Abstract:
An embodiment of the invention relates to a method of analyzing a substance comprising the steps of: fabricating a structure comprising said substance and at least one graphene layer; carrying out at least one measurement step with respect to said structure; and analyzing the measurement result of said measurement step to receive at least one analytical result concerning said substance.

Description:
[0001]    The present invention relates to a method of analyzing substances. More particularly, the invention relates to a method of analyzing substances of very small quantities. 
       BACKGROUND OF THE INVENTION 
       [0002]    The analysis of single molecules or thin layers thereof is of both technological and scientific importance. There are a few methods which can unambiguously identify a-priori unknown little quantities of material down to single molecules. However, all these methods suffer from some shortcomings, and therefore have not been technologically implemented yet. General reasons are low sensitivities of the analytical methods and the instability of thin molecular layers and especially single molecules with respect to their thermal, chemical and photo-degradation or to external mechanical fields caused, e.g., by scanning probe microscopy (SPM) tips. Therefore it would be advantageous to improve these methods. 
         [0003]    A few modifications of Surface Enhanced Raman Spectroscopy (SERS) can provide Raman spectra of small molecular quantities down to single molecules. A Raman spectrum is a “fingerprint” of a molecule. However, without surface enhancement the probability for a photon to be Raman scattered is very low, such that only large material quantities can be analyzed. On the one hand SERS is based on electromagnetic field enhancement by nano-structures made of plasmon active materials, and on the other hand on some chemical interaction between the surface and the molecule. A surface roughened in a certain manner has a few “active” sites, which enhance the Raman signal substantially to detect it even from single molecules located at the sites. One of the disadvantages of the above described method is the random nature of the active sites. One cannot analyze any molecule at will, since the active sites are usually not precisely known, and, e.g., the molecules have to interact in a particular way with the active sites. The above-mentioned disadvantage can be overcome by precisely fabricated nano-structures. Modern lithography methods do not allow, however, building highly efficient nano-structures reproducibly, since efficient field enhancement requires nanometer precision, which cannot be provided by, e.g., electron beam lithography. 
         [0004]    A further disadvantage of prior art SERS is the strong difference between the Raman spectra in solution and the SERS Raman spectra, which are attributed, to some extent, to a specific chemical interaction between the molecule and the SERS active material. Also the molecules have to be placed precisely into the nano-structures to be analyzed. 
         [0005]    The above-mentioned disadvantages may be overcome to some extent by the so called Tip Enhanced Raman Spectroscopy (TERS). TERS employs a sharp tip made of a plasmon active material. The tip is placed over an area of interest and enhances Raman scattering of molecules located in the nearest proximity to the tip apex. TERS has its own difficulties: one has to position the tip with high precision in order not to damage the molecules on the surface. 
         [0006]    Similarly to TERS and SERS, also infrared spectra of molecules can be enhanced with plasmon active structures. The enhancement is not as strong as for Raman spectroscopy in the visible range, and a single molecule analysis is yet to be demonstrated. 
         [0000]    A common disadvantage of all the above-described methods is the large amount of heat produced by the focused plasmon localized at the active site, i.e. high temperatures can cause oxidative damage to the molecules. 
       OBJECTIVE OF THE PRESENT INVENTION 
       [0007]    An objective of the present invention is to provide a method which allows analyzing substances with very accurate results. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    An embodiment of the invention relates to a method of analyzing a substance comprising the steps of: fabricating a structure comprising said substance and at least one graphene layer; carrying out at least one measurement step with respect to said structure, and analyzing the measurement result of said measurement step to receive at least one analytical result concerning said substance. 
         [0009]    On the one hand graphene is a monoatomically thick flexible and both electrically and thermally conductive layer of carbon atoms, and on the other hand it is optically transparent, impermeable to gases and liquids, and on solid substrates it protects against wear. Thus graphene can act as an optically transparent surface protective layer against local heating, wear and interactions with the ambient environment. Moreover, graphene exhibits its own plasmon resonance at ultraviolet wavelengths with a tail into the blue spectral region. 
         [0010]    Preferably, at least one bent surface section is fabricated in the graphene layer(s) in proximity of the substance. Bent surface sections may be used to increase the field strength of radiation generated during the measurements. 
         [0011]    The step of fabricating the structure may further comprise adding a field enhancing material, which is separated from the substance by the graphene layer(s). 
         [0012]    Further, the structure may be fabricated such that both the graphene layer(s) and the field enhancing material each have a bent surface section in proximity of the enclosed substance. 
         [0013]    The measurement step preferably comprises generating electromagnetic radiation and directing the electromagnetic radiation towards the substance. 
