Stain etchants made from ͑HF + V 2 O 5 ͒ or ͑HF + FeCl 3 •6H 2 O + HCl or H 2 SO 4 ͒ exhibited virtually no initiation time before the formation of porous silicon. Etching with Fe͑III͒ solutions for extended periods resulted in a unique dual layer structure that can reach a thickness Ͼ10 m and exhibited not only red-orange but also green photoluminescence ͑PL͒. Etching with ͑CeF 4 + H 2 SO 4 ͒ produced extremely uniform films. Visible PL was observed immediately after etching except for those films produced with ͑CeF 4 + H 2 SO 4 ͒, which required several days of air exposure before bright and robust PL developed.

Stain etching of silicon in aqueous solutions composed of FeCl3 + HF + concentrated (HClO4 or H2SO4) leads to highly luminescent porous Si with a unique dual layer structure. The upper layer (∼3 μm thick) exhibits bluer luminescence (peaked at 560–590 nm) and a porosity in excess of 84%. The lower layer exhibits redder luminescence (peaked at 630–645 nm) and a lower porosity. Both layers are crystalline and are composed predominantly of uniform micropores. Because of the substantially higher porosity of the upper layer, this layer is highly susceptible to cracking and exfoliation. Critical point drying greatly reduces cracking and exfoliation but does not eliminate cracking completely. Brushing easily removes the upper layer while leaving the lower layer intact and the removed material remains photoluminescent. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Silicon substrates exposed to laser ablation in a chemically reactive environment such as SF 6 or HCl can experience spontaneous formation of conical pillars. We use these pillars as a template to define the dimensions and order of macropores produced by etching such substrates in aqueous solutions of KOH or tetramethylammonium hydroxide. The pillars anchor the sidewalls of the pores during etching, and the interpillar spacing controls the width of the pores. The macropores have crystallographically defined shapes for which we develop an explanation based on the kinetics of etching. Si͑001͒ macropores can be etched such that they are rectangular with straight walls and an inverted pyramidal bottom. They have been etched as through holes, which is of interest for optical applications. On Si͑111͒, there is a transition from hexagonal to triangular macropores that are all aligned in one direction. The Si͑111͒ pores exhibit an optimum etch time before they begin to disappear. The behavior of the macropores is quite similar regardless of whether the pillars are produced by nanosecond or femtosecond lasers. Differences observed relate to the different initial structures ͑spacing and regularity͒ of these two different types of pillar-covered surfaces.There is increasing awareness of a new variant of synthetic chemistry that strives not so much to create a desired molecule/ composition, but rather, that seeks to make a desired architecture. Impetus for this derives from the observation that, particularly on the nanoscale, the properties of materials change with the size of the sample. Thus, for instance, the substantial interest in nanowires/tubes, 1-3 porous solids, 4-6 and metallorganic framework compounds, [7][8][9] which are examples of nanomaterials created by etching and growth. With a particular interest in silicon, we have embarked on a series of studies aimed at elucidating the dynamics of and determining the versatility of structural transformations by means of ͑laser-assisted͒ chemical processes beginning with solid substrates. 10 Silicon undergoes radical changes as a result of being restructured. Etching to create nanocrystalline silicon leads to quantum confinement of both electrons and phonons. This in turn facilitates the observation of strong visible photoluminescence and even optical gain 11-15 from a material that is normally an abysmally weak emitter in the bulk phase, 16 unless it is, for example, specially pumped to produce a Raman laser. [17][18][19] When covered with pillars produced by chemically enhanced laser ablation, polished Si changes from a shiny grayish mirror into a black material with virtually no reflectivity even below the bandgap. 20,21 Pillars can be created by either nanosecond or femtosecond laser pulses. Here we denote pillars made with a nanosecond laser as nanosecond pillars; similarly, femtosecond pillars are those pillars produced with a femtosecond pulsed laser.Pillars form as the result of the combination of laser ablation, etching, and growth. 10 Although both nanosecond a...

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Stain etching of silicon in aqueous solutions composed of FeCl3 + HF + concentrated (HCl, HClO4 or H2SO4) leads to brilliantly luminescent porous Si with a unique dual layer structure. The upper layer (~2 µm or more thick) exhibits bluer luminescence (peaked at 520-590 nm). The lower layer exhibits redder luminescence (peaked at 630-660 nm). The peak PL wavelength and the PL intensity both depend strongly on the excitation wavelength. Photoluminescence excitation (PLE) spectroscopy indicates that the upper and lower layers are distinct because the PLE spectra of the two layers are different. Additionally, we demonstrate that two completely new stain etchants can produce visibly luminescent microporous silicon: V2O5 dissolved in HF(aq) and CeF4 dissolved in H2SO4(aq). In all cases bubble formation is suppressed and layer homogeneity greatly exceeds that obtained with nitrate based stain etchants.