To get the good vibrational-mode signal in the Raman spectra perticularly in thin film. The thicknees, density and particle size should be of higher side. It is just like XRD patterns if you have the higher crystallaniety and higher particle size, sample shows nice intense XRD peak. This mechanism also work for vibration-mode in the Raman spectra.

I am not really sure that the problem is the thickness of your film. This is one possibility but not te only one. The texture of your film may be also affecting the intensity of your Raman signal due to light scattering.

What is the composition of your film? What is the substrate?

The signal(s) you are looking for should be intense?

I have some experimental suggestions on the acquisition of your spectra to enhamce the signals intensity:

1) Accumulate the spectra for more time until you get an appropriate intensity.

2) Enhance (if possible) the laser intensity to a limit value were your material is not sensible.

3) Try to make some analysis under or overfocus. Sometimes, specially with thin films, the focus affects a lot the intensity of te signals.

you can try SERS, which is highly sensitive. deposit the thin film on a suitable metal thin film (or metal nanopartilces) then do Raman measurements.

The question is too vague to get a repsonse which goes beyong simple advice. Specify wich fils are you referring to, the type of substrate, the condition of Raman collection (are you in resonance or far from it etc)

I agree with Mathias I think 300 nm is ways enough to get a good signal... you may have an idea about the signal you are aiming to measure.

* Try a thicker layer to be your reference.

* Calibrate your spectrometre with Silicium or other material to make sure that it is not a hardware problem.

* Clean the surface of your sample.

Dear Andre,

The problem you mentioned is not uncommon. Raman signal in thin film samples may be relatively weaker for several reasons, as also written by M. Strauss, viz. the roughness, texture, crystalline nature of the film, and so on. Thickness should also will be a crucial parameter in Raman signal because a sufficient scattering volume would be necessary to get 'sufficient' Raman signal.

Just to add a few suggestions/comments which you may think of:

(1) Is the substrate Raman-background dominating your thin film Raman-signal? (What is the substrate? If it dominates, you may think of choosing another substrate if possible). Moreover, if possible, increase the film thickness.

(2) You may consider using micro-Raman with objective having high magnification and numerical aperture (for a tighter focus and reduced depth-of-focus),

(3) You may consider using laser excitation of low wavelength (say, UV) which would help reducing the penetration depth,

(4) As also suggested by M. Strauss, consider integrating for longer time at a higher laser power (with out affecting the sample)

Regards,

Surajit

To get the good vibrational-mode signal in the Raman spectra perticularly in thin film. The thicknees, density and particle size should be of higher side. It is just like XRD patterns if you have the higher crystallaniety and higher particle size, sample shows nice intense XRD peak. This mechanism also work for vibration-mode in the Raman spectra.

Your problem should be more precisely defined for an accurate response. The intensity of Raman signals is depending on many factors (and one of them are the quantity of sample probed among other such as structural defects, cristallinity etc ...). Usually, when a material does not lead to a sufficient signal, you can change the wavelength of the incident beam (see Rayleigh's Law or to be in resonance ) or increase the power of the laser.

Raman signal intensity will depend on whether the thin film is crystal or amorphous, cross-section etc. If it is crystal, orientation will have make difference too. To enhance the signal intensity, from my experiences, 1)change wave length of the excitation light (Ar+ laser line) towards UV to see if you can get resonance, if you do, big enhancement may occur; 2) use multiple scan, the S/N is proportional to the square root of the scan times.

Take a look at this paper :

Substrate-induced modulation of the Raman scattering signals from self-assembled organic nanometric films

A Baptiste, A Bulou, JF Bardeau, J Nouet, A Gibaud… - Langmuir, 2004

I tried it on polymer thin film and it works...

the spot size of a He-Ne laser is between 8-9 um, then if you are not using the correct optical arrangement (magnification, focus point, signal-noise, etc), you will get too much information from the substrate.

If the sample is not destroyed you may: increase the laser power, increase the number of accumulation and generally play with the time to acquire the spectrum and eventually with the optical variables (lens and aperture).

Hi,

The lose of Raman signal in thin films materials is due to the focalised light dimensions. First you must use a higher laser light frequency and a higher NA probe lense. May be lateral scans could be more interesting with some deconvolution into the recorded signal.

Yours

Hi

The best way is to confined light inside the film. This way only the signal coming from the film is extracted with high internal reflections

The technique is then called Waveguided Raman Spectroscopy.

It will require to couple light into the film with a high Refractive Index prism and also that the RI of the film is higher than the one of its substrate !

