Diamond-like carbon nanorods and fabrication thereof

Abstract

Novel sp.sup.3 rich diamond-like carbon (DLC) nanorod films were fabricated by hot filament chemical vapor deposition technique. The results are indicative of a bottom-up self-assembly synthesis process, which results in a hierarchical structure that consists of microscale papillae comprising numerous nanorods. The papillae have diameters ranging from 2 to 4 m and the nanorods have diameters in the 35-45 nm range. A growth mechanism based on the vapor liquid-solid mechanism is proposed that accounts for the morphological aspects in the micro- and nano-scales.

Claims

1. A method of fabricating diamond-like carbon nanorods comprising: exposing a catalyst-coated substrate to a mixture of gases comprising at least: a carbon-containing gas, hydrogen gas and a sulfur-containing gas species, in the presence of an energy source that breaks the gaseous molecules into radicals effectively fabricating diamond-like carbon nanorods on said catalyst-coated substrate, wherein the ratio of the carbon-containing gas and hydrogen is about 0.1-3.0:100.

2. The method of claim 1, wherein said catalyst-coated substrate is exposed to said mixtures of gases at a substrate temperature of from about 750 C. to about 1000 C.

3. The method of claim 1, wherein said catalyst-coated substrate is exposed to said mixtures of gases for a period of from about 30 min to about 90 min.

4. The method of claim 1, wherein said carbon-containing gas comprises at least one of: methane, acetylene, and alcohol.

5. The method of claim 4, wherein said alcohol is selected from C1-C6 alcohol.

6. The method of claim 1, wherein the substrate material comprises one of: Si, quartz, alumina and Cu.

7. The method of claim 1, wherein the catalyst is coated on the substrate by one of: chemical vapor deposition, physical vapor deposition, ion-beam deposition, sputtering, e-beam evaporation, vacuum deposition, spin coating, and dipping.

8. The method of claim 1, wherein the catalyst comprises iron.

9. The method of claim 1, wherein the catalyst comprises a layer of more than 0 nm and less than 100 nm.

10. The method of claim 1, wherein the sulphur-containing gas species comprises at least one of: hydrogen sulfide or carbon disulphide.

11. The method of claim 1, wherein the concentration of sulfur-containing gas species in the gas phase is in the range of from about 100 ppm to about 1000 ppm.

12. The method of claim 1, wherein the total pressure of the gases is in the range of from about 10 Torr to about 100 Torr.

13. The method of claim 1, wherein a nanorod of diamond-like carbon fabricated by said method has a longer axis in the range of from about 5 nm to about 500 nm and a shorter axis in the range of from about 0.5 nm to about 50 nm.

14. A method of fabricating diamond-like carbon nanorods comprising: forming a patterned nanotemplate coating of catalyst material on a substrate; and exposing said catalyst-coated substrate to a mixture of at least: a carbon-containing gas, hydrogen gas and a sulfur-containing gas species in the presence of an energy source that breaks the gaseous molecules into radicals effectively fabricating diamond-like carbon nanorods on said catalyst-coated substrate, wherein the ratio of the carbon-containing gas and hydrogen is about 0.1-3.0:100.

15. The method of claim 14, wherein said catalyst-coated substrate is exposed to said mixtures of gases at a substrate temperature of from about 750 C. to about 1000 C.

16. The method of claim 14, wherein said catalyst-coated substrate is exposed to said mixtures of gases for a period of from about 1 min to about 60 min in an evacuated chamber.

17. The method of claim 14 wherein, said patterned nanotemplate formation is made by one of: lithography, ion beam patterning, the use of nanostencils, photolithography, inkjet printing, and guided and unguided self assembly.

18. The method of claim 14, wherein the carbon-containing gas comprises at least one of: methane, acetylene, and alcohol.

19. The method of claim 18, wherein said alcohol is selected from C1-C6 alcohol.

20. The method of claim 14, wherein the substrate material comprises one of: Si, quartz, alumina and Cu.

21. The method of claim 14, wherein the catalyst is coated on the substrate by one of: chemical vapor deposition, physical vapor deposition, ion-beam deposition, sputtering, e-beam evaporation, vacuum deposition, spin coating, and dipping.

22. The method of claim 14, wherein said catalyst comprises iron.

23. The method of claim 14, wherein the catalyst comprises a layer of less than 100 nm.

24. The method of claim 14, wherein the sulphur-containing gas species comprises at least one of: hydrogen sulfide or carbon disulphide.

