Metal-enhanced HF etching of Si is an electroless method used to produce porous Si. Such etching generally uses not only metal-modified Si but also an oxidizing agent, such as hydrogen peroxide or metal ions. Pd exhibits high activity in enhancing the HF etching of Si without an oxidizing agent even under dissolved-oxygen-free and dark conditions. Electrolessly deposited Pd particles on n-type Si enhance the HF etching of Si but produce no porous layer. Patterned Pd films localize the etching under the boundary of the Pd deposited areas, and thus Pd can produce a microetch pattern on Si with a simple immersion in the HF solution. This etching reaction is explained by electron injection into the conduction band of Si due to the Pd-enhanced anodic oxidization of Si with water and the cathodic hydrogen evolution on Pd with the injected electrons.Porous silicon ͑Si͒ is usually prepared by electrochemical etching under an anodic bias in a fluoride-containing solution. 1-3 Metalenhanced HF etching of Si has attracted considerable attention as a new electroless method that can produce porous Si by immersing metal-modified Si in an HF solution without bias. 4-15 Such etching generally uses not only metal-modified Si but also an oxidizing agent, such as hydrogen peroxide, 5,10,12-14 peroxodisulfate, 8 permanganate, 8 or metal ions. 7 We reported the preparation process 6,9,11,15 of porous Si using metal-particle-enhanced HF etching of Si without a particular oxidizing agent as well as its application for solar cells 6,9,16 and metal nanorod formation. 17 This etching generally proceeds by a local galvanic cell mechanism requiring photoillumination or dissolved oxygen. Palladium ͑Pd͒ exhibits high activity in enhancing etching under dissolved-oxygen-free and dark conditions. 15,18 In this paper, we describe the unique behavior of Pd on HF etching of Si and micropattern formation. 19 ExperimentalSingle-crystalline n-type Si wafers ͓Yamanaka Semiconductor, CZ ͑100͒, ca. 1 ⍀ cm͔ were cut into pieces ͑1 ϫ 1 cm͒, washed with acetone, and etched with CP-4A ͑a mixture of HF, nitric acid, acetic acid, and water͒ and a 7.3 M ͑M = mol dm −3 ͒ HF solution. Metal particles were deposited on Si wafers by electroless displacement deposition from a 1 mM metal-salt ͑PdCl 2 or H 2 PtCl 6 ͒ solution containing 0.15 M HF. 20 Pd thin films were deposited onto n-Si wafers by the electron-beam evaporation method. The thicknesses of the deposited films were measured using a quartz crystal microbalance ͑ULVAC, CRTM-5000͒ adjusted by gravimetry. The Pd films were patterned by attaching a mask ͑85 m square holes and 40 m space͒ to the n-Si wafers during deposition. The metaldeposited n-Si wafers were immersed in a 7.3 M HF aqueous solution at 298 K under dark conditions. The etching time was 24 h unless otherwise specified. Argon gas ͑Ͼ99.9999% purity, Taiyo Nippon Sanso, grade-1͒ bubbling was applied to the HF solution before and during etching to remove the dissolved oxygen from the solution. The average etch rate of Si was measured using a gravi...

Aerobic, site-selective C(sp3)–H oxygenation using a novel N-oxyl radical directing activator (chemically reactive directing group) is described.

Theo rganocatalytic aerobic oxidation of electron-deficient a-fluoroalkyl alcohols at room temperature is described. Ther esulting fluoroalkyl ketones are versatiles ynthetic intermediates for av ariety of fluorine-containingm olecules.T his otherwise difficult transformation has now been accomplished by the reactionofa-fluoroalkylalcohols with N-oxyl radicals,c atalytically generated from 9-azabicyclo[3.3.1]nonan-3-one N-oxyl/nitrogeno xide (keto-ABNO/NO x )a nd oxygeni na cetic acid (AcOH), affording the corresponding fluoroalkyl ketones in high yield. This operationally simple reaction can be performed under mild conditions,a nd was applied to aw ide range of alcohols( 20 examples), thus demonstrating ah igh functional group tolerance.M oreover, am odified one-pot protocol based on this method was able to convert an aldehyde to atrifluoromethyl ketone on ag ram scale.

Metal-enhanced HF etching of Si is an electroless method to produce porous Si. Such etching generally uses not only metal-modified Si but also an oxidizing agent, such as hydrogen peroxide. Pd exhibits high activity for enhancing the HF etching of Si without an oxidizing agent, even under dissolved oxygen free and dark conditions. Electrolessly deposited Pd particles on n-Si enhance the HF etching of Si, but produce no porous layer. Patterned Pd films localize the etching under and the boundary of the Pd deposited areas, and thus Pd can produce a micro-etch pattern on Si with simple immersion in the HF solution. This etching reaction is explained by electron injection into the conduction band of Si due to Pd-enhanced anodic oxidization of Si with water.

Metal-assisted HF etching of Si has attracted considerable attention as a new electroless method that can produce porous Si by immersing metal-modified Si in a HF solution without bias. Such etching generally uses not only metal-modified Si but also an oxidizing agent. Palladium exhibits high activity in assisted etching under dissolved-oxygen-free and dark conditions. In this study, we investigate the Pd assisted HF etching of n-Si by electrochemical measurements. The potential of Pd metal on Si is more negative than the potential of hydrogen evolution at open circuit conditions. Anodic current generation of Pd-modified Si electrodes at positive bias and the localization of etching under Pd films at low thickness indicate that Pd catalyzes the anodic dissolution of Si and the cathodic hydrogen evolution.