Raman spectroscopy Information from Raman Spectroscopy characteristic Raman frequencies composition of material e.g. MoS 2 , MoO 3 changes in frequency of Raman peak stress/strai n state e.g. Si 10 cm -1 shift per % strain polarisation of Raman peak crystal symmetry and orientation e.g. orientation of CVD diamond grains width of Raman peak quality of crystal e.g. amount of plastic deformation parallel perpendicular intensity of Raman peak amount of material e.g. thickness of transparent coating
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Raman spectroscopy - Chemistry · Raman spectroscopy Information from ... width of Raman peak quality of crystal ... coupling optics creates two additional conjugate image planes
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Raman spectroscopy
Information from Raman Spectroscopy
characteristicRaman frequencies
composition ofmaterial
e.g. MoS2,MoO3
changes infrequency ofRaman peak
stress/strainstate
e.g. Si 10 cm-1 shift per% strain
polarisation ofRaman peak
crystal symmetry andorientation
e.g. orientation of CVDdiamond grains
width of Ramanpeak
quality ofcrystal
e.g. amount of plasticdeformation
parallel
perpendicular
intensity ofRaman peak
amount ofmaterial
e.g. thickness oftransparent coating
The coupling of a Raman spectrometer withan optical microscope provides a number ofadvantages:
1) Confocal Light collection
2) High lateral spatial resolution
3) Excellent depth resolution
4) Large solid collection angle for theRaman light
Collecting the light
The basic function of a Raman system
• Deliver the laser to the sampling point– With low power loss through the system
– Illuminating an area consistent with sampling dimensions
– Provide a selection/choice of laser wavelengths
• Collect the Raman scatter– High aperture
– High efficiency optics
– High level of rejection of the scattered laser light
• Disperse the scattered light– Short wavelength excitation requires high dispersion spectrometers
• Detect the scattered light
• Graphically / mathematically present the spectral data
Laser wavelength selection concerns for classical Raman
As the laser wavelength gets shorter
Raman scattering efficiency increases
The risk of fluorescence increases (except deep UV)
• Illuminate a Sample with an Intense Single Frequency Light Source
• Measure the relative frequency shift of the inelastically scattered light
Generic Raman system flow diagram
detector
laser sample
diffractiongrating
Raman microscopy: Dispersive instrument basics
Multi-channelDetector
sampleHNF
laser
slitGrating
100
200
300
Cou
nts
System basics:
1) laser
2) Rayleigh rejection filter
3) grating (resolution)
4) CCD detector
18 8 -2 -12
14
4
-6
Dispersion
0 200
Intensity
The Renishaw Raman spectrometer is an imagingspectrograph
on-axis stigmatic design with a -70 oC Peltier cooledCCD detector. Advanced inverted mode, deep depletionand UV optimized detectors are available as options.
We can easily demonstrate the high quality imaging andsystem performance advantages as seen in the imageof the Si 520 cm-1 Raman band on the CCD detector.
Image of Si 520 cm-1
band
pixel number
pixe
l num
ber
Research Grade MicroRaman Spectrometer
Delivering the light
90 and 180 degree scattering
Porto notation
90 degree scattering x(z,z)y and
180 degree scattering x(z,z)x’
excitation direction (excitation polarization,
scattered polarization) scattering direction
The actual excitation and collection directions are the range of angles0 to γ
γ
mag N.A 2*γ(deg)
x5 0.12 11.5
x20 0.4 29
x50 0.75 97.2
x100 0.9 128.3
x
x’
y
γ
Delivering the light
Delivering the light (180 degree backscattering)
excitation
Raman
> 90% efficient
Holographic notch or edge filter
2
1
Delivering the light
Raman microscope systems typicallyoperate in with the excitation direction andcollected Raman scattering directionseparated 1800. This mode of collectionand excitation is referred to as “back-scattering”.
Typically back-scattered Raman collectionnecessitate special optics that operateboth as a Rayleigh filter and as a lasermirror. Holographic notch filters andspecial dielectric mirrors are often theoptics of choice, since they minimize laserintensity loss and Raman scattering lossesthat would otherwise occur when utilizing apartial reflector.
