Journal of Non-Crystalline Solids 357 (2011) 1473–1478

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Journal of Non-Crystalline Solids

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High resolution UHV-AFM surface analysis on polymeric materials: Baltic Amber

E. Barletta ⁎, K. WandeltInstitut für Physikalische und Theoretische Chemie, Universität Bonn, Wegelerstr. 12, 53115 Bonn, Germany

⁎ Corresponding author.E-mail address: barletta@pc.uni-bonn.de (E. Barletta

0022-3093/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.jnoncrysol.2010.12.039

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 January 2010Received in revised form 22 November 2010Available online 20 January 2011

Keywords:Amber;Polymers;UHV-AFM;Dielectric surfaces

In this paper we present, for the first time, the results from Atomic Force Microscopy (AFM) surface studiesfrom freshly fractured Baltic Amber samples, carried out under ultrahigh vacuum (UHV) conditions frommicrometer to nanometer resolution. Themicrometric AFM images provide a structural clue to the birefringentbehavior occasionally observed with amber samples. Two-dimensional pair-distance distributions of thenanometric AFM images prove the completely amorphous structure of the material. This, together with thedetection of individual motifs such as aromatic rings, supports the notion of amber being an amorphouspolymeric organic network, consistent with the accompanying X-Ray Photoelectron spectroscopy (XPS) data.No nanocrystalline inclusions could be found. The results also show that it is possible to obtain atomicallyresolved AFM images from amorphous dielectric surfaces.

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1. Introduction

In modern material's science, polymers are widely studied due totheir broad application as materials with peculiar optical, physical andmechanical properties [1–3]. For instance, they are promising candidatematerials for cost-sensitive optical communication components like;e.g. modern plastic optical fibers used in telecommunication willprobably substitute in the near future the delicate and expensive onesmade by glass or quartz. However, the materials processing technologyof polymeric optical structures has not yet reached full maturity. Foroptical structures, one of the most critical properties is the smoothnessof the surface [4]. For example forwaveguides, roughness at the sidewallleads to scattering losses that occur due to the interaction of thetransmitted modes at the boundaries of the waveguides. The increasedscattering loss could severely hamper the scaling down of thewaveguides and waveguide-based devices, as the scattering losses areproportional to the third power of the refractive index differencebetween core and cladding [5].

Another necessary characteristic that must be achieved in the nextgeneration of polymers is a good mechanical and chemical resistanceand stability since the environmental conditions, where the deviceswill be operated, could be quite severe. The real goal is thus to realizematerials that show all these properties at the same time, and thepossibility to tailor these optical or mechanical parameters during thesynthesis process of the polymer itself.

In this context it is important to study and understand the structureof natural polymeric materials that show very interesting behavior interms of optical properties (fluorescence, birefringence, transparence to

UV-radiation) joined with both exceptional mechanical and chemicalresistance [6,7]. The final aim is to reproduce and even improve theseproperties in the preparation of synthetic polymeric materials.

Amber is a quite unique material. Most of the world's amber is inthe range of 30–90 million years old. A common statement is thatamber is the fossil form of the original organic compounds(therpenoids and poly-labdanoids) present in the resin of trees atthat time [8,9]. This statement is actually not completely correct,because the “fossilization process” also involves the substitution ofthe original organic material with mineral compounds, usually watersoluble salts, until finally only the footprint of the original object willbe preserved.

Amber, however, has not suffered any lack of organicmaterial, only arearrangement of chemical bonds within the different compoundspresent in the original fresh resin has occurred. In this case one shouldnot talk about “fossilization” but rather “polymerization”, which seemsto follow a free-radicalmechanism [10–13]. In Table 1 a small summaryof typical chemical and physical properties of amber is shown.

Based on a variety of studies carried out in the past about the resultof this polymerization process, it is stated that amber has amacromolecular structure, and that it is composed of two mainconstituents: the insoluble component, a macromolecular polymericnetwork, and the soluble one, constituted by aromatic compounds oflow molecular weight. This terminology originates from the classicalmethods used to investigate the amber.