You can find many papers about it including a old one of ours in JOURNAL OF MOLECULAR STRUCTURE Volume: 481 Pages: 169-178

Try to choose a laser with a frequency close to an interband resonance of your film

Hi, 300nm is a reasonable thickness to get intense Raman signal but what is more relevant is materials and the incident wavelength of the laser which you did not mention. So all of us can only guess that its is far from resonance or the film is not crystalline.....etc. Provide more details if you want an solution for your problem.

Good luck.

Thanks everybody for the answers!

I would like to inform some important things about the equippament and how the analysis were done.

In relation of the equippament:

The Raman spectra were carried out using a Sanning Near-field Optical Microscopy Witec Alpha 300 S which features a Confocal Microscope (CM), a Scanning Near-Field Optical Microscope (SNOM) and an Atomic Force Microscope (AFM) in the same instrument and the machine is equipped by a Laser Nd: YAG with wavelength 514 nm and 4cm-1 of spectral resolution. I can not change the wavelength light because we have only the Argon laser. The calibration of the equippament is done twice or three times a week with silicon, not every time before doing analysis.

In relation of the analysis:

1. It was used the maximum of power of the laser, e.g., 50mW and i can't change the power because we don't have a filters system;

2. The lens of the microscope used is 20x/0.4/0 WD 3.8.

3. The incidence of the laser was perpendicular to the surface of the thin films;

4. It was accumulated the spectra for long time;

In relation of the material:

1. The thin films are based on perovskite-type oxides (titanates and stannates);

2. I was expecting good signals for the lattice formers (Ti and Sn) of the materials, but the noise in the Raman spectra is so high. and it seems that it isoverlapping and reducing the signal;

3. For the preparation of the thin films, different substrates were used (amorphous silica, LAO and sapphire single crystals) and the substrates vibrational-modes are more intense than the thin films ones. A broad fluorescence band was also observed in all the samples.

4. By using different kinds of substrates, different types of growth were observed (polycrystalline, textured and epitaxial growths) with nice crystallinity for all the samples.

5. I didn`t prepare a thicker filme to be my reference, i have prepared also powders of my material and i got different intensity of Raman vibrational-mode intensities and the conditions of analysis were the same for all the samples;

Even doing all of that, the mean signals were not so good. So I will try some different as it was suggested to see if i get better results.

Thanks again.

André

Guillaume Vignaud and Jean-Marie Nedelec thanks for the papers suggested!

Hi, With titanate thin-films using 514 nm laser it is difficult to get intense Raman signal. You could probably try low temperature (depending on your sample's phase transition)

The Raman spectra were done at room temperature, i didn`t vary the temperature during the analysis. For information, there is no phase transition in the my materials, neither in the titanates neither in stannates.

So if you have low temperature facility try reducing the temperature to get more intensity

I wouldn't like to change the temperature of the analysis because i didn`t do it for any other characterization of my materials.

@André -- Titanates and stanates are a large family of materials, can't really speak for all of them. However, things like sodium titanates should give decent Raman at 514nm excitation and 300nm thick film should be workable. Depending on crystallinity of the film you may get average to good Raman signal using NSOM. Sometimes its easier to get rid of substrate signals if you use reflection mode, provided your instrument carry such an attachment. Alternative is to try TERS, that should be easily doable given the instruments you are using. However I suggest you try doing micro Raman in an environmental SEM to investigate homogeneity and quality of Raman signals from your sample.

Palash, I know titanates and stannates are a large family and each one presents a specific behaviour because mainly because of their different crystalline strucutres and also because of the defects presented on them, so for being more precise I am working with alkaline-earth (Ca, Sr and Ba) Stannates and Titanates and their solid solutions. The Raman vibrational-modes for the solid solution, for instance, were not so evident in comparison to the pure titanates or stannates when done in the same conditions of analysis. I don't know about TERS analysis, what this method consist? How could i work with it? Could this method change the characteristics of my samples?

TERS = Tip Enhanced Raman Spectroscopy where you use a tip that is coated with silver or gold (typically). Talk to your instrument tech support and see if they have any TERS tip, I am sure though there are other vendors who will have compatible tips. Analysis part will be same as before. Like before TERS should not change anything in your sample.

Palash Gangopadhyay,

would I need another suitable acessory for doing TERS or only the this special tip for the analysis?

Seyyed Ali Shojaee,

unfortunately there is no possibility to change the polarization with this machine.

Dear Andre,

I am preparing films around 300 nm, But i have no idea how to prepare it would be reasonable. Could you please tell me how to prepare a film around 300nm? What technique did you use to determine the thickness of the film?