Abstract:
The present invention provides a method for producing carbon structures by laser irradiation, the method comprising: (i) providing a substrate, at least a portion of whose surface being covered with a sample comprising one or more thermally degradable organic compounds, said sample being in the form of homogeneous solution, suspension or emulsion; (ii) irradiating said covered surface portion locally by applying a focused laser beam, thus resulting in local deposition of carbon, and (iii) repeating step (ii) by moving either the laser beam or the sample, thus creating a desired pattern of carbon structures. The present invention further provides carbon structures produced by the method of the invention.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method for producing patterns of carbon.  
           [0002]    List Of References  
           [0003]    In the following description reference will be made to several prior art documents shown in the list of references below. The reference will be made by indicating in brackets the first author name from the list.  
           [0004]    A. E. Aleksenskii, M. V. Baidakova, A. Ya. Vul, V. Yu. Davydov, Yu. A. Pevtsova, Phys. Solid State 1997, 39, 1007.  
           [0005]    J. C. Angus, H. A. Will, W. S. Stanko, J. Appl. Phys. 1968, 39, 2915.  
           [0006]    F. P. Bundy, H. T. Hall, H. M. Strong, R. H. Wentorf Jr., Nature 1955, 176, 51.  
           [0007]    M. Elbaum, D. Zbaida, E. Klein, A. Lachish-Zalait, WO 01/38940 A2.  
           [0008]    Y. Gogotsi, S. Welz, D. A. Ersoy, M. J. McNallan, Nature 2001, 411, 283.  
           [0009]    A. Lachish-Zalait, D. Zbaida, E. Klein, M. Elbaum, Adv. Funct. Mater. 2001, 11, 218.  
           [0010]    Y. Namba, E. Heidarpour, M. Nakayama, J. Appl. Phys. 1992, 72, 1748.  
           [0011]    L. C. Qin, D. Zhou, A. R. Krauss, D. M. Gruen, NanoStructured Materials 1998, 10, 649.  
           [0012]    J. Robretson, Prog. Solid State Chem. 1991, 21, 199.  
           [0013]    K. Uetake, N. Sakikawa Chem Abstr., 1974, 81:5175t  
           [0014]    X. Wang, J. Chen, Z. Zheng, Z. Sun, F. Yan, J. Crystal Growth 1997, 181, 308.  
           [0015]    M. Yoshikawa, Y. Mori, H. Obata, M. Maegawa, G. Katagiri, H. Ishida, A. Ishitani, Appl. Phys. Lett. 1995, 67, 694.  
         BACKGROUND OF THE INVENTION  
         [0016]    Carbon is a very versatile element, due to the different ways in which carbon atoms can bond to each other and to other elements. The most common naturally occurring forms of pure carbon are graphite and diamond.  
           [0017]    In graphite, the atoms are threefold coordinated as sp 2  hybrids, forming planes of six-member rings. Carbon atoms bond strongly to each other within a plane but weakly between adjacent planes. Graphite is opaque, soft, flexible, and an excellent conductor of heat and electricity. These properties are exploited in foundries, lubricants, brake linings, crucibles and pencils.  
           [0018]    The diamond allotrope consists of four fold-coordinated carbon atoms (sp 3  hybrids) and the atomic bonding is strong in all directions. Diamond is the hardest known material, electrically insulating, and transparent from the far ultra-violet to the far infrared. Beside being a precious stone, potential applications include wear-resistant coatings, thin films semiconductor devices, heat sinks, abrasives and cutting tools.  
           [0019]    The currently preferred method for preparing artificial diamond crystals is based on high-pressure high-temperature (HPHT) (Bundy et al., 1955), which simulates conditions like those of natural creation of diamond. Although pure diamond is obtained, bulk crystallites are produced, and due to its hardness it is difficult to model them into shapes required in many applications. In order to facilitate the use of the diamond properties, the chemical vapor deposition (CVD) method (Angus et al., 1968) was developed, according to which the carbon atoms condense, out of the vapor phase, in diamond configuration on a heated substrate at low pressure.  
           [0020]    Another form of carbon known as diamond-like carbon (DLC) films (Robretson, 1991) consists of amorphous carbon that contains a significant fraction of sp 3  bonding. DLC films or films prepared by CVD contain mixtures of sp 2  and sp 3  carbon forms in different proportions. These films have important technological applications due to their hardness and chemical inertness, low electronic affinity and wide optical gap. The films can be exploited for optoelectronic device applications, panel display, protective coating, wear resistant coating, abrasive for semiconductors, reinforce for polymer and rubber, and heat sinks.  
           [0021]    The exploitation of carbon forms for microelectronics and sensor applications as well as protective coating of electrical circuits, is still limited by the lack of a suitable means of carbon lithography or direct carbon patterning.  