Indeed, in the past the first step in the analysis of amber was theattempt to dissolve it in different strong organic solvents [14,15], andafterwards to analyze the obtained solution and/or the residual insolublematerial. The techniques used in the past for this kind of analysis weretypically infrared absorption spectroscopy [16,17], Raman spectroscopy[18–20], Nuclear Magnetic Resonance [21], Gas Chromatography MassSpectroscopy (GC–MS) [22–24], Positron Annihilation Spectroscopy for

Table 1A small summary of the general chemical and physical properties of amber.

Elementary composition C: 61–81%H: 8.5–11%O: 15%S: up to 0.5%Fe: up to 0.2%

Hardness 199–290 MPa or 2.0–2.5 on Mohs scaleDensity 0.96–1.096 g/cm3

Melting temperature 287–300 °CLight refraction coefficient 1.539–1.542Other optical properties Fluorescence, UV transparence, anomalous birefringence

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Chemical Analysis (PASCA), UV-Fluorescence Spectroscopy (UV-FS)[25,26], and Dynamic Light Scattering (DLS) [27].

However, to the best of our knowledge, no structural studies werecarried out on “bulk” samples of amber at sub-micrometric scale, inorder to get direct insight into its macromolecular network. This wasdue to the dielectric nature of amber that excludes the possibility toemploy some of the widely used methods, like Scanning ElectronMicroscopy or Scanning TunnelingMicroscopy, in order to get images atthis dimensional scale. In this work, for the first time, we use AtomicForce Microscopy (AFM) to achieve high resolution pictures of thestructure of thismaterial. But evenwith thismethod themeasurementsare not easy because of the surface charge that is always presentwhen adielectric material is broken.

2. Experimental

The amber sampleswere obtained from a big piece of Baltic Amber,which was cut in 4×5×6 mm small blocks by means of a precisiondiamond wire saw. After that, every piece was fractured, in the AFMlaboratory, bymean of ametal wedge in order to obtain a fresh surfaceto be analyzed (see Fig. 1). Finally the sample was fitted in an AFMsample-holder (Omicron) and immediately inserted into vacuumenvironment, in order to minimize surface contamination.

The following AFM measurements of the fractured surface weretaken under UHV conditions at a residual pressure during imaging ofbetter than 3.5×10−9 mbar.

The images were recorded by mean of an UHV-AFM/STM(Omicron Vakuumphysik GmbH). The instrument is equipped witha single tube high resolution piezo-scanner that allowsmeasurementsfrom the range of 6×6 μm2 down to atomic resolution. The vertical(Z) resolution is 0.01 nm.

Fig. 1. Sample breaking system.

The AFM measurements were carried out in “contact mode” usingsilicon-cantilevers (Fa. Nanosensors) and loading forces of 2.5–3 nN.Under these conditions no obvious modification of the surfacestructure could be detected within the scanned areas upon repetitiveimaging.

In order to study the chemical composition XPS spectra wereacquired from the samples by a VG ESCALAB system, using the MgKαradiation (1253.6 eV) of a conventional twin-anode Al/Mg source. Theemitted photoelectronswere analyzed byaVGCLAM100hemisphericalanalyzer, with a resolution (error) of 0.5 eV.

3. Results and discussion

Atomic Force Microscopy is a local probe technique that revealslocal structural features, which are not necessarily representative forthe whole sample surface. Because of this reason it is appropriate torecord and process many images from different areas of the surface.This furnishes local structure information with high resolution butalso enables to determine an average behavior. All the evaluationsthat are described in the following have, thus, been carried outquantitatively and averaged over a large number of pictures recordedin different areas of the samples.

An immediate impression from the freshly fractured surface of thismaterial during the picture recording was their high roughness. Infact, even though the AFM piezo-scanner was able to image areas upto 6×6 μm2, the largest area that was possible to be scanned on thissample was only 1 μm×1 μm due to the limited z-range. This isbecause of the very high roughness of the surface, compared, e.g., tothat of cleavage surfaces of crystalline materials.

