Editing Raman spectroscopy (section)

=== Enhanced (or near-field) Raman spectroscopy ===
Enhancement of Raman scattering is achieved by local electric-field enhancement by optical [[near and far field|near-field]] effects (e.g. localized [[surface plasmon]]s).
* ''[[Surface Enhanced Raman Spectroscopy|Surface-enhanced Raman spectroscopy]] (SERS)'' – Normally done in a silver or gold colloid or a substrate containing silver or gold. Surface [[plasmons]] of silver and gold are excited by the laser, resulting in an increase in the electric fields surrounding the metal. Given that Raman intensities are proportional to the electric field, there is large increase in the measured signal (by up to 10<sup>11</sup>). This effect was originally observed by [[Martin Fleischmann]] but the prevailing explanation was proposed by Van Duyne in 1977.<ref>{{cite journal |author = Jeanmaire DL |author2 = van Duyne RP | title = Surface Raman Electrochemistry Part I. Heterocyclic, Aromatic and Aliphatic Amines Adsorbed on the Anodized Silver Electrode |journal = [[Journal of Electroanalytical Chemistry]] | volume = 84 | pages =1–20  | date = 1977 | doi = 10.1016/S0022-0728(77)80224-6 }}</ref> A  comprehensive theory of the effect was given by Lombardi and Birke.<ref>{{cite journal|author=Lombardi JR|author2=Birke RL|title= A Unified Approach to Surface-Enhanced Raman Spectroscopy |journal = [[Journal of Physical Chemistry C]]| volume=112|issue=14|pages= 5605–5617 | date = 2008 | doi = 10.1021/jp800167v}}</ref>
* ''Surface-enhanced resonance Raman spectroscopy (SERRS)'' – A combination of SERS and resonance Raman spectroscopy that uses proximity to a surface to increase Raman intensity, and excitation wavelength matched to the maximum absorbance of the molecule being analysed.
* ''[[Tip-enhanced Raman spectroscopy]] (TERS)'' – TERS combines the chemical sensitivity of SERS with the high spatial resolution of scanning probe microscopy techniques, enabling chemical imaging of surfaces at the nanometre length-scale with high detection sensitivity.<ref>{{Cite journal |last1=Shi |first1=Xian |last2=Coca-López |first2=Nicolás |last3=Janik |first3=Julia |last4=Hartschuh |first4=Achim |date=2017-02-17 |title=Advances in Tip-Enhanced Near-Field Raman Microscopy Using Nanoantennas |url=http://dx.doi.org/10.1021/acs.chemrev.6b00640 |journal=Chemical Reviews |volume=117 |issue=7 |pages=4945–4960 |doi=10.1021/acs.chemrev.6b00640 |pmid=28212025 |issn=0009-2665}}</ref> It uses a metallic (usually silver-/gold-coated AFM or STM) tip to enhance the Raman signals of molecules situated in its vicinity. The spatial resolution is approximately the size of the tip apex (20–30&nbsp;nm). TERS has been shown to have sensitivity down to the single molecule level <ref>{{Cite journal|last1=Hou|first1=J. G.|last2=Yang|first2=J. L.|last3=Luo|first3=Y.|last4=Aizpurua|first4=J.|last5=Y. Liao|last6=Zhang|first6=L.|last7=Chen|first7=L. G.|last8=Zhang|first8=C.|last9=Jiang|first9=S.|date=June 2013|title=Chemical mapping of a single molecule by plasmon-enhanced Raman scattering|journal=Nature|volume=498|issue=7452|pages=82–86|doi=10.1038/nature12151|pmid=23739426|issn=1476-4687|bibcode=2013Natur.498...82Z|s2cid=205233946}}</ref><ref>{{Cite journal|last1=Lee|first1=Joonhee|last2=Tallarida|first2=Nicholas|last3=Chen|first3=Xing|last4=Liu|first4=Pengchong|last5=Jensen|first5=Lasse|last6=Apkarian|first6=Vartkess Ara|date=2017-10-12|title=Tip-Enhanced Raman Spectromicroscopy of Co(II)-Tetraphenylporphyrin on Au(111): Toward the Chemists' Microscope|journal=ACS Nano|volume=11|issue=11|pages=11466–11474|doi=10.1021/acsnano.7b06183|pmid=28976729|issn=1936-0851|doi-access=free}}</ref><ref>{{Cite journal|last1=Tallarida|first1=Nicholas|last2=Lee|first2=Joonhee|last3=Apkarian|first3=Vartkess Ara|date=2017-10-09|title=Tip-Enhanced Raman Spectromicroscopy on the Angstrom Scale: Bare and CO-Terminated Ag Tips|journal=ACS Nano|volume=11|issue=11|pages=11393–11401|doi=10.1021/acsnano.7b06022|pmid=28980800|issn=1936-0851|doi-access=free}}</ref><ref>{{Cite journal|last1=Lee|first1=Joonhee|last2=Tallarida|first2=Nicholas|last3=Chen|first3=Xing|last4=Jensen|first4=Lasse|last5=Apkarian|first5=V. Ara|date=June 2018|title=Microscopy with a single-molecule scanning electrometer|journal=Science