Compositions comprising epitaxial nanowires on graphene substrates and methods of making thereof

Abstract

A composition of matter comprising at least one nanowire on a graphitic substrate, said at least one nanowire having been grown epitaxially on said substrate, wherein said nanowire comprises at least one group III-V compound or at least one group II-VI compound or comprises at least one non carbon group (IV) element.

Claims

1. A composition of matter comprising at least one epitaxial nanowire on a graphene substrate having a thickness of 20 nm or less, wherein said at least one epitaxial nanowire comprises AlN, GaN, or Al(In)GaN.

2. The composition as claimed in claim 1, wherein said at least one epitaxial nanowire grows in the [111] or [0001] direction.

3. The composition as claimed in claim 1, wherein said at least one epitaxial nanowire comprises AlGaN.

4. The composition as claimed in claim 1, wherein said graphene substrate comprises 10 or fewer layers.

5. The composition as claimed in claim 1, wherein said graphene substrate is a CVD-grown graphene.

6. The composition as claimed in claim 1, wherein a surface of said graphene substrate is modified with a plasma treatment.

7. The composition as claimed in claim 1, wherein a surface of said graphene substrate is modified with a plasma treatment with a gas of oxygen, hydrogen, NO.sub.2, or their combinations.

8. The composition as claimed in claim 1, wherein a surface of said graphene substrate is modified by chemical doping using a solution of FeCl.sub.3, AuCl.sub.3 or GaCl.sub.3.

9. The composition as claimed in claim 1, wherein said at least one epitaxial nanowire has a diameter of no more than 200 nm and has a length of up to 5 microns.

10. The composition as claimed in claim 1, wherein the composition comprises a plurality of said epitaxial nanowires wherein said plurality of said epitaxial nanowires are substantially parallel to each other.

11. The composition as claimed in claim 1, wherein said at least one epitaxial nanowire is grown in the presence of a catalyst.

12. A process for preparing the composition as claimed in claim 1, the process comprising the steps of: (I) providing Al and/or Ga, N, and optionally In to a surface of said graphene substrate; and (II) epitaxially growing at least one epitaxial nanowire from the surface of said graphene substrate.

13. The process as claimed in claim 12, wherein a catalyst is deposited on the substrate.

14. The process as claimed in claim 13, wherein said catalyst is Au or the metal of said at least one nanowire to be grown.

15. The process of claim 13, wherein said catalyst is Ga.

16. The process as claimed in claim 12, wherein said graphene substrate is coated with a hole-patterned mask.

17. The process as claimed in claim 16, wherein the hole-patterned mask comprises SiO.sub.2 or Si.sub.3N.sub.4.

18. The process as claimed in claim 16, wherein the surface of the graphene substrate exposed through the hole pattern is modified with a plasma treatment.

19. The process of claim 16, wherein the hole patterned mask is deposited by e-beam evaporation, CVD, PE-CVD, or sputtering.

20. The process of claim 16, wherein a surface of the graphene substrate exposed through the hole pattern of the hole-patterned mask is modified with plasma treatment with a gas of oxygen, hydrogen, NO.sub.2, or their combinations.

21. The process of claim 12, wherein the Al and/or Ga, N, and optionally In are provided to the surface of said graphene substrate via a molecular beam or by using metal organic CVD.

22. An optical, optoelectronic, or electronic device comprising the composition as claimed in claim 1.

23. The optical, optoelectronic, or electronic device of claim 22, wherein the optical, optoelectronic, or electronic device comprises a solar cell or a light emitting diode.

24. The composition as claimed in claim 1, wherein said at least one epitaxial nanowire is grown without the presence of a catalyst.

25. The composition as claimed in claim 1, wherein said at least one epitaxial nanowire is doped.

26. The composition as claimed in claim 1, wherein the graphene substrate is coated with a hole-patterned mask through which the at least one epitaxial nanowire was grown.