A general definition of roughness that we will use in this paper is:

Rq =

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1N

∑N

I=1z−zð Þ2

s

In others words the roughness is a statistical evaluation of themagnitude of the difference of height of the various points of thesample surface compared with their mean value.

In crystalline materials, for areas of similar size, this value is aboutsome fractions of a nanometer. By contrast the roughness values Rq

calculated for large areas of the amber samples like the one shown inFig. 2 is about 10 nm, and the difference between the minimum andthe maximum values of height in this area is even about 60 nm.

Another important remark is that, at this scale, the amber shows amorphology which is absolutely different from that of crystalline ormicrocrystalline solids. There is no hint of preferred planes on theanalyzed surface, which, in turn, is typical of ordered materials.

The amorphous nature of bulk amber was indirectly concluded inthe past in several papers by using e.g., spectroscopic methods. From

Fig. 2. 1000×1000 nm area in 3-D representation.

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the analysis of our real space topographical images, at the aboveshown as well as at lower scales (see Fig. 6), we can also exclude theexistence of any short range microcrystalline order.

From the many pictures recorded at the scale as shown in Fig. 2 wefound a further interesting feature of that natural polymer, namely ahighly uniform and homogeneous topography present in all imagedareas, without appreciable differences between the pictures taken inmany different areas of the sample surface. Moreover, by evaluatingthe roughness value for several pictures, recorded at various scales inthree different areas of the sample, it is possible to see that theroughness scales with the lateral dimension of the pictures. Thisbehavior can be seen in Fig. 3, where we present data from only threeregions on the sample, but the same trend was found in all analyzedareas.

This means that the apparent roughness changes with thegeometric zooming process, but ultimately converges towards alimiting value. This behavior can be explained in two different ways.

The first one, shown in Fig. 3 by fitting the roughness data by anexponential curve (black), is a general effect when considering veryrough samples. Imaging small areas of the sample results in a lowerprobability to meet big irregularities on the surface, which could hidethe smaller ones. This interpretation is consistent with the fact that forthe largest pictures the roughness approaches a saturation value(Fig. 3), which indicates that now also the biggest irregularities on thesample contribute to the height distribution and that the “realmacroscopic value” of the roughness is obtained.

The data (Fig. 3) were fitted using an exponential curve in theform:

y = ad expbx

� �

where y is the roughness and x is the lateral size of the analyzedimage. For higher values of x the fit converges to a roughness value ofabout 12.6 nm.

The second effect may possibly disclose a specific property of thismaterial. Disregarding the last data points for very large scan areas(N600×600 nm2) all other data points can likewise be fitted with astraight line (purple lines in Fig. 3). Keeping in mind that, even if thesize of the imaged areas changes, the number of pixel (measuredpoints per image) is constant, we are changing the “grid” onwhich weare taking data points. In our case all recorded pictures are 250 by250 pixels, independent from the lateral size of the picture. Borrowing

Fig. 3. The roughness of imaged areas scales with the lateral size of the AFM pictures. Thedata error is equal to the vertical (Z) resolution of the instrument, namely ±0.01 nm.

a concept from fractal geometry we can say that when we change thescanning size area without changing the number of pixel in this area,we are changing the “measuring rule”. The smaller the measured areathe higher is the resolution within the recorded images. As aconsequence, a linear behavior of the surface roughness as a functionof the “resolution”, as indicated by the data points in Fig. 3 for imagessized up to 600×600 nm2, suggest good “similarity” of the datarecorded in several different areas in two respect. First at all, for givenscan area this behavior indicates a high degree of homogeneity of thisnatural polymer at different locations on the surface. Secondly, itelucidates the uniformity of the material at any location independentof resolution.

Another significant outcome of our AFM measurements is thepresence, at different scales, of parallel and longish structures formingawavy pattern. The angle between the longish structures and both thefracturing and scanning directions seem to change randomly in thedifferent areas of the sample.

Fig. 4 shows two pictures, recorded at different scales, whichexemplify this characteristic feature. Below the images line profilesare reported, from which it is possible to estimate the sizes of thesestructures.