           [0022]    It has been published earlier by co-inventors (Lachish-Zalait et al., 2001 and Elbaum et al in WO 01/38940) that microscale-patterned surfaces can be generated by applying a tightly focused single-mode laser beam tightly focused through an optical lens directly on a homogeneous solution containing soluble metal salts or metal compounds. The laser beam strikes the dissolved chemicals in a confined volume and a localized microchemistry process takes place (oxidation-reduction, chemical or thermal decomposition) at the glass/solution interface. Consequently, the product deposits as a solid metal or metal compound and firmly attaches to the substrate that holds the solution. Operating the laser while moving the microscope stage or the laser beam draw continuous micro-scale lines. This direct micropatterning was applied on a variety of precursor solutions leading to patterns of metallic silver, gold, platinum, oxidized copper as well as compounds containing transition metals-II, for example Mo and W.  
         SUMMARY OF THE INVENTION  
         [0023]    It has been found, according to the present invention, that carbon microstructures may be produced by applying a focused laser beam directly onto a sample in the form of a homogeneous solution, dispersion or emulsion, of heat degradable precursors. The carbon structures so formed, show evidence of both sp 3  and sp 2  hybridization and their size is in the micron range or even smaller. The laser beam induces thermal decomposition of the heat degradable precursor in solution, thus affording the precipitation of carbon patterns that attach firmly to the surface that holds the irradiated solution.  
           [0024]    Thus, the present invention relates to a method for producing carbon structures by laser irradiation, the method comprising:  
           [0025]    (i) providing a substrate, at least a portion of whose surface is covered with a sample comprising one or more thermally degradable organic compounds, said sample being in the form of homogeneous solution, suspension or emulsion;  
           [0026]    (ii) irradiating said covered surface portion locally by applying a focused laser beam, thus resulting in local deposition of carbon, and  
           [0027]    (iii) repeating step (ii) by moving either the laser beam or the sample, thus creating a desired pattern of carbon structures.  
           [0028]    The deposition of carbon is obtained from the decomposition of the thermally degradable compounds comprised by the irradiated sample.  
           [0029]    The carbon patterns formed by the method of the present invention may contain various amounts of sp 3  and sp 2  bonding, thus having physical characteristics ranging from diamond, diamond-like carbon and graphite.  
           [0030]    In the above method, the scanning of the surface with the laser may also be carried out in a predetermined manner as a direct-write technique. Direct-write patterning is ideal for sample-specific marking, such as serial numbers, codes, identification cards, etc.  
           [0031]    Examples of thermally degradable organic compounds are peroxides, azo compounds, acids, ketones, diketones, biphenyls and polyphenyl compounds.  
           [0032]    The wavelength of the laser beam may be in the visible, UV, IR or near IR range. The patterning with IR lasers was accomplished with success, as described below and this result is surprising since from a mechanistic point of view the infrared laser photons do not have sufficient energy to break chemical bonds.  
           [0033]    The major factors influencing the patterning rate are the precursor, the solvent, the laser power and the type of the substrate. The size and shape of the structure generated on the surface is dictated by the width of the laser spot and the thermal diffusion rate. Arrays of any desired shape may be built by serial production of a local pattern (e.g. dot or line).  
           [0034]    The carbon patterns produced by the method of the present invention may be utilized in a number of novel applications. Patterns of carbon with sp 3  hybridization can serve as nucleation sites for growing diamond on surfaces not amenable to direct deposition. In addition, carbon patterns can be drawn on a substrate and cast into a polymer matrix. Peeling off the matrix from the glass substrate will afford 3D micron size channels suitable for micro-fluidic applications. Also, the ability to pattern carbon with sp 3  hybridization can be used in electron emitters of high quantum efficiency. The ability to pattern then opens applications in device development for detectors. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:  
         [0036]    [0036]FIG. 1 shows an optical image with low magnification of the carbon pattern.  
         [0037]    [0037]FIG. 2 shows imaging in a field emission scanning electron microscope (SEM; FEI model XC-30) using the environmental mode with 1 torr water vapor pressure without any coating.  
         [0038]    [0038]FIG. 3 shows a typical electron diffraction of the carbon line with sp 3  configuration and spotty morphology.  
         [0039]    [0039]FIG. 4 shows a typical selected site in the carbon line of an ordered array of graphene sheets with lattice fringes (marked between arrows) of 3.35 Å, corresponding to carbon with sp 2  configuration.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0040]    It has been found according to the present invention that carbon with various amounts of sp 3  and sp 2  hybridization may be obtained, by applying a tightly focused laser beam directly onto a sample comprising heat degradable precursors, the sample being in the form of homogeneous solution, suspension, or emulsion. The sample may be held by various surfaces, both hydrophilic and hydrophobic in nature, and the surfaces do not require any pretreatment such as cleaning, degreasing, and the like, before the application of the sample.  