Advances|volume=4|issue=6|pages=eaat5472|doi=10.1126/sciadv.aat5472|pmid=29963637|pmc=6025905|issn=2375-2548|bibcode=2018SciA....4.5472L}}</ref> and holds some promise for [[bioanalysis]] applications <ref>{{cite journal | last1 = Hermann | first1 = P | last2 = Hermeling | first2 = A | last3 = Lausch | first3 = V | last4 = Holland | first4 = G | last5 = Möller | first5 = L | last6 = Bannert | first6 = N | last7 = Naumann | first7 = D | year = 2011 | title = Evaluation of tip-enhanced Raman spectroscopy for characterizing different virus strains | journal = Analyst | volume = 136 | issue = 2| pages = 1148–1152 | doi = 10.1039/C0AN00531B | pmid = 21270980 | bibcode = 2011Ana...136.1148H }}</ref> and DNA sequencing.<ref name="He 753–757"/> TERS was used to image the vibrational normal modes of single molecules.<ref>{{Cite journal|last1=Lee|first1=Joonhee|last2=Crampton|first2=Kevin T.|last3=Tallarida|first3=Nicholas|last4=Apkarian|first4=V. Ara|date=April 2019|title=Visualizing vibrational normal modes of a single molecule with atomically confined light|journal=Nature|volume=568|issue=7750|pages=78–82|doi=10.1038/s41586-019-1059-9|pmid=30944493|issn=0028-0836|bibcode=2019Natur.568...78L|s2cid=92998248}}</ref>
* ''[[Surface plasmon polariton]] enhanced Raman scattering (SPPERS)'' – This approach exploits apertureless metallic conical tips for near field excitation of molecules. This technique differs from the TERS approach due to its inherent capability of suppressing the background field. In fact, when an appropriate laser source impinges on the base of the cone, a TM0 mode<ref>{{cite journal|last1=Novotny|first1=L|last2=Hafner|first2=C|title=Light propagation in a cylindrical waveguide with a complex, metallic, dielectric function|journal=Physical Review E|volume=50|issue=5|pages=4094–4106|date=1994|doi=10.1103/PhysRevE.50.4094|pmid=9962466|bibcode = 1994PhRvE..50.4094N }}</ref> (polaritonic mode) can be locally created, namely far away from the excitation spot (apex of the tip). The mode can propagate along the tip without producing any radiation field up to the tip apex where it interacts with the molecule. In this way, the focal plane is separated from the excitation plane by a distance given by the tip length, and no background plays any role in the Raman excitation of the molecule.<ref>{{cite journal|display-authors=4|last1=De Angelis|first1=F|last2=Das|first2=G|last3=Candeloro|first3=P|last4=Patrini|first4=M|last5=Galli|first5=M|last6=Bek|first6=A|last7=Lazzarino|first7=M|last8=Maksymov|first8=I|last9=Liberale|first9=C|title=Nanoscale chemical mapping using three-dimensional adiabatic compression of surface plasmon polaritons|journal=Nature Nanotechnology|volume=5|issue=1|pages=67–72|date=2010|doi=10.1038/nnano.2009.348|pmid=19935647|bibcode = 2010NatNa...5...67D |last10=Andreani|first10=Lucio Claudio|last11=Di Fabrizio|first11=Enzo}}</ref><ref>{{cite journal|display-authors=4|last1=De Angelis|first1=F|last2=Proietti Zaccaria|first2=R|last3=Francardi|first3=M|last4=Liberale|first4=C|last5=Di Fabrizio|first5=E|title=Multi-scheme approach for efficient surface plasmon polariton generation in metallic conical tips on AFM-based cantilevers|journal=Optics Express|volume=19|issue=22|pages=22268–79|date=2011|doi=10.1364/OE.19.022268|pmid=22109069|bibcode = 2011OExpr..1922268D |doi-access=free}}</ref><ref>{{cite journal|display-authors=4|last1=Proietti Zaccaria|first1=R|last2=Alabastri|first2=A|last3=De Angelis|first3=F|last4=Das|first4=G|last5=Liberale|first5=C|last6=Toma|first6=A|last7=Giugni|first7=A|last8=Razzari|first8=L|last9=Malerba|first9=M|title=Fully analytical description of adiabatic compression in dissipative polaritonic structures|journal=Physical Review B|volume=86|issue=3|page=035410|date=2012|doi=10.1103/PhysRevB.86.035410|bibcode = 2012PhRvB..86c5410P |last10=Sun|first10=Hong Bo|last11=Di Fabrizio|first11=Enzo}}</ref><ref>{{cite journal|display-authors=4|last1=Proietti Zaccaria|first1=R|last2=De Angelis|first2=F|last3=Toma|first3=A|last4=Razzari|first4=L|last5=Alabastri|first5=A|last6=Das|first6=G|last7=Liberale|first7=C|last8=Di Fabrizio|first8=E|title=Surface plasmon polariton compression through radially and linearly polarized source|journal=Optics Letters|volume=37|issue=4|pages=545–7|date=2012|doi=10.1364/OL.37.000545|pmid=22344101|bibcode = 2012OptL...37..545Z }}</ref>