A very interesting feature is that, in these two pictures, thelongitudinal direction of the (bright) structures is independent ofboth the scanning and the fracturing direction of the sample. This wastrue in all images recorded in different areas of the sample. Thisindicates that the longitudinal structures are not a consequence of thebreaking stress, but are due to an internal anisotropy of the sample.

In the broad literature on amber this phenomenon was alreadyrecognized from the interpretation of some optical properties ofamber. In fact, it is often reported [28] that some samples of ambershow an anomalous birefringence that could be detected in polarizedlight experiments. Because this phenomenon is well known and wasexplained only for crystalline materials, several hypotheses about theexistence of some kind of patterning in the amber were put forward.However, it was never possible in the past to document the evidenceof such patterns by structural studies.

Fig. 5 shows a bigger area where these patterns are evident. Wefound that these kinds of structures are common in our samples, butthat the separations between adjacent structures are variable.Probably this is the reason why the anomalous birefringence is notalways presented in amber samples (also our samples, measured by apolarimeter, did not show this phenomenon), since the necessarycondition that the pattern must be regular, i.e. a diffraction grating,and comparable with the light wavelength, is not always met.

The origin of this patterning is not clear and it is hypothesized thatit could be explained in terms of an anisotropy in the process of theexudation of the resin stabilized in the following polymerizationprocess [6].

In the freshly prepared samples it was not possible to analyze areassmaller than 80–60 nm, because the pictures appeared blurred and,hence, it was not possible to distinguish any smaller structures on thesurface. This is probably caused by the surface electrostatic chargethat is always present when a dielectric material was broken. Usuallythis charging is quickly eliminated by annealing the respective samplefor some hours at high temperature. Unfortunately this is not possiblewith the amber samples, because at temperatures higher than 80 °Camber starts to outgas with unwanted consequences for the vacuumchamber and the sample surface itself; i.e. the latter would sufferirreversible modifications. An alternative solution is to let the samplesrest in vacuum for some weeks. In this way the surface charge will beslowly dissipated and it will be finally possible to record highresolution pictures. This second strategy was exploited for oursamples.

Fig. 6 shows a 7×7 nm2 area of an amber sample. The surface looksto be constituted by a disordered networkwithout any perceptible longrange organization. However, several circular objects of subnanometer

Fig. 4. Two areas of different size and relative line profiles.

1476 E. Barletta, K. Wandelt / Journal of Non-Crystalline Solids 357 (2011) 1473–1478

size can be seen on the surface (arrows). These circular objects aresuggested tobe aromatic rings (see inset Fig. 7). The internal diameter ofthese rings was measured, and was found to be on average about

Fig. 5. 1000×1000 nm large pattern and a line profile showing the dimensions of thesurface structures.

0.28 nm (e.g. see the line profile in Fig. 6). This length is consistent withthe diameter of the benzene-like aromatic ring which is always presentin the complexpolymericmolecules (therpenoids and poly-labdanoids)that constitute a big percentage of the tree resin.

In order to check this hypothesis, a pair distance correlationanalysis wasmade on several atomically resolved pictures like the oneshown in Fig. 6. The pair distance correlation method consists in theestimation of the mutual distances between hypothetical atoms in thepicture. A software program analyzes the image highlighting theposition of each object (atoms) on the surface. Then a histogram isbuilt considering the number of atoms found at a fixed distance fromeach object. The result is a statistical evaluation of interatomicdistance on the analyzed surface. When some distance values appearwith higher frequency (peaks), and when these distances statisticallymatch with the bonding distances expected for the investigatedmaterial, this may be taken as evidence that our high resolutionpictures represent the real molecular structure [29,30].

This method has successfully been used in investigations ofamorphous material, where any imbedded nanocrystalline structurecan help to judge the quality of the recorded pictures [e.g. 33].

In the present case the knowledge of the chemical composition ofour samples, namely, the presence of aromatic rings, which actuallyappear in our AFM data, lends support to our interpretation of theAFM images as showing the surface structure with atomic resolution.

The correlation histogram shown in Fig. 7 represents the sum ofthe histograms resulting from analyzing six different atomicallyresolved areas, in order to improve the signal to noise ratio and reducethe error that could be introduced by random occurrences in the pairdistance calculation.

