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46
Раздел II. Наноматериалы
УДК 53.086
А.М. Алексеев, Й. Лоос
АКУСТИЧЕСКАЯ АТОМНО-СИЛОВАЯ МИКРОСКОПИЯ ПОЛИМЕРНЫХ
МАТЕРИАЛОВ
Проводилось исследование влияния различных условий измерения полимеров, на примере поликарбоната, полистирола и полипропилена, с помощью атомно-силовой акустической микроскопии (АСАМ) для нахождения оптимальных условий для определения локального модуля Юнга исследуемого материала. Описываются особенности измерения АСАМ зондами с различными значениями жесткости балки, а также длины и ширины кантилеве-ра. Обнаружено, что только зонды с параметрами в определенном диапазоне могут быть использованы для измерений, так как применение слишком мягких зондов затруднено низкой чувствительностью к упругим свойствам поверхности, а использование слишком жестких зондов приводит к разрушению полимерных образцов и возникновению дефектов на их поверхности. Предложена процедура измерения полимеров методом АСАМ, включающая в себя измерения амплитуды и частоты колебания кантилевера, а также расстояния между зондом и исследуемым образцом в каждой точке поверхности.
АСАМ; модуль Юнга; полимеры; кантилевер.
A.M. Alekseev, J. Loos ATOMIC FORCE ACOUSTIC MICROSCOPY ON POLYMER MATERIALS
There was the research of influence various conditions of measurement polymers to _ find optimum conditions for the definition of the local Young's modulus this studied material. The experiment was carried out on the example of polycarbonate, polystyrene and polypropylene, with the using of the atomic force acoustic microscopy (AFAM). The peculiarities of AFAM measurements by probes with various values of a beam rigidity and also length and width of a cantilever are described. It was_ found that only probes parameters in a certain range can be used_ for measurement, since the application of too soft probes is complicated by low sensitivity to elastic surface properties. At the same time the using of too rigid probes leads to the destruction of polymeric samples and emergence of defects on their surface. The procedure of polymers measurement by AFAM method was proposed, involving the amplitude measurements and fluctuation frequency of a cantilever, as well as the distance between the probe and the test sample at each point of the surface.
AFAM; Young's modulus; polymers; cantilever.
Introduction. One of the main disadvantages of Atomic Force Microscopy (AFM) is difficult quantification of measured signal. Atomic Force Acoustic Microscopy (AFAM) is one of few AFM methods, which can obtain quantitative information about sample properties, namely it is able to quantify Young's modulus (also called modulus of elasticity) at nanoscale [1-9]. The method is based on detection of acoustic waves transmitted from the transducer to the sample surface by AFM probe equipped with flexible cantilever. The cantilever working in contact mode is used as a sensor to detect acoustic waves in AFAM setup. The vibrations of the cantilever are detected by standard AFM optical scheme. The resonant frequency of the cantilever coupled with surface depends on the elastic modulus of sample. The resolution of AFAM can be as small as a tip-sample contact area (less than 10nm). The mathematical model developed by the group of Prof. W. Arnold (Saarbrücken, Germany) allows for calculating of absolute
value of Young's modulus from contact resonant frequency of cantilever. Mapping of the cantilever resonant frequency over sample surface enables to evaluate lateral distribution of the Young's modulus. Initially, AFAM was invented specially for stiff materials with the high value of Young's modulus. However, the problem of nondestructive high-resolution mechanical testing is also important for polymers and other low modulus materials. Possibility of the elastic modulus quantification is very attractive for polymer systems. Our study is related to understanding the peculiarities of the polymer measurements with AFAM. For this purpose set of three standard samples was prepared: polycarbonate (PC), polystyrene (PS) and isotactic polypropylene (iPP). All these standard samples were studied by nanoindenter before AFAM measurements. The elastic modules obtained from indentations by standard Oliver and Parr method [10-11] were used as reference for comparison with AFAM data.
1. Experimental details. The SPM SOLVER P47H (NT-MDT, Russia) equipped with ultrasonic transducer (Panametrics-NDT, USA) was used for AFAM measurements. The commercially available cantilevers of NSG and CSG series (NT-MDT, Russia) and CSC (Micromash, Estonia) were tested on polymers. The range of loads with these probes depends on force constant of lever; we tested both contact and noncontact levers.
For indentation testing the combination of SOLVER P47 (NT-MDT) and Triboscope transducer (Hysitron Inc., USA) was used. Triboscope transducer was equipped with diamond Berkovich tip.
The standard AFAM setup is shown in Fig. 1. The SPM with "scanning by tip" design measures surface of the sample coupled with ultrasonic transducer by its back side. Oscillations of sample surface induced by transducer are transmitted to the cantilever, which is placed in contact with surface. Cantilever oscillations are registered by the standard optical scheme of AFM. It is also possible to induce cantilever oscillations by AFM piezodriver placed under cantilever base (so called ultrasonic AFM or UAFM) [12-14]. However, with our set-up quality of cantilever excitation is much better when transducer is used. The theory for both methods is the same.