         [0014]    The wavelength of the electromagnetic radiation preferably falls within a wavelength range from the ultraviolet through the visible to the infrared region where the electromagnetic radiation may excite the field enhancing material and/or the graphene layer(s) to generate secondary electromagnetic fields which increase the amplitude of the electromagnetic radiation inside the substance. 
         [0015]    The field enhancing material may consist of or may comprise one or more of the following materials: metal (preferably gold), graphite, and MoS 2 . 
         [0016]    Alternatively or additionally, the measurement step may comprise the steps of: generating electromagnetic radiation in the visible and/or infrared range and directing said electro-magnetic radiation towards the covered substance, and exciting the graphene layer to generate secondary fields which increase the amplitude of the radiation inside the substance. 
         [0017]    The step of fabricating the structure preferably comprises the steps of: providing a carrier, depositing said substance on top of the carrier, and covering the carrier and the substance with said at least one graphene layer. 
         [0018]    The carrier may consist of monoclinic silicate or may at least comprise a monoclinic silicate (mica) surface. In this case, the substance may be deposited on the monoclinic silicate surface of the carrier, and the substance may be covered by at least one graphene layer. 
         [0019]    The step of fabricating the structure may also comprise placing the substance between two or more graphene layers. 
         [0020]    Further, the step of fabricating the structure may comprise the steps of: providing a carrier consisting of or comprising a field enhancing material, said carrier having at least one hole capable of localizing the substance on the carrier; placing said at least one graphene layer on top of the carrier; and filling said substance into said hole wherein said at least one graphene layer separates the substance from the carrier. 
         [0021]    Furthermore, the substance to be analyzed may comprise one or more of the following substances: single molecules, molecular self-assembled layers, a polymeric molecule, a substance which is thermally, chemically and/or mechanically instable, DNA (Deoxyribonucleic acid), RNA (Ribonucleic acid), a protein, a synthetic polynucleotide, a polypeptide. 
         [0022]    The measurement step may include one or more of the following measurement procedures: Scanning Probe Technique, Raman spectroscopy, Infrared spectroscopy, Scanning Tunneling Microscopy, Scanning Tunneling Spectroscopy, Scanning Probe Microscopy, and Surface Enhanced Raman Spectroscopy. 
         [0023]    The structure is preferably fabricated using a carrier consisting of or at least comprising one or more of the following materials: graphite, a substrate coated with one or more graphene layers, materials of the mica group, MoS 2 , glass, plated gold, and a silicon wafer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    In order that the manner in which the above-recited and other advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended figures. Understanding that these figures depict only typical embodiments of the invention and are therefore not to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail by the use of the accompanying drawings in which: 
           [0025]      FIG. 1  shows in an exemplary fashion a contact mode SFM (SFM: scanning force microscopy) surface height image of ds-DNA (double-stranded DNA) replicas in graphene. 
           [0026]      FIG. 2  shows in an exemplary fashion a sketch of the sample cross section marked in  FIG. 1 . 
           [0027]      FIG. 3  shows in an exemplary fashion a tapping mode image of the area imaged in  FIG. 1  recorded a few minutes after contact mode imaging, indicating no damage is done to ds-DNA during contact mode imaging. 
           [0028]      FIG. 4  shows in an exemplary fashion a tapping mode SFM height image of ds-DNA replicas in graphene of different thickness (number of graphene layers indicated on the image). The heights of DNA molecules and their replicas in the graphene layer (s) are on the order of one nanometer as can be estimated from the SFM images. 
           [0029]      FIG. 5  shows in an exemplary fashion height images of samples with ds-DNA on a mica surface, coated and uncoated, respectively, with few layer graphene. 
           [0030]      FIG. 6  shows a first exemplary embodiment of a structure comprising a substance to be analyzed, and at least one graphene layer. 
           [0031]      FIG. 7  shows a second exemplary embodiment of a structure comprising a substance to be analyzed, and at least one graphene layer. 
           [0032]      FIG. 8  shows a third exemplary embodiment of a structure comprising a substance to be analyzed, and at least one graphene layer. 
           [0033]      FIG. 9  shows a fourth exemplary embodiment of a structure comprising at least one graphene layer. 
           [0034]      FIG. 10  shows a fifth exemplary embodiment of a structure comprising a substance to be analyzed, and at least one graphene layer. 
           [0035]      FIG. 11  shows a sixth exemplary embodiment of a structure comprising at least one graphene layer. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0036]    The preferred embodiments of the present invention will be best understood by reference to the drawings, wherein identical or comparable parts are designated by the same reference signs throughout. 
