Anisotropic porous germanium nanostructures achieved with fast bipolar electrochemical etching and chemical etching

A method for producing a battery comprising:

• providing an anode comprising at least one monolithic nanostructured germanium substrate by:
• setting up nucleation etching to induce nucleation sites with a desired nucleation site spacing on a first side of a germanium substrate;
• etching said germanium substrate to produce porous germanium on said first side of said germanium substrate with pores extending into said first side at said nucleation sites with a fast bipolar electrochemical etching step comprising;
• submerging said germanium substrate in an etching electrolyte;
• applying etching pulses on said first side;
• and
• applying passivation pulses;
• wherein said etching pulses creates an intensified electrical current at a bottom of said pores with reduced current along sidewalls of said pores, said passivation pulses create a passivation layer on said sidewalls of said pores that protects said sidewalls of said pores from said etching electrolyte during said etching pulses, and said etching pulses causing said pores to assume tubular shape;
• and
• etching said porous germanium to produce columnar germanium with a chemical etching step comprising;
• controlling a concentration of a chemical etching solution comprising an acid and an oxidization agent;
• and
• controlling a dipping time of said porous germanium in said chemical etching solution,
• wherein said concentration and said dipping time are controlled to produce columnar germanium by enlarging said tubular pores to at least partly merge with neighboring ones of said pores to leave columns of germanium extending upwardly from said first side of said germanium substrate;
• providing a battery electrolyte;
• and
• providing a cathode with said cathode in electrical contact with said battery electrolyte and said battery electrolyte in contact with said anode.