         [0041]    In practice, a glass slide and cover slip enclose the sample. The focused laser radiation passes through the glass cover slip and strikes the confined sample at the glass/solution interface. Deposition may occur on either glass surface on which the laser is focused. Thermal decomposition of the heat degradable precursor in solution, suspension or emulsion form is induced, thus affording the precipitation of carbon that contains various amounts of sp 3  and sp 2  hybridization.  
         [0042]    The lasers used in the method of the present invention may operate in the visible, UV or infra red ( 1 R) region, more preferably in the IR region (830, 980 nm) and are ideally focused to a narrow spot by a microscope objective. The intensity of the laser beam at the sample interface was measured and found to be on the order of 10 mW.  
         [0043]    Patterning of the substrate takes place only if the laser is focused directly at the substrate-solution interface. Defocused light or focusing within the bulk solution has no apparent effect. Both the near and the far surface with respect to the objective can be patterned.  
         [0044]    In the initiation step, it was observed that directing the laser beam to a very small amount (about 0.5-1 micron) of freshly precipitated crystals of dibenzoyl peroxide could induce initiation of carbon deposition, although the starting material has no significant absorption at the laser wavelength of irradiation. The same applies to all starting materials used in the method of the invention, when the irradiating laser operates in the IR or near IR region.  
         [0045]    However, once initiated, it appears that the deposited product absorbs the laser radiation. The deposition of the carbon involves intense local heating. Violent bubbling was observed in the fluid, and a trace of molten borosilicate glass substrate was seen in the SEM after mechanical removal of the deposited carbon line. Local melting was also observed on quartz cover slip, indicating a local temperature exceeding 1400° C. Thus, once the process of deposition begins, its propagation is self-sustaining.  
         [0046]    For the case of dibenzoyl peroxide as a precursor, it was observed that an alcohol solution of this compound undergoes thermal decomposition at around 70-80° C. Thermal decomposition of dibenzoyl peroxide is known to produce carbon dioxide, benzoic acid, biphenyl, phenyl benzoate, benzene and terphenyls (Uetake et al., 1974). The products, most probably, are obtained through the appropriate radicals, optionally followed by dimerization step. In the present case of carbon patterning, it is suggested that a complete pyrolysis takes place leading to the deposition of pure carbon on the glass surface. Operating the laser momentarily forms isolated spots, while moving the microscope stage or the laser spot in the x-y plane draws continuous lines. The rate of deposition depends on the identity of the precursor solution, laser power, and deposition speed.  
         [0047]    Examples of thermally degradable compounds are peroxides, azo compounds, acids, ketones, diketones, biphenyls and polyphenyl compounds. More specifically, compounds suitable to be used in the method of the invention are dibenzoyl peroxide, di-tertbutyl peroxide, azo-bis-isobutyronitril, benzophenone, benzoic acid, dibenzoyl, benzhydrol, ethyl benzoate, benzoyl benzoate, biphenyl, p-terphenyl, naphthalene, anthracene, camphor, etc.  
         [0048]    In practice, a precursor solution is obtained by dissolving one or more thermally degradable compounds into solvents capable to dissolve these compounds, such as alcohol (e.g. ethanol or isopropyl alcohol), toluene, benzene, xylene, etc. For example, a dibenzoyl peroxide solution was prepared by dissolving dibenzoyl peroxide (30% in water) (90 mg) in ethanol (3 ml) or in toluene (1 ml). Alternatively, the sample is prepared in the form of suspension or emulsion, by dissolving one or more thermally degradable compounds into a suitable solvent or solvent system.  
       EXAMPLES  
       [0049]    Experimental  
         [0050]    An IR diode laser source was used in a configuration of optical tweezers. The laser operated at two wavelengths: at 830 and 980 nm and the output power at the sample was 5 and 10 mW, respectively.  
         [0051]    Scanning electron microscope was performed after coating the sample with carbon and gold in a JEOL GMC  6400  equipped with an Oxford Link EDS spectrometer. Alternatively, imaging was performed in a field emission scanning electron microscope (SEM; FEI model XC-30) using environmental mode with 1 torr water vapor pressure without any coating.  
         [0052]    Electron diffraction of the removed lines adsorbed dry onto carbon/collodion coated Cu grid, were carried out by transmission electron microscope (TEM; Philips model CM 120).  
         [0053]    The sample cell consisted of a long cover slip (22×40 mm), fixed crosswise to an ordinary glass microscope slide (25×76 mm) using wax spacers, leaving final dimensions approximately 5×25×0.1 mm).  