Relative laser excitation efficiency andRaman transmission efficiencies can beeasily calculated for most configurations
The minimum laser focus isdetermined by:
1. the focusing optic N.A.2. laser wavefront (distortionor M2)3. How the back aperture ofthe objective is filled
Laser focused spot size
Delivering the light
Raman spectroscopy utilizing a microscopefor laser excitation and Raman lightcollection offers that highest Raman lightcollection efficiencies.
When properly designed, Ramanmicroscopes allow Raman spectroscopywith very high lateral spatial resolution,minimal depth of field and the highestpossible laser energy density for a givenlaser power.
It is important to note that the laserminimum focused spot size is not typicallythe same size as the coupled Ramanscattered spot size.
The minimum laser focused spot size isoften compromised by improperly matchingthe laser size to the back aperture of anobjective and by wavefront errors inherentto the laser and introduced by the laserdelivery optics.
Without consideration of the laser mode quality and wavefront, orsource size the minimum laser focused spot for any optic isdescribed by equation 1:
Minimum laser focus
1) Excitation wavelength: λ
2) effective numerical aperture : N.A.
3) dl is determined by twice the Rayleigh criteria of the adjacent distancerequired to spatially resolve the presence of an identical size spots
The laser focused spotsize does not necessarilydefine the lateral spatialresolution of the Ramansystem. The lateralspatial resolution, is oftendiscussed in terms of theRayleigh criteria for thecollected Raman light.The Rayleigh criteriarequires that the distancebetween two pointssources of light of equalintensity be greater thanthe distance from thepeak to the first airy diskminimum. Completediscrimination of twoadjacent materials occursat twice the Rayleighcriteria
Laser focused spot size
Delivering the light
Airy disk pattern
0
0.2
0.4
0.6
0.8
1
1.2
-2.5 -1.5 -0.5 0.5 1.5 2.5
Distance/microns
Rel
ativ
e In
ten
sity
objective N.A.: 0.75
Excitation wavelength/nm: 514.5
Separation distance 0.44 um
Diffration limited focus
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
-4 -3 -2 -1 0 1 2 3 4
distance/microns
rela
tiv
e e
ne
rgy
de
ns
ity
x50 (0.75) x20 (0.40) x5 (0.10)
It’s important toremember that theobjective used todeliver the laser lightaffects the laserenergy density.
The relative energydensity and peakpower for the X5,X20 and X50objectives areshown relative tothe X50 objective.
The peak energydensity decreases by~50% for the X20and 87% for the X5objective
Airy disk calculation for X5, X20 and X50 objective calculated for 514.5 nm l
2
*53.2
=
ll D
fh λ
The system laser focus depth (hl) is determined by:
1) Excitation wavelength: λ
2) Microscope objective focal length : f
3) Effective laser beam diameter at the the objective backaperture: Dl
Laser focus and depth of field
Delivering the light
DO NOT CONFUSE LASER FOCUS DEPTH WITH CONFOCAL COLLECTION DEPTH
4
3*21.3
=
ll D
fλτ
The system laser focus volume (τl) is determined by:
1) Excitation wavelength: λ
2) Microscope objective focal length : f
3) Effective laser beam diameter at the the objective backaperture: Dl
Laser focus and illuminated volume
Delivering the light
DO NOT CONFUSE LASER FOCUS VOLUME WITH CONFOCAL COLLECTION VOLUME
Solid collection angle is proportional to (N.A.)2 not 1/(f/#)^2
Measured vs. calculated
σ = 4/π *(N.A.)2
Collecting the light
Oil immersion objectiveincrease is likely due toreduced reflection losses
Relative collection volume
0
1
2
3
4
5
6
400 500 600 700 800 900
Wavelength (nm)
Re
lati
ve
vo
lum
e
Macro-sampling isimproved with longerwavelength excitation
Collecting the light
4
3*21.3
=
ll D
fλτ
The system laser focus volume (τl)
Extended scanning(Renishaw patent EP 0638788)
From the Renishaw Raman software the user can select:
• a fixed grating measurement with a spectrum 'window'of 400 cm-1 to 1000 cm-1 (configuration dependent)
• a unique 'extended scanning' facility allowing the userto choose any Raman shift range up to about 10000 cm-1
(configuration dependent). Essential for extended rangescanning for Raman and photoluminescence
Extended scanning is implemented by moving the grating andthe charge generated on the CCD camera synchronously.