         [0037]    It will be readily understood that the present invention, as generally described herein, could vary in a wide range. Thus, the following more detailed description of the exemplary embodiments of the present invention, is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention. 
         [0038]      FIGS. 1 to 4  show in an exemplary fashion a single to a few layers of graphene which are exfoliated mechanically onto a mica surface covered with double stranded (ds) vector DNA. It can be seen that the topography of the graphene layers replicates the underlying ds-DNA rings, as shown by scanning force microscopy (SFM) imaging in intermittent contact (also known as tapping) and contact modes. 
         [0039]    Attempts to image unprotected ds-DNA molecules on mica in contact mode under similar conditions destroy the molecules, proving that graphene may protect ds-DNA molecules on mica from wear and breakage.  FIG. 5  shows in an exemplary fashion height images of samples with ds-DNA (including some contamination islands) on mica, coated and uncoated, respectively, with few layer graphene. 
         [0040]    Sections a) and b) of  FIG. 5  were taken in contact mode with the same scan parameters (˜30 nN normal force). Section a) of  FIG. 5  shows that ds-DNA coated with few layer graphene can be imaged reproducibly, while section b) of  FIG. 5  shows that uncoated ds-DNA cannot be imaged reproducibly. Section c) of  FIG. 5  shows an image taken subsequently in tapping mode. Arrows relate the sample areas recorded first in contact and then in tapping mode. It is apparent that contact mode imaging of ds-DNA not covered by graphenes destroyed the molecules, while ds-DNA covered by graphene remained intact. This implies the ability to perform profiling analysis of relatively soft and instable molecules even in contact mode SFM, which typically destructs macromolecules such as ds-DNA. The thickness of graphene is only 0.34 nm, allowing for precise profiling. In addition, both graphene and mica are optically highly transparent, i.e. the encapsulated molecules are accessible to optical excitation. Thus all methods described above, including TERS, will in principle still work on molecules encapsulated under graphene. 
         [0041]    Moreover, graphene will protect the molecules against interactions with ambient, i.e. oxygen or water vapor, and thereby protect them from oxidative damage. In addition, graphene allows using a broad range of tip-surface distance control mechanisms including contact mode SFM, friction and shear force microscopy, as well as scanning tunneling microscopy (STM), the latter due to the high conductivity of graphene. Also a broad range of different spectroscopic techniques can be employed including scanning tunneling spectroscopy (STS) and conductive or Kelvin probe force microscopy. 
         [0042]    Different exemplary structures comprising substances to be analyzed and protective graphene layers are shown in  FIGS. 6-11 . 
         [0043]      FIG. 6  shows a first embodiment of a structure  5  comprising a carrier  10  having a mica surface  20 . A substance  30 , like DNA for instance, is deposited on the mica surface  20 . Then, at least one graphene layer  40  is arranged on top. Further, a layer of a field enhancing material  50  is deposited on the surface of the graphene layer  40 . 
         [0044]    The field enhancing material  50  may be a plasmon active material, preferably gold or another conductor such as silver, graphite etc. The graphene layer  40  will protect the substance  30  from any chemical or physical interaction with the field enhancing material  50 . 
         [0045]    It can be seen in  FIG. 6  that both the graphene layer  40  and the field enhancing material  50  have a bent surface section  41  and  51 , respectively, both in proximity of the enclosed substance  30 . The bent surface sections  41  and  51  form a cavity where the substance  30  is localized. 
         [0046]    In order to analyze the substance  30 , electromagnetic radiation may be generated e.g. for Raman or infrared spectroscopy. If the field enhancing material  50  is gold the preferred wavelength of the electromagnetic radiation ranges from 400 nm to 1.2 μm. The external radiation will excite the field enhancing material  50  to generate secondary fields which interfere with the external radiation such that the resulting electromagnetic field inside the substance  30  has an increased radiation amplitude. 
         [0047]    Furthermore, depending on the wavelength of the external radiation, the graphene layer  40  may also exhibit plasmon activity in the ultraviolet with the plasmon tail even in the blue spectral region. Thus, the graphene layer  40  may also provide some enhancement of the measurement signal (e.g. a Raman signal) by increasing the radiation amplitude inside the substance  30 . 
         [0048]    In summary, the structure  5  of  FIG. 6  allows avoiding any direct contact of the substance  30  (analyte) with the field enhancing material  50 , while still keeping the substance  30  very close to the field enhancing material  50 , which can therefore provide plasmon enhancement of the measured signal (e.g. a molecular Raman signal). 