In this histogram several peaks are present emphasizing theenhanced abundance of the corresponding interatomic distances overothers. Yet, in the case of amber it is not easy to identify the origin ofall these bond lengths because of the complex stoichiometry of thismaterial. However, three of the four marked peaks match almostperfectly with the expected mutual distances between the carbonatoms on the aromatic rings (see inset Fig. 7).

Fig. 6. Atomically resolved 7×7 nm2 AFM image. The arrows point to some of thesupposed aromatic ring (see text).

Fig. 7. A pair correlation histogram of atomically resolved areas of the amber surface. Inthe inset, an aromatic ring ant the distances between carbon atoms corresponding tothe peaks in the histogram.

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The theoretical values for the lengths a, b and c are 0.14 nm,0.24 nm and 0.28 nm [31], which also appear in the pair correlationanalysis shown in Fig. 7.

Finally Fig. 8 shows a survey XPS spectrum of a freshly preparedsurface of our amber samples.

In good agreement with the organic nature of this material, themain components of our sample are carbon, oxygen and nitrogen.Hydrogen cannot be detected by this technique, and the intensity ofheavier elements (Fe, Ca, Cr) is very small.

A particularly interesting piece of evidence is the presence ofmultiple components in both the C and the O peak, a typical feature ofpolymericmaterials. In particular the 289 eV component in the carbonpeak can be correlated with the presence of O=C–O double bonds[35], which matches with the bond length of about 0.12 nm,represented by the first peak in Fig. 7. Vice versa this correspondencecan be regarded as a further confirmation of the reliability of our AFMmeasurements and that our atomically resolved pictures may beconsidered realistic.

4. Conclusions

The AFM results and their consistency with accompanying XPSdata from the surface of freshly broken amber samples presented anddiscussed in this paper enable us to claim the following conclusions.

Firstly, the similarity of all AFM images independent of the scannedarea, i.e. the linear scaling of the surface roughness with the scandimension, indicates homogeneity of the material throughout.

Secondly, the presence of regular wavy patterns in the ambertopography, that could explain the phenomenon of the anomalousbirefringence, sometime shown from amber, is for the first timedocumented from our AFM recorded pictures.

Thirdly the lack of any crystalline order in the samples at all sizesand locations of the scanned area indicates the completely amorphouscharacter of our amber samples.

Fourthly, highly resolved images of the amber surface, registeredafter allowing the freshly prepared surfaces to discharge over a longperiod of time, reveal an amorphous structure on the atomic scale.Two-dimensional pair correlation histograms obtained from theseatomically resolved images; however, yield not only inter-atomicdistances but even structural motifs, i.e. aromatic rings, which areconsistent with the organic compounds typically found in amber. As a

Fig. 8. Survey XPS spectrum of a freshly prepared Amber sample. The insets show themulti-structured peaks of oxygen and carbon, typical of all polymeric materials. Inparticular both the 289 eV peak of carbon and the 535.5 eV peak of oxygen can berelated to an O=C–O configuration, respectively.

1478 E. Barletta, K. Wandelt / Journal of Non-Crystalline Solids 357 (2011) 1473–1478

consequence, the chemically and structural homogeneity togetherwith the amorphous nature of the material supports the notion thatamber can, indeed, be regarded as a polymeric network.

This is, to the best of our knowledge, the first “direct” proof of theexistence of such homogeneous polymeric network. This, in turn,demonstrates the power and the versatility of the Atomic ForceMicroscopy (AFM) to unravel the structure of complex materials evenwithatomic resolution. Inparticular, the resolutionof short-rangeorderedmotifs, such as the aromatic rings, resolved within the same image as theamorphousmatrix is an important result by itself; it lends reliability to theinterpretation of these AFM images in terms of real atomic structure.These results, together with other results obtained in our group [32–34]from completely different kinds of amorphous surfaces demonstrate that,if the microscope is well isolated from external interferences and thesample surfaces are well prepared, even amorphous materials can besuccessfully studied by means of the UHV-AFM technique.

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