25. The method of claim 14, wherein the concentration of sulfur-containing gas species in the gas phase is in the range of from about 100 ppm to about 1000 ppm.

26. The method of claim 14, wherein the total pressure of the gases is in the range of from about 10 Torr to about 100 Torr.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the invention, in which:

(2) FIG. 1 shows SEM images of (a) PS spheres deposited on Si Substrate (b) Fe nanoisland (c) Uniform and random distribution of flappele and (d) magnified image of single flappele, according to the present invention.

(3) FIG. 2 shows ERTEM images of DLC rods.

(4) FIG. 3 shows Raman spectra of the DLC nanorod film, according to the present invention.

(5) FIG. 4 shows EEL spectrum of the DLC nanorods film, according to the present invention.

(6) FIG. 5 shows a Gaussian fit of the EEL spectrum, according to the present invention.

(7) FIG. 6 illustrates a representation of the inventive mechanism, according to the present invention.

(8) Throughout the figures, the same reference numbers and characters, unless otherwise stated, are used to denote like elements, components, portions or features of the illustrated embodiments. The subject invention will be described in detail in conjunction with the accompanying figures, in view of the illustrative embodiments.

DETAILED DESCRIPTION OF THE INVENTION

(9) The present invention will be explained in more detail with reference to the following example. However, the example is given for the purpose of illustration of one of the various embodiments of the present invention and in no way be construed as limiting the scope of the invention.

(10) I Experimental Procedure

(11) A self-assembly of iron nanostructures was fabricated on an n-type (100) Si substrate using spin coated polystyrene spheres. A major problem encountered in the monolayer coverage of polystyrene spheres was the lack of areal uniformity throughout the wafer surface which in turn was due to non adhesion of polystyrene spheres on the substrate surface. Thus, prior to spin coating the Si substrates were ultrasonicated in 20 mL of HF (47-51%) for 20 min and subsequently boiled in 20 mL HNO.sub.3 for 10 min so that a uniform thin layer of SiO.sub.2 is formed on the surface, resulting in better adhesion of polystyrene spheres to the substrate.

(12) The water dispersed polystyrene spheres were spin coated at 3000 rpm for 30 sec. A thin Fe layer (100 nm) was deposited on the polystyrene coated Si substrate using RF sputtering. Eventually, the polystyrene spheres were ultrasonicately etched in trichloroethanol for 4 min. The iron template was used as the catalytic compound for the deposition of desired uniform diamond-like carbon nanorod film. The Si substrate was introduced in a HFCVD chamber employing two Rhenium filaments of 24 cm each so as to cover a large area of the substrate surface. Growth parameters were a gas mixture of 0.3% methane in hydrogen and 97.7% hydrogen sulphide in hydrogen, deposition pressure of 20 torr, substrate temperature of 900 C. and Filament temperature of about 4000 C.

(13) II Results and Discussion

(14) A) SEM and TEM

(15) FIG. 1 shows the SEM micrograph of the polystyrene coated Si substrate and DLC nanorod film. A major problem encountered in the monolayer coverage of polystyrene spheres is the lack of areal uniformity throughout the wafer surface which in turn is due to non adhesion of polystyrene spheres on the substrate surface. To overcome this problem the Si substrates were ultrasonicated in HF and subsequently boiled in HNO.sub.3 solution so that a uniform oxide layer is formed on the surface, resulting in good adhesion of polystyrene spheres.

(16) FIG. 1a shows the Fe deposited polystyrene spheres and FIG. 1b shows the Fe nanoisland with a uniform distribution FIG. 1c shows the image of a non uniform distribution of papillae which is composed of nanorods that grow radial to the substrate and self assemble into micro- and nanoscale hierarchical structures. FIG. 1d displays the uniform distribution of nanorod film all over the substrate surface. Various papillae of diameters ranging from 0.2 to 0.4 m are found arranged in a random pattern on the substrate surface.

(17) A number of samples were prepared under the same conditions and an insight into their structural properties revealed that the constitution of the deposited film was consistent for all the samples but a slight inconsistency was observed in terms of uniformity of the film.

(18) FIG. 2 represents the EFTEM of the deposited film. TEM samples were prepared by scratching the nanorod film from the substrate using a sharp tungsten tip and then deposited on a carbon TEM grid. The Figure distinctly shows the aggregation of Fe nanoparticles at the rod tip. The average diameter of the rods was estimated to be in the range of 35 nm-45 nm.