         [0049]      FIG. 7  shows a second embodiment of a structure  5  comprising a carrier  10  having a mica surface  20 . A substance  30 , like DNA for instance, is deposited on the mica surface  20 . Then, at least one graphene layer  40  is arranged on top. The graphene layer  40  will protect the substance  30  from chemical interaction with the external atmosphere. 
         [0050]    The graphene layer  40  has a bent surface section  41  in proximity of the enclosed substance  30 . The bent surface section  41  forms a cavity where the substance  30  is localized. 
         [0051]    In order to analyze the substance  30 , electromagnetic radiation may be generated e.g. for Raman or infrared spectroscopy. In order to excite the graphene layer  40  to generate secondary fields the preferred wavelength of the external electromagnetic radiation ranges from 1 nm to 1 mm. The secondary fields interfere with the external electromagnetic radiation such that an increased radiation amplitude is generated inside the substance  30 . 
         [0052]    In summary, the graphene layer  40  of  FIG. 7  protects the substance  30  and simultaneously provides a field enhancing effect. 
         [0053]      FIG. 8  shows a third embodiment of a structure  5  comprising a carrier  10  consisting of a field enhancing material  50  such as gold. The carrier  10  has at least one hole  70 , the size of which being adapted to the volume of a substance to be analyzed. At least one graphene layer  40  is arranged on top of the field enhancing material  50 . Then, the hole  70  is filled with the substance  30 . The graphene layer  40  separates the substance  30  from the carrier  10 . 
         [0054]    The graphene layer  40  and the hole  70  have bent surface sections  41  and  51 , respectively, in proximity of the carried substance  30 . The bent surface sections form a cavity where the substance  30  is localized. 
         [0055]    In order to analyze the substance  30 , electromagnetic radiation may be generated e.g. for Raman or infrared spectroscopy. Depending on the wavelength of the electromagnetic radiation, the field enhancing material  50  and/or the graphene layer  40  will generate secondary fields which interfere with the external radiation such that an increased radiation amplitude is generated inside the substance  30 . 
         [0056]    In summary, the structure of  FIG. 8  allows avoiding direct contact of the substance  30  (analyte) with the field enhancing material  50 , while still keeping the substance  30  very close to the field enhancing material  50 , which can therefore provide the plasmon enhancement with respect to the measured signal (e.g. a molecular Raman signal). 
         [0057]    Referring again to  FIG. 6 , the structure  5  of  FIG. 6  may be modified by removing the carrier  10 . Then, the remaining structure consisting of the graphene layer  40 , the substance  30 , and the layer of a field enhancing material  50  may be used as a “SERS chip”. On the one hand the protrusions created by the substance  30  will provide SERS active sites and on the other hand graphene will protect adsorbates from direct contact with the surface of the field enhancing material  50 . However, after removal of the carrier  10  the graphene layer  40  might relax into a less stretched, i.e. plane conformation. In order to prevent the graphene layer  40  from relaxation, an adhesive layer  75  is preferably arranged between the graphene layer  40  and the field enhancing material  50 . The adhesive layer  75  may also consist of a field enhancing material. The resulting structure  5  having an additional adhesive layer  75  is shown in an exemplary fashion in  FIG. 9 . 
         [0058]      FIG. 10  shows an exemplary embodiment of a structure  5  where a graphene layer  40  is relaxed into a planar conformation. Here, plasmon resonance properties of graphene may superpose with the plasmon resonance of the field enhancing material  50  of the carrier  10  in the area of the hole  70 . The hole  70  may be a controlled quantum-sized defect in the carrier  10 . 
         [0059]      FIG. 11  shows an exemplary embodiment of a structure  5  where—in addition to the substance to be analyzed—particles  80  (e.g. nano-particles) of a plasmon active material are encapsulated in or carried by the graphene layer  40 . The resulting structure  5  may be used to tune local surface plasmon activity for SERS applications. The substance to be analyzed is not shown in  FIG. 11 . 
       LITERATURE 
       [0000]    
       
         [1] C. Lee et al.,  Physica Status Solidi B - Basic Solid State Physics  246, 2562 (2009). 
         [2] T. Eberlein et al.,  Physical Review B  77, 233406 (2008). 
         [3] J. R. Lombardi, R. L. Birke,  Journal of Physical Chemistry C  112, 5605 (2008). 
       
     
       REFERENCE SIGNS 
       [0000]    
       
           5  structure 
           10  carrier 
           20  surface 
           30  substance 
           40  graphene layer 
           41  bent surface section 
           50  field enhancing material 
           51  bent surface section 
           70  hole 
           75  adhesive layer 
           80  particle