Confocal Raman microscopy in 3-dimensional shape and composition

determination of heterogeneous systems

M. Pastorczaka,*, M. Wiatrowskia, M. Kozaneckia, M. Lodzinskib, J. Ulanskia

aDepartment of Molecular Physics, Technical University of Lodz, Zeromskiego street 116, 90-924 Lodz, PolandbDepartment of Mineralogy, Petrography and Geochemistry, AGH—University of Science and Technology, 30-059 Krakow, Poland

Received 19 November 2004; accepted 2 December 2004

Available online 29 January 2005

Abstract

Heterogeneous materials with special properties dedicated for specific applications attract a lot of interest due to the possibility of

designing tailor-made products with strictly desirable properties. Wide range of new heterogeneous systems requires fast and non-destructive

techniques for their characterization. The confocal 3D Raman micro-spectroscopy seems to be a powerful method for investigations of such

materials. Its major advantage is a wide range of information simultaneously gained from the Raman spectra about the chemical structure,

morphology, intermolecular interactions, internal stresses, etc. In this paper we focus our attention on the combination of two confocal

Raman microscopy techniques: depth-profiling and mapping. Presented results show that such 3D Raman micro-spectroscopy allows to

estimate shapes and sizes of distributed particles even for materials giving very weak Raman signal.

q 2004 Elsevier B.V. All rights reserved.

Keywords: Confocal Raman micro-spectroscopy; 3D mapping; Composites

1. Introduction

Modern technologies make use of wide variety of

heterogeneous materials (such as composites, blends, gels,

alloys, etc.) with very specific properties, dedicated for

special applications. This group of materials attracts so much

interest due to the possibility of designing and making the

products with strictly desirable chemical and physical

properties (optical, electrical, magnetic, mechanical,

thermal or chemical resistivity and many other) [1,2]. Most

of these materials have specific architecture on the micro- or

even nanometric level. Due to that for characterisation of

these materials techniques providing the information on their

structure with high spatial resolution are needed. Besides of

many advantages and technical improvements commonly

used techniques allowing to study materials at nanometric

level, such as: Atomic Force Microscopy (AFM), Scanning

Electron Microscopy (SEM), Secondary Ionic Mass

0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.molstruc.2004.12.001

* Corresponding author. Tel.: C48 42 6313216; fax: C48 42 6313218.

E-mail address: pasta@p.lodz.pl (M. Pastorczak).

Spectrometry (SIMS) have many limitations. For example,

AFM and SEM allow only for investigation of surface

morphology, with SIMS one can only characterise the

chemical composition without any information on the

supramolecular structure, etc. The confocal Raman micro-

spectrometers, although it has spatial resolution only around

1 mm, seems to be good complementary method. The major

advantage of Raman spectroscopy is a wide range of

information simultaneously gained from the Raman spectra

about the chemical structure, morphology, intermolecular

interactions, internal stresses, etc. [3–5]. Using spatially

resolved Raman techniques, such as mapping or depth-

profiling one can characterise distribution of the components

or stresses as well as the intermolecular interactions in

multiphase systems [6,7]. Using special procedures it is

possible to detect and to investigate particles even with

nanometric size [8,9].

In this paper we describe the combination of two Raman

techniques: depth-profiling and x–y mapping. The main goal

of this work was to check the possibility of using the

confocal Raman microscopy to produce 3D images; there-

fore, the studies were performed on some chosen model

Journal of Molecular Structure 744–747 (2005) 997–1003

www.elsevier.com/locate/molstruc

M. Pastorczak et al. / Journal of Molecular Structure 744–747 (2005) 997–1003998

heterogeneous systems: natural minerals (rock with

different inclusions) and synthetic polymer composites.

2. Experimental

2.1. Materials

The polystyrene/micro-diamonds composite (PS/C) was

studied as a model organic/inorganic system. It was

prepared by compressing the commercial atactic PS

(MwZ149,000) with synthetic monocrystalline diamond

particles (average diameter ca. 1 mm and the purity 99.9%)

from Sigma-Aldrich Co. After compression-moulding at a

temperature of 170 8C the sample was conditioned for 5 min

at 60 8C.

A second studied heterogeneous material was natural

beryl (emerald) with an inclusion. This sample originates

from Izumrudnyje Kopii mine in Russia.

2.2. Techniques

All the spectra were collected using the Jobin-Yvon

Raman spectrometer T64000 equipped with the Olympus

Fig. 1. 3D Raman mapping technique as a combination of Raman depth-profilin

confocal microscope and motorized x–y table. The 514.5 nm

Ar line (LEXEL 95 ion laser) was used as the excitation

light. To approach the best spatial resolution the measure-

ments were performed using 100! microscope objective

(NA 0.9) and confocal pinhole of 0.1 mm width. The laser

power and the time of sample excitation were adjusted to

obtain high quality Raman spectra, enabling creation of the

spectral maps. Calibration of the confocal system was

checked by performing the depth-profile for silicon wafer

(peak 521 cmK1). The obtained results (full width at half

maximum (FWHM) of the depth-profileZ2.5 mm) are

comparable with the data given in literature, e.g. published

by Hajatdoost [10,11].