This feature is NOT available on any other instrument and isKEY to system performance
CCD Basics
Extended scanning: how it works
Extended scanning vs stitched scanning
Advantages of extended scanninguse a single grating no stitching required and no “discontinuities” at joinsflexible wavenumber coverage (up to 10000 cm-1 )pixel-to-pixel variation is averaged out - enhancing noise reductionno compromise on resolution across the scanned rangesimple to use
Adequate S/N
To acquire useful Raman spectra all you need is:
Sufficient spectral and spatial resolution and coverage
Repeatability
The ability to collect and detect enough photons to distinguish theirelectronic signal above system generated noise before the sample changesor dies.
The ability to separate spectral peaks narrower than the narrowestanticipated spectral features of your sample
The ability to collect all of the spectral data required for the analysis
The ability to optically restrict the data collection to an area / volumesmall enough to eliminate acquisition of unwanted spectral data ofnearby substances
The ability to consistently get the same right or wrong values
Confocal Raman microscopy without pinhole optics
The use of a stigmatic spectrograph and stigmatic microscope-spectrometercoupling optics creates two additional conjugate image planes at the slit andCCD eliminating the need for pinhole optics!
1. Read noise: How many photon generated electrons are required to achievea signal level greater than the read noise?
Raman systems that require off chip binning increase read noise to thesquare root of the number of pixels binned.
2. Dark Charge rate: How long can you integrate before the binned CCDpixels generate a charge equivalent to the read noise?
At the integration time that the dark charge signal contributes to the noiseeither through shot noise or uniformity of response, it must be subtracted.
3. Uniformity of response: How many photon generated electrons can bemeasured before the shot noise is exceeded by the non-uniformity ofresponse?
At the point uniformity of response noise exceeds shot noise the pixels mustbe read out individually (without binning) for response correction.
Optimal CCD operating temperature
The best CCD temperature operation is determined by the CCD darkcharge rate and the requirements for operation near the detectorlimit of ~1050 nm
• Low temperatures decrease the CCD dark charge rate. The CCD darkcharge rate decreases ~50% for each 6-9 degree decrease in operatingtemperature.
e*T*122 QdQd T
-64003
0
=
Dark Charge e/p/s
0.001
0.01
0.1
1
10
-80 -60 -40 -20 0
Temperature C
Dar
k C
harg
e ra
te e
/p/s
Qd dark charge rate (e/p/s) atoperating temperature T
Qdo - dark charge rate at reference
temperature (typically 23-25 oC)
A1A-CCD02-06 Deep Depletion Sensor Issue 3, January2000
Select response uniformity rather than QE
Renishaw CCDtypicalresponsecurve. Thepeak QE is~50%, butthe responseuniformity isan order ofmagnitudebetter thanwith higher QECCD chips
If the wavelength of the laser is close to an excited electronic state of a bond in molecule, i.e. where it is strongly absorbed or fluoresces, the signal enhancement can be increased by a factor between 100 and 10,000.
Resonance Raman
Advantage: You can select a wavelength to enhance the sensitivity toa particular type of bond or vibrations.
In the study of carbon nanotubes multiple laserwavelengths are used to increase sensitivity tospecific vibrational modes within a molecule.
0
.2
.4
.6
.8
1
Cou
nts
500 1000 1500 2000 Raman Shift (cm-1)Time: 10secs
File # 1 : RICE(8)C
Solid Film Laser: 12834.5cm-1
244 nm and 780 nm excited Raman spectra of nanotubes
244 nm
RBMC-C
Resonance Raman spectroscopy of SWNT
Since the Raman spectral measurment for nano-tubes is typically a resonance Raman measurementthe excitation wavelength can dramatically affect the spectral feature intensity and shift.
785 nm
Resonance Raman of SWINTs
500 1000 1500 2000 2500 3000 3500
Raman shift / cm-1
0
5000
10000
15000
20000
25000
Cou
nts
514 nm excited and 488 nm excited Raman spectra MWNT material
Raman spectroscopy of MWNT
Raman and Fraud
Lewis, I. R.; Edwards, H. G. M., Lewis, I. R.; Edwards, H. G. M., Handbook of Raman Spectroscopy: From the Research Laboratory to theHandbook of Raman Spectroscopy: From the Research Laboratory to theProcess Line, Process Line, Marcel Dekker, New York: 2001.0Marcel Dekker, New York: 2001.0
Ivory or Plastic?