27. The composition as claimed in claim 1, wherein a surface of the graphene substrate is modified by chemical doping.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1a shows the hexagonal positions of the carbon atoms (gray circles) of the graphitic substrate and the hexagonal positions of the semiconductor atoms (yellow circles) in the (111) and (0001) plane of a cubic or hexagonal crystal structure, respectively. The spacing between the semiconductor atoms (4.266 =31.422 (carbon atom distance)) needed in order to achieve exact lattice match with the graphitic substrate is depicted. In this example the semiconductor atoms are placed above some particular hollow (H-site) centres of the hexagonal carbon rings. Instead of being placed on top of H-sites, all the semiconductor atoms may also be rigidly shifted so that they are above a bridge (B-site) between carbon atoms or all centred above top (T-site) of carbon atoms in a way that a hexagonal symmetric pattern is still maintained.

(2) FIG. 1b shows the positions of the semiconductor atoms in the (111) and (0001) plane of a cubic or hexagonal crystal structure, respectively, on top of H- and B-sites of the carbon atoms of the graphene surface. The spacing between the semiconductor atoms (3.694 =3/2sqr(3)1.422 (carbon atom distance)) needed in order to achieve exact lattice match with the graphitic substrate is depicted.

(3) FIG. 1c shows the positions of the semiconductor atoms in the (111) and (0001) plane of a cubic or hexagonal crystal structure, respectively, on top of H- and T-sites of the carbon atoms of the graphene surface. The spacing between the semiconductor atoms (2.844 =21.422 (carbon atom distance)) needed in order to achieve exact lattice match with the graphitic substrate is depicted.

(4) FIG. 2 shows a MBE experimental set up.

(5) FIG. 3a is an idealised depiction of Ga (self) catalysed GaAs nanowires grown on graphite.

(6) FIG. 3b is a 45.sup.0 tilted view SEM image of two vertical Ga assisted GaAs nanowires grown by MBE on a flake of Kish graphite. The spherical particles are Ga droplets.

(7) FIG. 3c is a cross sectional TEM image of the graphite/nanowire interface of a vertical Ga-assisted GaAs nanowire grown epitaxially on top of Kish graphite.

(8) FIG. 4 shows a depiction of a mask on the graphite surface, which has been etched with holes.

(9) The invention will now be described with reference to the following non limiting examples.

Example 1

Experimental Procedure

(10) Nanowires (NWs) were grown in a Varian Gen II Modular molecular beam epitaxy (MBE) system equipped with a Ga dual filament cell, an In SUMO dual filament cell, and an As valved cracker cell, allowing to fix the proportion of dimers and tetramers. In the present study, the major species of arsenic were As.sub.4. Growth of NWs is performed either on a Kish graphite flake or on a graphene film (1 to 7 monolayers thick) grown by a chemical vapor deposition (CVD) technique directly on a Ni film deposited on an oxidized silicon wafer. The CVD graphene films were bought from Graphene Supermarket, USA. The samples were prepared using two different procedures. In the first procedure, the samples were cleaned by iso-propanol followed by a blow dry with nitrogen, and then In-bonded to the silicon wafer. In the second procedure, a 30 nm thick SiO.sub.2 layer was deposited in an e-beam evaporator chamber on the samples prepared using the first procedure where after holes of 100 nm in diameter were fabricated in the SiO.sub.2 using e-beam lithography and plasma etching.

(11) The samples were then loaded into the MBE system for the NW growth. The Ga/In flux was first supplied to the surface during a time interval typically in the range 5 s to 10 minutes, dependent on Ga/In flux and desired droplet size, while the As shutter was closed, to initiate the formation of Ga/In droplets on the surface. The substrate temperature was increased to a temperature suitable for GaAs/InAs NW growth: i.e. 610 C./450 C., respectively. GaAs/InAs NW growth was initiated by simultaneously opening the shutter of the Ga/In effusion cell and the shutter and valve of the As effusion cell. The temperature of the Ga/In effusion cell was preset to yield a nominal planar growth rate of 0.1 m per hour. To form the GaAs NWs, an As.sub.4 flux of 1.110.sup.6 Torr is used, whereas the As.sub.4 flux is set to 410.sup.6 Torr to form InAs NWs.