Raman Spectroscopy of NanostructuresRaman studies of nanostructuresOutlineSlide 4Optical optionsLight scatteringElastic scatteringInelastic scatteringSlide 9Energy scalesScattering processesScattering spectroscopyCharacteristic frequenciesSlide 14Chemical fingerprintingSlide 16Slide 17Slide 18Implementation issuesSlide 20Raman instrumentationSlide 22micro-Raman spectroscopySlide 24Raman scattering in crystalsSlide 26Wurtzite crystal modesSlide 28Raman mode polarizationSlide 30Slide 31Summary: Raman in crystalsSlide 33Electronic resonanceResonant Raman - GaAs nanocrystalsResonant Raman - GaAs nanocrystalsResonant RamanSlide 38Slide 391974: Pyridine on rough silver surface1974: Pyridine on Ag surfaces1977: Pyridine on Ag surfacesSkip forward 30 years…Slide 44Slide 45EE235 16 March 20091Raman Spectroscopy of NanostructuresEE 23516 March 2009EE235 16 March 20092Raman studies of nanostructuresMultiple meanings!Study of the nanostructures themselvesStudy of a chemical analyte on top of nanostructuresEE235 16 March 20093OutlineGeneral principlesLight scatteringElectronic and vibrational energy levelsExperimental implementationRaman scattering in crystalsResonant enhancementSignal enhancementResonant enhancementSurface enhancement (SERS)EE235 16 March 20094OutlineGeneral principlesLight scatteringElectronic and vibrational energy levelsExperimental implementationRaman scattering in crystalsResonant enhancementSignal enhancementResonant enhancementSurface enhancement (SERS)EE235 16 March 20095Optical optionsRaman scattering: Inelastic light scatteringAbsorbedTransmittedReflectedScatteredIncident light 000000ElasticInelasticEE235 16 March 20096Light scattering Rayleigh: Elastic scatteringwww.phy.cam.ac.uk/history/historypictures/LordRayleigh.jpgwww.iisc.ernet.in/images/CVRaman.gif Raman: Inelastic scatteringEE235 16 March 20097Elastic scatteringElectronic ground stateElectronic excited stater0Induced polarizationP = E (: polarizability)- +-+rDipole re-radiates at same frequency iE=E0cos(it)“Virtual state”EE235 16 March 20098Frequency of scattered light differs from the incident light by an amount corresponding to a normal mode of the mechanical oscillatorvibisInelastic scatteringm1 m2- +Vibration changes polarizability:- +- +vib0Qmax|Qmin tQQvibcos0Periodic displacementr0EE235 16 March 20099Induced polarizability change from vibrations(first order)EQdQdEEPoindRayleigh term: ... QdQdoClassical scattering model tQQvibcos0 tEEicos0Taylor expand)cos(00tEPiind ttQQEPvibivibiQind)cos()cos(000Raman term: Stokes Anti-StokesEE235 16 March 200910Energy scalesVisible light (red), 620 nm ↔ 2 eV ↔ 484 THz ↔ 16130 cm-1CO2 symmetric mode, 1335 cm-1↔ 41 THz ↔ 0.165 eV ↔ 7.5 m Vibrational energies << Optical energiesO C OEE235 16 March 200911Scattering processesElectronic ground stateElectronic excited stateRayleighVibrational levelsFIR- MIREnergyRamanStokes Anti-StokesisvibisvibisEE235 16 March 200912Scattering spectroscopyMeasure the wavelength-shifted scattered light as a probe of vibrational energiesKnown characteristic bond frequenciesEE235 16 March 200913Characteristic frequenciescm-1EE235 16 March 200914Scattering spectroscopyMeasure the wavelength-shifted scattered light as a probe of vibrational energiesKnown characteristic bond frequencies“Chemical fingerprinting”: every molecule has a unique vibrational spectrumEE235 16 March 200915Chemical fingerprintingIR absorption and Raman scattering complementarySame transitions, different selection rules - all has to do with symmetrycm-1EE235 16 March 200916Scattering spectroscopy“Chemical fingerprinting”: every molecule has a unique vibrational spectrumCheck the aptly-named Journal of Raman spectroscopy:Chemical analysis tool!http://www3.interscience.wiley.com/journal/3420/homeEE235 16 March 200917OutlineGeneral principlesLight scatteringElectronic and vibrational energy levelsExperimental implementationRaman in crystalsResonant enhancementSignal enhancementResonant enhancementSurface enhancement (SERS)EE235 16 March 200918Scattering processesElectronic ground stateElectronic excited stateRayleighVibrational levelsFIR- MIREnergyRamanStokes Anti-StokesisvibisvibisIn principle, any laser wavelength shorter (i.e. higher energy) than the vibrational mode of interest will work as a sourceEE235 16 March 200919Implementation issuesWeak Raman-scattered light close to strong source laser + Rayleigh-scattered lightNeed high rejection of unwanted “carrier frequency” light very close to Raman-shifted signal lightO C O0.165 eV2 eVeVIntensityEE235 16 March 200920Implementation issuesNeed: very clean source lineVery sharp edge discriminationSpectrometerDispersive stageDetectorNarrow excitation source SampleFilterEE235 16 March 200921Raman instrumentationMulti-stage spectrographsSpectrometer3Spectrometer2Spectrometer1Disperse Recompress DisperseInputDetectorExcellent stray light rejection, can measure very close to laser line (few cm-1)Large and expensiveEE235 16 March 200922Raman instrumentationSingle spectrometer, strong filtersLongpass filter in collection pathLine-pass filter (shortpass for tunable lasers) for laser line clean-up in excitation pathFinal resolution depends on spectrograph and detector; filter limitation ~tens of cm-1InputDetectorSpectrometersemrock LP-633RELong-pass filter to transmit only Stokes-scattered lightEE235 16 March 200923micro-Raman spectroscopySpatial imaging down to diffraction limit of excitation wavelength used Confocal setup - small sample volume probedVibrational map of sampleJobinYvonEE235 16 March 200924OutlineGeneral principlesLight scatteringElectronic and vibrational energy levelsExperimental implementationRaman scattering in crystalsResonant enhancementSignal enhancementResonant enhancementSurface
View Full Document
Unlocking...