2.3. Methodology

3D Raman mapping applied in our studies is a

combination of two known Raman techniques: depth-

profiling and x–y mapping. In the depth-profiling method

the Raman spectra are collected point-by-point along the

z-axis (normal to the surface of the sample). Raman

mapping consists in scanning (point-by-point) some

selected area of the sample in the x–y plane. In both

techniques the Raman spectra should be collected in

g method (bottom scheme) and Raman mapping method (upper scheme).

M. Pastorczak et al. / Journal of Molecular Structure 744–747 (2005) 997–1003 999

properly chosen range of wavelengths covering the selected

band characteristic for the component of interest. Depth-

profiles are obtained by integration of intensity of selected

Raman band. Marking borders in FWHM of selected peaks

is an usual procedure in creating spectral x–y maps. 3D

Raman mapping as a combination of depth-profiling and

mapping allows for creation of a 3D spectral view of the

sample, as it is shown schematically in Fig. 1.

3. Results and discussion

The PS/C composite was chosen as a model system for

its high optical transparency and strong intensity of

characteristic Raman peaks: 1332 cmK1 for diamond and

1002 cmK1 for polystyrene. Raman spectra of this system

were collected with 0.25 mm step in x and y directions and

0.5 mm step along the z-axis. The maps were prepared for

both the characteristic diamond peak and the characteristic

polystyrene peak as shown in insets Fig. 2. In Fig. 3a,

showing the 2D map of the sample surface, one can notice

the area corresponding to the micro-diamonds which shapes

are very similar to those visible by optical microscopy

(compare Fig. 2). We have found, that the diameter of the

micro-diamonds estimated using this Raman map, will be

compatible with the optical microscopy pictures and with

the information provided by Sigma-Aldrich Co. (ca. 1 mm),

Fig. 2. Microphotograph of the polystyrene/micro-diamond composite from opti

by arrows.

when we assume, that the grain boundary is situated in the

place where the diamond peak intensity fall down to 25% of

its maximum value.

Analysis of the maps corresponding to the planes below

the surface (Fig. 3b1–e1) shows that the diamond peak

intensity decreases down to 25% for map acquired for 2 mm

depths. It allows estimating the sizes of particular diamond

grains to be ca. 1–1.5 mm, which is in agreement with the

in-plane sizes estimated from the x–y map shown in

Fig. 3a1. The ‘negative’ maps based on the 1002 cmK1

polystyrene peak also reflect correctly the shape of the

diamond particles. For the depth 2 mm (map corresponds to

the plane under the diamond grain), the x–y map of

polystyrene peak is uniform (see Fig. 3e2), which means

that the presence of diamond grain above the confocal plane

does not interfere the Raman signal of the polystyrene

matrix below the grain.

Fig. 4 presents a micro-photography of the second

investigated object, the natural inclusion in the beryl crystal.

Using Raman micro-spectrometry it was found that the

inclusion contains methane and carbon dioxide—see inset

in Fig. 4. The 3D Raman mapping technique was used in

order to determine a distribution of these two substances in

the inclusion. The inclusion was mapped with the x–y table

step of 1.5 mm and such 2D Raman maps were collected at

different depths, with 3 mm step along the z-axis, starting at

1 mm below the sample surface. So large x, y and z steps

cal microscopy; inset—Raman spectra from different places, as indicated

Fig. 3. Raman maps of the polystyrene/micro-diamond composite made basing on the diamond peak (1332 cmK1)—column 1 and the polystyrene peak

(1002 cmK1)—column 2. Maps a–e in each column are obtained at every 0.5 mm along z-axis starting from the sample surface.

M. Pastorczak et al. / Journal of Molecular Structure 744–747 (2005) 997–10031000

Fig. 4. Microphotograph of the studied inclusion in the beryl crystal from the optical microscopy. Inset—Raman spectra obtained from different places, as

indicated by arrows.

M. Pastorczak et al. / Journal of Molecular Structure 744–747 (2005) 997–1003 1001

were chosen because of relatively big size of inclusions. The

Raman maps created basing on the 1070 cmK1 beryl peak

(Fig. 5a1–e1), 1352 cmK1 carbon dioxide peak (maps

5a2–e2) and 2917 cmK1 methane peak (maps 5a3–e3)

reflect spatial distribution of each compound. An absence of

the CO2 and CH4 peaks in the maps 5e2 and e3,

respectively, suggests that at this depth (13 mm) there is

no more the inclusion. Fig. 5a3–e3 shows that the methane

is placed only in the bubble well visible (bottom part of the

inclusion) on the microphotography—see Fig. 4.

Contrary to methane, carbon dioxide fulfils the whole

inclusion. Obtained results prove that the investigated

inclusion is two phases and it contains liquid CO2 which

is in thermodynamical equilibrium with the mixture of

gases: CO2 and CH4.

