Black GE based on crystalline/amorphous core/shell nanoneedle arrays
Direct growth of black Ge on low-temperature substrates, including plastics and rubber is reported. The material is based on highly dense, crystalline/amorphous core/shell Ge nanoneedle arrays with ultrasharp tips (˜4 nm) enabled by the Ni catalyzed vapor-solid-solid growth process. Ge nanoneedle arrays exhibit remarkable optical properties. Specifically, minimal optical reflectance (<1%) is observed, even for high angles of incidence (˜75°) and for relatively short nanoneedle lengths (˜1 μm). Furthermore, the material exhibits high optical absorption efficiency with an effective band gap of ˜1 eV. The reported black Ge can have important practical implications for efficient photovoltaic and photodetector applications on nonconventional substrates.
1. A method for forming a nanoneedle array of germanium on a substrate, the method comprising;
evaporating a thin film of nickel onto a substrate;
introducing GeH 4 gas into a CVD chamber into which the nickel coated substrate has been placed; and thereafter,
heating the chamber to between 270° C. and 320° C. for a period of time, while maintaining a pressure of 1.8-280 Torr during the heating step.
2. The method of claim 1 wherein the nickel film is grown to a thickness of 0.5 nm.
3. The method of claim 1 wherein said GeH 4 gas is introduced as a mixture of 10% GeH 4 in H 2 .
4. The method of claim 1 wherein the pressure is maintained at about 280 Torr.
5. The method of claim 1 wherein the substrate is glass.
6. The method of claim 1 wherein the substrate is a deformable substrate.
7. The method of claim 1 wherein the deformable substrate is selected from the group comprising flexible Kapton and stretchable rubber.
8. An array of germanium cone shaped nanoneedles formed on a substrate according to the process of claim 1 , wherein the tip diameter of the nanoneedles is between 4 to 13 nm.
9. The array of claim 8 wherein the tip diameter of the nanoneedles is approximately 4 nm.
10. The array of claim 8 wherein the nanoneedles are of near vertical orientation.
11. The array of claim 10 wherein the nanoneedles have a length of between 0.4 μm and 3.4 μm.
12. The array of claim 11 wherein the nanoneedles exhibit a gradual reduction of effective refractive index from the bottom to the top of the needle.
13. The array of claim 11 wherein the nanoneedles have a length of ≧1.0 μm.
14. The array of claim 8 wherein the substrate is a deformable substrate.
15. The array of claim 14 wherein the deformable substrate is selected from the group comprising Kapton and stretchable rubber.