Lewis, I. R.; Edwards, H. G. M., Lewis, I. R.; Edwards, H. G. M., Handbook of Raman Spectroscopy: From the ResearchHandbook of Raman Spectroscopy: From the ResearchLaboratory to the Process Line, Laboratory to the Process Line, Marcel Dekker, New York: 2001.Marcel Dekker, New York: 2001.
The Vinland Map: Genuine or Forged?
Brown, K. L.; Clark, J. H. R., Brown, K. L.; Clark, J. H. R., Anal. Chem. Anal. Chem. 20022002, 74,, 74,3658.3658.
The Vinland Map: Forged!
Brown, K. L.; Clark, J. H. R., Brown, K. L.; Clark, J. H. R., Anal. Chem. Anal. Chem. 20022002, 74,, 74,3658.3658.
Surface-Enhanced Raman Scattering (SERS)
Haynes, McFarland, and Van Duyne, Anal. Chem.,77, 338A-346A (2005).
SERS: Surface Enhanced Raman Scattering
Discovered in 1977, Jeanmire et al. & Albrecht et al.
--Strongly increased Raman signals from molecules attachedto metal nanostructures
--SERS active substrates: metallic structures with sizeabout 10--100 nm (e.g. colloidal Ag, Au, roughened surfaces)
General contributions:
1)Electromagnetic field enhancement
2) Chemical ‘first layer’ effect
SERS Enhancement Mechanisms
Chemical Mechanism:Laser excites (a) new electronic states arising fromchemisorption or (b) shifted or broadened adsorbateelectronic states yielding a resonance condition.
• Short range (1-5 Å)• No roughness requirement• Contributes EF ~ 102 – 104
Electromagnetic Mechanism:LSPR induces large electromagnetic fields at roughenedmetal surface where molecules are adsorbed.
• Long range (2-4 nm)• Affected by all factors determining LSPR• Contributes EF > 104
Localized Surface Plasmon Resonance
The resonance results in (1) wavelength-selective extinction and (2) enhanced EMfields at the surface.
Spectral location of the LSPR is dependent upon particle size, shape, composition,and dielectric environment.
Localized Surface Plasmon Resonance
Non-resonant Resonant
1) Resonant λ is absorbed2) EM fields localized at nanoparticle surface
D3 produces the Klarite range of substrates for SurfaceEnhanced Raman Spectroscopy. Klarite substrates enable faster,higher accuracy detection of biological and chemical samples atlower detection limits for a wide range of applications inhomeland security, forensics, medical diagnostics andpharmaceutical drug discovery. Manufactured using techniquesfrom semiconductor processing Klarite substrates offer highlevels of enhancement and reliability.
References
Raman Microscopy: Developments and ApplicationsRaman Microscopy: Developments and Applications, G., G. Turrell Turrell, J. Corset,, J. Corset, eds eds. (Elsevier Academic Press,. (Elsevier Academic Press,1996)1996)
Raman Spectroscopy for Chemical AnalysisRaman Spectroscopy for Chemical Analysis, R.L., R.L. McCreery McCreery (Wiley (Wiley Interscience Interscience, 2000)., 2000).
Raman Technology for TodayRaman Technology for Today’’ss Spectroscopists Spectroscopists, 2004 Technology primer, Supplement to Spectroscopy, 2004 Technology primer, Supplement to Spectroscopymagazine.magazine.
FT Raman spectroscopy, P. Hendra et al., Ellis Horwood.
Raman and IR spectroscopy in biology and chemistry, J. Twardowski and P. Anzenbacher, Ellis Horwood.
Ch 18 in Skoog, Holler, Nieman, Principles of Instrumental Analysis, Saunders.
Raman Websites and On-Line Databases
www.spectroscopynow.com/coi/cda/landing.cda?chId=6&type=Education(many links including,An Introduction to Raman Spectroscopy: Introduction and Basic Principles, by J. Javier Laserna,)