         [0054]    A precursor solution was prepared by mixing one or more heat degradable precursor in a suitable solvent or mixture of solvents and then it was injected into the sample cell.  
         [0055]    Patterns deposited on the glass slide were thoroughly washed with ethanol (×1), toluene (×3), incubated in toluene ×2 times for 1 hour each, to remove the precursors and then dried in air.  
         [0056]    Pattern Characterization  
         [0057]    Patterns were generated with a typical width of 5-7 microns and 1.5-2 micron height. Typical deposition speed is about 1 micron per second depending on the laser power, precursor solution, and the substrate glass. In all cases, gray colored lines were observed. FIG. 1 shows an optical image of a carbon continuous pattern in the form of a line. Moving the microscope stage or the laser beam, continuous deposits are formed to a length of centimeters.  
         [0058]    The elemental analysis as revealed by the electron dispersion microscopy (EDS) showed that, in the pattern formed by the method of the invention, carbon is the dominant element with some traces of oxygen (0.7%). Highly ordered pyrolitic graphite (HOPG) as a standard calibration for EDS, showed carbon with a content of 0.5-0.7% oxygen.  
         [0059]    [0059]FIG. 2 shows a deposited line as imaged in field-emission scanning electron microscope (SEM; FEI model XC-30), using the environmental mode with 1 torr vapor pressure shows a densely packed line.  
         [0060]    Carbon patterns were deposited on borosilicate glass as well as quartz and mica. In order to establish the electron diffraction pattern, deposited lines were scraped from the glass surface and adsorbed dry onto Cu grid for transmission electron microscopy. Selected area electron diffraction (SAED) patterns were recorded from regions where the grains were thin enough to remain electron-transparent. Alternatively, the lines were embedded in epoxy resin and sectioned into thin (50-70 nm) slices by an ultramicrotome.  
         [0061]    Two different morphologies have been observed and distinct electron diffraction (ED) patterns from different sites on the carbon pattern. FIG. 3 shows the most dominant image of the spotty morphology and two diffraction rings in the ED pattern. The intense ring corresponds to the interplanar d-spacing of 2.06 Å and a more diffuse line to d=1.22 Å, corresponding to the Miller indices of (111) and (220) of cubic (sp 3 ) diamond, respectively (Namba et al., 1992 and Qin et al., 1998).  
         [0062]    [0062]FIG. 4 shows a selected site, one of several, in a patterned sample with an ordered array of graphene sheets with lattice fringes (marked between arrows) of 3.35 _ (Gogotsi et al.,  2001 ). The diffraction pattern showed an intense ring at spacing of 3.35 _ corresponding to the (002) lattice plane of graphite. The other diffuse rings are at 2.13 and 1.23 _ corresponding to (100) and (110) lattice planes of (sp 2 ) graphite, respectively (Qin et al., 1998).  
         [0063]    For comparison, electron diffraction of three samples was analyzed as standard controls. Highly ordered pyrolytic graphite (HOPG), showed a typical spotty diffraction pattern corresponding to interplanar d-spacing 3.35, 2.07, 1.50, 1.22, 1.15 and 1.10 Å, corresponding to the Miller indices (002), (101), (103), (110), (112) and (006) respectively, typical to sp 2  graphitic carbon form. In addition, we have recorded the diffraction pattern of synthetic polycrystalline diamond, gray powder with 1-micron crystallite size. Five spotty rings with d-spacing of 2.06, 1.26, 1.07, 0.82, and 0.71 Å. Natural monocrystalline diamond powder with 1-micron crystallite size showed patterns of single crystals. Typical diffraction patterns of several crystals in the sample with different orientations were with d-spacing at 1.25 Å for one crystal, 2.07 Å from another, and 1.26 and 1.08 Å from the third one.  
         [0064]    The crystallite size in a patterned sample of the invention was estimated by fitting the diffraction maximum at 2.06 Å to a Gaussian line shape. The average size calculated from the peak half width using the Selyakov-Scherrer expression (Aleksenskii et al., 1997) was found to be 9-30 Å. Therefore, the derived particle size and crystal quality influence the electron diffraction pattern.  
         [0065]    The Raman analysis is known as a convenient tool to characterize carbon materials. The Raman spectra (using Renishaw microscope while excitation of the sample with HeNe laser at 633 nm or UV at 244 nm) is not applicable for small (9-30 Å) crystallite (Yoshikawa 1995 and Wang 1997) and does not contradict the existence of both sp 2  and sp 3  carbon form.  
         [0066]    In summary, the present invention provides a method for deposition of carbon patterns on surfaces. The deposited material contains high proportion of sp 3  and sp 2 -bonded carbon. The ability to deposit micro-scale patterns opens possibilities for applications to micromechanical, microelectronic and sensing devices. The method of the invention may also be used as a direct-write technique.