPD was an order of magnitude smaller than that in APD without barrier layer.

Light-induced tumor theranostics based on chemical-exfoliated borophene

Among 2D materials(Xenes)which are at the forefront of research activities,borophene,is an exciting new entry due to its uniquely varied optical,electronic,and chemical properties in many polymorphic forms with widely varying band gaps including the lightest 2D metallic phase.In this paper,we used a simple selective chemical etching to prepare borophene with a strong near IR light-induced photothermal effect.The photothermal efficiency is similar to plasmonic Au nanoparticles,with the added benefit of borophene being degradable due to electron deficiency of boron.We introduce this selective chemical etching process to obtain ultrathin and large borophene nanosheets(thickness of ~4 nm and lateral size up to ~600 nm)from the precursor of AlB_(2).We also report first-time observation of a selective Acid etching behavior showing HCl etching of Al to form a residual B lattice,while HF selectively etches B to yield an Al lattice.We demonstrate that through surface modification with polydopamine(PDA),a biocompatible smart delivery nanoplatform of B@PDA can respond to a tumor environment,exhibiting an enhanced cellular uptake efficiency.We demonstrate that borophene can be more suitable for safe photothermal theranostic of thick tumor using deep penetrating near IR light compared to gold nanoparticles which are not degradable,thus posing longterm toxicity concerns.With about 40 kinds of borides,we hope that our work will open door to more discoveries of this top-down selective etching approach for generating borophene structures with rich unexplored thermal,electronic,and optical properties for many other technological applications.

Lead–halide perovskites for next-generation selfpowered photodetectors:a comprehensive review

Metal halide perovskites have aroused tremendous interest in optoelectronics due to their attractive properties,encouraging the development of high-performance devices for emerging application domains such as wearable electronics and the Internet of Things.Specifically,the development of high-performance perovskite-based photodetectors(PDs)as an ultimate substitute for conventional PDs made of inorganic semiconductors such as silicon,InGaAs,GaN,and germanium-based commercial PDs,attracts great attention by virtue of its solution processing,film deposition technique,and tunable optical properties.Importantly,perovskite PDs can also deliver high performance without an external power source;so-called self-powered perovskite photodetectors(SPPDs)have found eminent application in next-generation nanodevices operating independently,wirelessly,and remotely.Earlier research reports indicate that perovskite-based SPPDs have excellent photoresponsive behavior and wideband spectral response ranges.Despite the high-performance perovskite PDs,their commercialization is hindered by long-term material instability under ambient conditions.This review aims to provide a comprehensive compilation of the research results on self-powered,lead–halide perovskite PDs.In addition,a brief introduction is given to flexible SPPDs.Finally,we put forward some perspectives on the further development of perovskite-based self-powered PDs.We believe that this review can provide state-of-the-art current research on SPPDs and serve as a guide to improvising a path for enhancing the performance to meet the versatility of practical device applications.

Recent progress in GeSn growth and GeSn-based photonic devices

The GeSn binary alloy is a new group IV material that exhibits a direct bandgap when the Sn content ex- ceeds 6%. It shows great potential for laser use in optoelectronic integration circuits （OEIC） on account of its low light emission efficiency arising from the indirect bandgap characteristics of Si and Ge. The bandgap of GeSn can be tuned from 0.6 to 0 eV by varying the Sn content, thus making this alloy suitable for use in near-infrared and mid-infrared detectors. In this paper, the growth of the GeSn alloy is first reviewed. Subsequently, GeSn photode- tectors, light emitting diodes, and lasers are discussed. The GeSn alloy presents a promising pathway for the mono- lithic integration of Si photonic circuits by the complementary metal-oxide-semiconductor （CMOS） technology.

First-principles calculations of nitrogen-doped antimony triselenide： A prospective material for solar cells and infrared optoelectronic devices

This study is focused on calculation of the electronic structure and optical properties of non-metal doped Sb2Se3 using the first-principles method. One and two N atoms are introduced to Sb and Se sites in a Sb2Se3 crystal. When one and two N atoms are introduced into the Sb2Se3 lattice at Sb sites, the electronic structure shows that the doping significantly modifies the bandgap of Sb2Se3 from 1.11 eV to 0.787 and 0.685 eV, respectively. When N atoms are introduced to Se sites, the material shows a metallic behavior. The static dielectric constants el（0） for Sb16Se24, SblsN1Se24, Sb14N2Se24, Sb16Se23N1, and Sb16Se22N2 are 14.84, 15.54, 15.02, 18.9, and 39.29, respectively. The calculated values of the refractive index n（0） for Sb16Se24, SblsN1Se24, Sb14N2Se24, Sb16Se23N1, and Sb16Se22N2 are 3.83, 3.92, 3.86, 4.33, and 6.21, respectively. The optical absorbance and optical conductivity curves of the crystal for N-doping at Sb sites show a significant redshift towards the short-wave infrared spectral region as compared to N-doping at Se sites. The modulation of the static refractive index and static dielectric constant is mainly dependent on the doping level. The optical properties and bandgap narrowing effect suggest that the N-doped Sb2Se3is a promising new semiconductor and can be a replacement for GaSb due to its very similar bandgap and low cost.

Research progress of Ge on insulator grown by rapid melting growth

Ge is an attractive material for Si-based microelectronics and photonics due to its high carries mobility, pseudo direct bandgap structure, and the compatibility with complementary metal oxide semiconductor （CMOS） processes. Based on Ge, Ge on insulator （GOI） not only has these advantages, but also provides strong electronic and optical confinement. Recently, a novel technique to fabricate GOI by rapid melting growth （RMG） has been described. Here, we introduce the RMG technique and review recent efforts and progress in RMG. Firstly, we will introduce process steps of RMG. We will then review the researches which focus on characterizations of the GOI including growth dimension, growth mechanism, growth orientation, concentration distribution, and strain status. Finally, GOI based applications including high performance metal-oxide-semiconductor field effect transistors （MOSFETs） and photodetectors will be discussed. These results show that RMG is a promising technique for growth of high quality GOIs with different characterizations. The GOI grown by RMG is a potential material for the next-generation of integrated circuits and optoelectronic circuits.

Preface to the Special Issue on Si-Based Materials and Devices

It has been well known that the development of microelectronic and integrated circuit （IC）, mainly based on silicon materials, have changed the way of our life dramatically and accelerated the development and innovation of new technologies. With the increase of integration density in ICs, the gate lengths of transistors are now scaled down to 7 nm, leading to fundamental challenges to keep up with the Moore＇s law. One possible solution is to integrate optical circuits into the Si microelectronic platform to achieve high density electronic-photonic integration.