Selective electrochemical etching of cantilever-type SOI-MEMS devices

It is possible to achieve selective electrochemical etching between different materials,such as p-and n-type silicon.However,achieving selective electrochemical etching on two different regions of the same p-type silicon material is a problem that has rarely been considered.Herein,a novel selective electrochemical etching technique for cantilever-type silicon-on-insulator(SOI)wafer-based microswitches is proposed.In this study,a p-type handle layer was selectively etched,and a p-type device layer was passivated.This was achieved using a circuit with two voltage sources:voltages of−1.2 and 0 V were applied to the handle and device layers,respectively.It was found that the proposed etching process can effectively prevent the in-use sticking of a cantilever-type switch.This is accomplished by increasing the gap between the device layer and its underlying handle layer and increasing the roughness of these layers.The technique is applicable to the fabrication of various cantilever-type SOI microelectromechanical systems,irrespective of the resistivity of the SOI wafer.

Green and scalable electrochemical routes for cost‐effective mass production of MXenes for supercapacitor electrodes

One of the most unique properties of two-dimensional carbides and nitrides of transition metals(MXenes)is their excellent water dispersibility and yet possessing superior electrical conductivity but their industrial-scale application is limited by their costly chemical synthesis methods.In this work,the niche feature of MXenes was capitalized in the packed-bed electrochemical reactor to produce MXenes at an unprecedented reaction rate and yield with minimal chemical waste.A simple NH4F solution was employed as the green electrolyte,which could be used repeatedly without any loss in its efficacy.Surprisingly,both fluoride and ammonium were found to play critical roles in the electrochemical etching,functionalization,and expansion of the layered parent materials(MAXs)through which the liberation of ammonia gas was observed.The electrochemically produced MXenes with excellent conductivity,applied as supercapacitor electrodes,could deliver an ultrahigh volumetric capacity(1408 F cm^(−3))and a volumetric energy density(75.8 Wh L^(−1)).This revolutionary green,energy-efficient,and scalable electrochemical route will not only pave the way for industrial-scale production of MXenes but also open up a myriad of versatile electrochemical modifications for improved functional MXenes.

Efficient nanostructured heterogeneous catalysts by electrochemical etching of partially crystallized Fe-based metallic glass ribbons

Although an increasing interest has been attracted to further develop heterostructured catalysts from metallic glasses(MGs) by heat treatment, overcoming surface oxidation effect is still a critical problem for such environmental catalysts. Herein, a short-time electrochemical etching of partially crystallized Febased ribbons in 0.3 M H3 PO4 electrolyte enables the formation of honeycomb-like nanoporous structure as effective catalytic active sites in Fenton-like process. Studies of structure and surface morphologies reveal that the formation of nanoporous structure by potentiostatic etching originates from electrochemical potential difference of nanocrystals(a-Fe(Si) and Fe2 B) and residual amorphous phase in partially crystallized ribbons, where Fe2 B having a lower open circuit potential tends to be selectively dissolved.Simultaneously, thin oxide layer after electrochemical etching exposes more active sites for H2 O2 activation and provides an effective protection of nanocrystals from massive loss during etching. Investigation of optimal processing conditions suggests that the selection of electrolyte plays an important role;dye degradation rates of etched ribbons in HNO3 and Na2 SO4 electrolytes can also achieve at least 2 times higher than that of as-annealed ribbons. This work holds the promise to develop novel environmental catalysts by effective electrochemical etching of partially crystallized ribbons.

Electrochemical liftoff of freestanding GaN by a thick highly conductive sacrificial layer grown by HVPE

Separation technology is an indispensable step in the preparation of freestanding GaN substrate. In this paper, a largearea freestanding GaN layer was separated from the substrate by an electrochemical liftoff process on a sandwich structure composed of an Fe-doped GaN substrate, a highly conductive Si-doped sacrificial layer and a top Fe-doped layer grown by hydride vapor phase epitaxy(HVPE). The large difference between the resistivity in the Si-doped layer and Fe-doped layer resulted in a sharp interface between the etched and unetched layer. It was found that the etching rate increased linearly with the applied voltage, while it continuously decreased with the electrochemical etching process as a result of the mass transport limitation. Flaky GaN pieces and nitrogen gas generated from the sacrificial layer by electrochemical etching were recognized as the main factors responsible for the blocking of the etching channel. Hence, a thick Si-doped layer grown by HVPE was used as the sacrificial layer to alleviate this problem. Moreover, high temperature and ultrasonic oscillation were also found to increase the etching rate. Based on the results above, we succeeded in the liftoff of ~ 1.5 inch GaN layer. This work could help reduce the cost of freestanding GaN substrate and identifies a new way for mass production.