(19) Based on the TEM studies, it is proposed that the growth of DLC nanorods is based on Physical vapor deposition process. As illustrated in FIG. 6, with an increase in temperature, the Fe on the substrate surface transforms into nanosized droplets and the active chemical species C, CH, CH.sub.2 and CH.sub.3 as well as C.sub.2 and C.sub.2H interact with Fe surface producing Fe.sub.3C. The incoming C, CH vapors from the filament diffuse and condense at the solid/liquid interface and rate of such process is dependent on concentration of the active chemical species C, CH, CH.sub.2 and CH.sub.3 as well as C.sub.2 and C.sub.2H, which are in the gas phase and are dependent on temperature. As the vapors are continuously absorbed by the Fe droplets the concentration of C increases leading to saturation and eventually initiating the nucleation of nanorods, thus, a mixture of sp.sup.2 and sp.sup.3 hybridized carbon (DLC) precipitates out at the solid-liquid interface and is pushed to form nanorods. This invention confirms the validity of the VLS growth mechanism at the nanometer scale.

(20) The last step enthalpy is determined by CC band energy and difference of interaction energy between C atom with substrate surface and Fe.sub.3C surface. If such difference is larger than 4.5 eV, the process of interest is exothermic, and we can expect that process of the DLC nanorods will have rate higher than process of growing of the carbon (diamond) thin layer as well as rate of the mentioned active chemical species interaction with DLC opened surface (cylinder surface) should be much less than discussed mechanism growing rate due to such processes are endothermic. So it is proposed the mechanism of the DLC growing by carbon substitution process in Fe.sub.3C compound (substitution diffusion of C in Fe layer). We may also assume that during of such growing process C atom in the forming material has tetrahedral configuration with many defects, because surface of substrate is not ideal plan, and the Fe layer has thickness distribution in the layer, that gives diffusing flax distribution by the layer surface.

(21) III Micro Raman Analysis

(22) Raman spectroscopy is largely used to study Carbon materials. The structural phases of the films were characterized by micro-Raman spectroscopy (RS). The deconvoluted visible Raman spectrum of the deposited sample shows four characteristic bands. As shown in FIG. 3, the peak around 1131 cm.sup.1 corresponds to the presence of diamond in nanocrystalline form. A broad peak at 1331.5 cm.sup.1 which is a typical signature of the presence of sp.sup.3 bonded diamond phase. Graphitic carbon shows a G peak which is located at 1591 cm.sup.1 corresponding to E.sub.2g symmetry of graphite perfect crystal. The D-peak centered at 1350 cm.sup.1 corresponds to breathing mode A.sub.1g activated by disordered and forbidden in perfect graphite.

(23) IV Electron Energy Loss Spectroscopy (EELS)

(24) Electron Energy-loss spectroscopy (EELS) study was carried out in the area shown in FIG. 2. FIG. 5 displays the carbon K-edge energy loss spectrum comprising two maxima at 286 eV and at 294 eV corresponding to 1s-* (sp.sup.2 C atom) and 1s-* (sp.sup.3 C atom), respectively. The EELS spectrum of DLC nanorod film is dominated by plasmon exciton consisting of * Plasmon. The sp.sup.3 fraction of the DLC film was estimated from the EELS spectrum. The C K-edge portion of the spectrum was deconvoluted to compare the intensities of * and * features. The percentage of the sp.sup.3 hybridized carbon was calculated from the ratio of the core sp.sup.3 peak (294 eV) area over the Carbon K-edge area in the range of from 280 eV to 300 eV.

CONCLUSION

(25) A crystalline DLC nanorod film with a substantial amount of sp.sup.3 bonded carbon atom has been synthesized by the CVD method using iron as catalyst. The most significant result of the present work is the growth of a DLC nanorod film. The film is characterized by special micro- and nano scale hierarchical surface structures having high purity which is evident from the D and G Raman peaks obtained at 1347.8 cm.sup.1 and 1590 cm.sup.1, respectively. The sp.sup.3 carbon content was found to be around 88.80% as determined from Electron Energy Loss Spectroscopy.

(26) Although the present invention has been described herein with reference to the foregoing exemplary embodiment, this embodiment does not serve to limit the scope of the present invention. Accordingly, those skilled in the art to which the present invention pertains will appreciate that various modifications are possible, without departing from the technical spirit of the present invention.