A series of the Raman maps 5a1–e1 presents the spatial

distribution of intensity of the characteristic ‘matrix’ (beryl)

peak at 1070 cmK1. Not surprisingly the ‘matrix’ peak

intensity is the lowest in the area of the presence of the

inclusion.

It is necessary to note that although the shape of the

inclusion is not perfectly imaged by the 3D Raman maps

shown in Fig. 5, it would be possible to get more precise

results by using smaller in-plane x–y steps and the in-depth

z-step; however, it is time-consuming procedure in a case of

such relatively large objects. Another reason of the different

shape of the inclusion determined by the confocal Raman

spectroscopy can be the differences of refraction indexes

between environment (here air), matrix (here beryl) and the

inclusion. Due to such differences the focusing plane of the

excitation light (it means the real depth from which

the Raman spectra is taken) is shifted up or down comparing

to the depth determined by the microscope stage shift along

the z-axis [12]. As studies for PS/C composite shows,

for very small objects, focusing plane shift is smaller then

the depth resolution and the size estimation error is

negligible. In studies of the inclusion in emerald when

diameter of the inclusion is equal ca. 10 mm, the refraction

index of the material in the inclusion plays an important role

and it makes precise analysis of the Raman maps difficult.

Moreover, precise determination of the refractive indexes of

several phases in inclusion requires quantitative character-

ization of the composition and internal pressure. Contrary to

the laminates, where the suitable corrections can be

successfully done [12,13], for the objects with irregular

shape corrections and simultaneous analysis are very

difficult.

4. Conclusions

Presented results have shown that the confocal micro-

Raman spectroscopy is very powerful method for 3D

mapping of heterogeneous systems. For both natural and

synthetic materials the 3D Raman mapping allows to

estimate shapes and sizes of distributed particles in a proper

way without any special sample preparation. This is true for

the ‘positive’ as well as the ‘negative’ images, which means

that e.g. in a case of inclusions giving very weak Raman

signal their shapes and distribution may be determined by

making 3D map with peak characteristic for the matrix.

The estimation of the grain sizes along z-axis causes

problems due to an effect of refractive phenomena.

Similarly to the depth profiling studies of laminates, the

results from 3D Raman mapping require suitable correc-

tions for refraction. Nevertheless, for objects of a few

micrometers the influence of refraction is negligible. For

bigger objects with irregular shapes suitable corrections can

be difficult to introduce.

Fig. 5. Raman maps of the studied inclusion in the beryl crystal. The maps in the column 1 (a1–e1) based on the beryl peak (1070 cmK1), the column 2 (a2–e2)

based on the carbon dioxide peak (1352 cmK1) and the column 3 (a3–e3) based on the methane peak (2917 cmK1). The maps a–e in each column are obtained

along z-axis at every 3 mm starting from 1 mm under the sample surface (maps a).

M. Pastorczak et al. / Journal of Molecular Structure 744–747 (2005) 997–10031002

M. Pastorczak et al. / Journal of Molecular Structure 744–747 (2005) 997–1003 1003

Acknowledgements

This work was supported by KBN project No. 3 T08A 018

26 (Poland) and was done in a frame of the 6FP NoE project

NANOFUN-POLY, contract No. NMP3-CT-2004-500361.

References

[1] M.F. Ashby, Y.J.M. Brechet, Acta Mater. 51 (2003) 5801–5821.

[2] T.R. Jackson, H. Liu, N.M. Patrikalakis, E.M. Sachs, M.J. Cima,

Mater. Des. 20 (1999) 63–75.

[3] M. Clayburn, A. Luget, K.P.J. Williams, Raman Microscopy and

Imaging of Polymers, ACS Symposium Series 598 41, 1995 (Chapter 3).

[4] P. Smidt, M.R. Fernandez, J.M. Pastor, Polymer 38 (9) (1997)

2067–2075.

[5] J. Parthenios, D.G. Katerelos, G.C. Psarras, C. Galiotis, Eng. Fract.

Mech. 69 (2002) 1067–1087.

[6] C. Krafft, et al., Vib. Spectrosc. 32 (2003) 75–83.

[7] W. Schrof, E. Beck, R. Koniger, W. Reich, R. Schwalm, Prog. Org.

Coat. 35 (1999) 197–204.

[8] W.X. Sun, Z.X. Shen, Ultramicroscopy 94 (2003) 237–244.

[9] M.S. Anderson, S.D. Gaimari, J. Struct. Biol. 142 (2003) 364–368.

[10] S. Hajatdoost, J. Yarwood, Appl. Spectrosc. 50 (5) (1996) 558.

[11] S. Hajatdoost, M. Olsthoorn, J. Yarwood, Appl. Spectrosc. 51 (1997)

12.

[12] N.J. Everall, Appl. Spectrosc. 54 (10) (2000) 1515–1520.

[13] H. Reinecke, S.J. Spells, J. Sacristan, J. Yarwood, C. Mijangos, Appl.

Spectrosc. 55 (2001) 12.