Halogen chlorine triggered oxygen vacancy-rich Ni(OH)_(2) with enhanced reaction kinetics for pseudocapacitive energy storage

Two-dimensional (2D)Ni(OH)_(2) nanosheets can theoretically expose their active sites of 100%.Whereas,their intrinsic easy accumulation and low conductivity lead to weak and unsustainable reaction kinetics.Herein,we propose a novel halogen chlorine-triggered electrochemical etching strategy to controllably manage the reaction kinetics of 2D Ni(OH)_(2) nanosheets(EE/Cl-Ni(OH)_(2)).It is found that halogen chlorine doping can adjust the interlamellar spacing flexibly and promote the lattice oxygen activation to achieve controlled construction of superficial oxygen defects at the adjustable voltage.The optimal EE/Cl-Ni(OH)_(2) electrode exhibits a high rate capability and excellent specific capacity of 206.9 mA h g^(-1) at 1 A g^(-1) in a three-electrode system,which is more than twice as high as the pristine Ni(OH)_(2).Furthermore,EE/Cl-Ni(OH)_(2) cathode and FeOOH@rGO anode are employed for developing an aqueous Ni-Fe battery with an excellent energy density of 83 W h kg^(-1),a high power density of 17051 W kg^(-1),and robust durability over 20,000 cycles.This strategy exploits a fresh channel for the ingenious fabrication of highefficiency and stable nickel-based deficiency materials for energy storage.

An integrated strategy towards the facile synthesis of core-shell SiC-derived carbon@N-doped carbon for high-performance supercapacitors

Porous active core-shell carbon material with excellent synergistic effect has been regarded as a prospective material for supercapacitors.Herein,we report an integrated method for the facile synthesis of carbide-derived carbon(CDC)encapsulated with porous N-doped carbon(CDC@NC)towards highperformance supercapacitors.Polydopamine(PDA)as nitrogen and carbon sources was simply coated on SiC nanospheres to form SiC@PDA,which was then directly transformed into CDC@NC via a onestep molten salt electro-etching/in-situ doping process.The synthesized CDC@NC with hierarchically porous structure has a high specific surface area of 1191 m^(2) g^(-1).The CDC core and NC shell are typical amorphous carbon and more ordered N-doped carbon,respectively.Benefitting from its unique dual porous structures,the CDC@NC demonstrates high specific capacitances of 255 and 193 F g^(-1) at 0.5 and20 A g^(-1),respectively.The reaction mechanism of the electro-etching/in-situ doping process has also been investigated through experimental characterizations and theoretical density functional theory calculations.It is suggested that the molten salt electro-etching/in-situ doping strategy is promising for the synthesis of active core-shell porous carbon materials with synergistic properties for supercapacitors without the need for additional doping/activation processes.

Optical absorption and photoelectron collection properties of silicon wafers with conical quantum nanocrystals structure

A conical form of nano-sized quantum cluster was formed on the surface of p-type crystalline silicon [111] wafer by anode electrochemical etching in HF-based solution.The conical surface is highly effective in absorbing sunlight and transporting photoelectrons to semiconductor material.These are because each cone has a graded band gap with the energy level in the range from 1.1 to 3 eV which can be considered as consisting of quantum dots in different sizes.Since the boron concentration on the surface of each cone gradually decreases from top to bottom,a continuously varying electrical field is created along the cone height.This electric field is forcing photoelectrons generated in the cone to move rapidly to the direction perpendicular to wafer surface.Hence the drift time of photoelectrons can be less than their recombination time within the thin layer close to the bottom of the cone.

Colloidal silicon nanocrystallites for low-cost solar cell development

In this letter a formation of solution based of bulk-heterojunction based on freestanding silicon nanocrystals(Si-ncs) and conjugated((poly(3-hexylthiophene)(P3HT) polymer is demonstrated. Surfactant free Si-ncs prepared by low-cost electrochemical etching are applied for fabrication of bulk-heterojunction and photo-conductive blends. We show that the optimum blend performance is at 40 wt% nanocrystallites concentration within the P3 HT polymer matrix. Furthermore, we illustrate that solar cell transport properties can be improved by nanosecond laser fragmentation of the nanocrystallites micrograins in ethanol. It argues that the Si-ncs/polymer blend with refine nanocrystaline structure may impact the development of low-cost solar cells by environmental- friendly mean.

Cell osteogenic bioactivity mediated precisely by varying scaled micro-pits on ordered micro/nano hierarchical structures of titanium

Hierarchical surface structures with micro–nano scale play a crucial role in regulation of cell proliferation and osteogenic differentiation.It has been proven that cells are extremely sensitive to the nanoscaled structure and show multifarious phenotypes.Though a vital function of microstructure on osseointegration has been confirmed,the cell performances response to different microscaled structure is needed to be further dissected and in depth understood.In this work,the ordered micro–nano hierarchical structures with varying micro-scaled pits were precisely fabricated on titanium successfully by the combination of electrochemical,chemical etching and anodization as well.In vitro systematical assessments indicated that the micro–nano multilevel structures on titanium exhibited excellent cells adhesion and spreading ability,as well as steerable proliferation and osteogenic differentiation behaviors.It is shown that smaller micro-pits and lower roughness of the hierarchical structures enabled faster cell propagation.Despite cell growth was delayed on micro–nano titanium with relatively larger cell-match-size micro-pits and roughness,osteogenic-specific genes were significantly elevated.Furthermore,the alkaline phosphatase activity,collagen secretion and extracellular matrix mineralization of MC3T3-E1 on multiscaled titanium were suppressed by a large margin after adding IWP-2(an inhibitor of Wnt/b-catenin signal pathway),indicating this pathway played a crucial part in cell osteogenic differentiation modulated by micro–nano structures.