Porous and non-porous nanostructures

A method of forming a porous nanowire or nanowire-like structure, comprising:

• obtaining an etchable semiconductor substrate;
• performing metal-assisted electroless etch comprising the steps of:
• (i) forming on a surface of the substrate a patterned porosification assisting metal layer that has at least one opening, wherein said opening has at least one lateral dimension of no more than 5 microns;
• and
• (ii) subsequently exposing the substrate to a first etching solution and a second etching solution to form respectively a first region and a second region of a porous nanowire or nanowire-like structure, wherein said exposing results in dissolving the etch assisting metal in at least one of the first and second etching solutions to form a metal containing etching solution,
• wherein the metal containing etching solution porosifies a portion of the substrate to form a first porous region having a cross-section determined by the opening, wherein the cross-section is substantially perpendicular to the axis, the cross-section has at least one lateral dimension of no more than 5 microns, and
• wherein said second region is adjacent to the first region along an axis of the nanowire or nanowire-like structure and has at least one porous property different from that of the first region,
• wherein said property is selected from the group consisting of a pore size, porosity and pore orientation.

A method of making a porous object comprising:

• (i) obtaining a silicon substrate having a resistance higher than 0.02 Ω·cm;
• (ii) depositing a porosification assisting metal on a surface of the substrate;
• and
• (iii) subsequently exposing the substrate to an etching solution,
• wherein the etching solution comprises HF and an oxidant,
• and wherein the HF and the oxidant are at concentration levels effective to form a porous object.

A method of making a nanowire or nanowire-like structure comprising:

• (i) obtaining a non-porous object that has a cross-section that is substantially perpendicular to an axis of the object,
• wherein said cross-section has at least one lateral dimension of no more than 5 microns, and
• wherein an aspect ratio of said object is at least 4;
• and
• (ii) forming a nanowire or nanowire-like structure from the non-porous object by metal-assisted electroless etch, wherein the forming step comprises:
• depositing a porosification assisting metal on a surface of the non-porous object;
• and
• exposing the non-porous object to an etching solution that dissolves the etch-assisting metal to form a metal containing solution that etches away a portion of the non-porous object to form a non-porous elongated structure,
• wherein the porosifying solution comprises:
• an effective concentration of HF,
• an oxidizer, and
• at least one of ions and atoms of a porosification assisting metal.

A method of making a multi-doped structure comprising:

• obtaining a semiconductor substrate that comprises a first doping region and a second doping region,
• wherein the second doping region is different from the first doping region in at least one of a dopant type and a dopant concentration;
• and
• performing metal-assisted electroless etch comprising the steps of:
• (i) depositing an etching assisting metal on a surface of the substrate and exposing the substrate to an etching solution, wherein the etching solution dissolves the etching assisting metal to form a metal containing etching solution, wherein the metal containing etching solution etches away a portion of the substrate to form a multi-doped structure that has a cross-section that is substantially perpendicular to an axis of the structure, wherein said cross-section has at least one lateral dimension of no more than 5 microns and an aspect ratio of at least 4, and wherein said structure comprises a first doping region formed from the first doping region of the substrate and a second doping region formed from the second doping region of the substrate.