Fig. 1. Sheme of AFAM measurements
2. Test samples. Three polymers were measured in our study: polycarbonate (PC), polystyrene (PS) and isotactic polypropylene (iPP). Last two samples (PS and iPP) were obtained by cooling of the melt. PC sample is commercially available (G.E. Plastics, The Netherlands). The surface of studied samples was preliminary checked in tapping mode AFM. Surface of all three samples is rather smooth (Fig. 2). However, some variations of surface are still present that can lead to the topography influence on value of elastic modulus measured with AFAM. Unfortunately, control of surface during sample preparation on the scale of the tip size (10-200 nm) is very difficult.
Fig. 2. Topography of the test samples: a) PC, b) PS, c) iPP
We used PC and fused silica for calibration of an area function of Berkovich in-denter for measurements with nanoindentor. It was found that there is difference between these two calibrations originating from peculiar mechanical behavior of polymers. Both calibrations were used for evaluation of elastic modulus of the polymers by the Oliver and Parr method [10-11] implemented into Triboscope software. Calculations based on PC calibration give more consistent elastic modulus for polymeric samples. Finally, results of nanoindentation for the samples studied here are summarized in table 1. Each value is an average calculated from several tens measurements performed at different load (in the range of 10-130pN) (Fig. 3) and different loading rate. It was found that loading rate has only little influence on measured elastic modulus. The data in Table 1 were used as reference for AFAM measurements.
Table 1
Results of nanoindentation for different test samples
sample PC PS iPP
Reduced elastic modulus, GPa 3.0 4.0 1.5
Elastic modulus, GPa 2.6 3.6 1.2
3. AFAM basics. Theory of cantilever vibrations in contact with surface was developed in works of the group of Prof. Arnold [ 1 -9]. The theory is much simpler for rectangle cantilevers than for V-shape lever or other complicated geometries. The rectangle lever allows for obtaining analytical expressions for contact stiffness k* depend-
d F (z )
ing on contact resonant frequency. The contact stiffness is k* = — ■
д z
where F(z)
is the tip-sample force, z0 is the equilibrium position.
z=z
Fig. 3. Theoretical model of AFAM
Complete model of AFAM used in this article is shown in Fig. 3 [9]. Elastic properties of the sample are modeled by the spring with force constant k*. The model shown in Fig. 3 takes into account slope of the cantilever, position of the tip on the lever Le, and the tip length Lt. The lateral spring kl describes movement of sloped cantilever in a lateral direction. The analytical equations for such model can be found in [9]. Calculations of k* from experimental data requires knowledge about geometry of probe.
The procedure of Young's modulus calculation with AFAM includes two steps: 1. Measurements of the reference sample with known elastic modulus, calculations of k* from analytical expressions by using measured contact frequencies fn (n is number of mode). Then tip radius can be determined from Hertzian contact theory for spherical tip and flat sample surface. Within this theory the contact stiffness is expressed as [2]:
8 z
36E *2 RFt, (1)
where R is the tip radius, Ft is the normal force (calculated from force-distance curve), E* is the reduced elastic modulus:
_L=Izvvl+Izvv:, (2)
E * Ec E,
Ec, vc, Es, vs are modulus of elasticity and Poisson's ratio of the tip and the sample, respectively. For rectangular cantilever the force constant kc used in calculations is hc = Ect3w /4L3, where Ec is cantilever Young's modulus (i.e. Young's modulus of
silicon for probes used in this study), and t, w, L are thickness, width and length of cantilever.
2. Measurements of the sample of interest with the same tip at the same tip-sample contact conditions; calculation of k* and determination of E* from (1) with R known from reference measurements.
If ratio k*/kc is too high or too small the resonant frequency is not sensitive to the changes of the tip-sample contact properties, therefore properties of the tip are extremely important for AFAM measurements.
4. Cantilevers. Theory of AFAM measurements, which is used in this work, is valid only for rectangle cantilevers [9]. In our study we have performed AFAM measurements with the three types of commercial cantilevers: CSG11, NSG11 (both from NT-MDT Co) and CSC12/Cr/Au (Micromash). All these cantilevers are rectangular in plane. CSC12/Cr/Au type has gold layer on tip deposited on chromium sublayer. SEM images of the cantilevers were obtained in order to have all important geometrical parameters, namely: length L, width w and thickness t of the lever, tip length Lt, tip position at the lever Le. These parameters were used for calculations of contact stiffness by using software provided by group of Prof. W. Arnold (Saarbrucken, Germany). Average parameters of cantilevers of each type determined from SEM data are listed in table 2.
Table 2
Cantilever parameters determined from SEM
Type of cantilever Length L, |im Width w, |m Thickness t, |m Tip position from free end Le, |im Tip length Lt, |im
NSG10 150 35 2.6 7.8 14
CSG10 290 35 1 6.8 13
CSC12 (A, B, C) 100, 80, 120 35 Not uniform 7 19
Average force constants kc are: 6 N/m for NSG10, 0.05 N/m for CSG10. Cantilevers of CSC12 type (3 levers on each probe) have different thickness t depending on position along lever (see Fig. 4). Data of manufacturer for kc are: 0.95, 1.75, 0.6 N/m for A, B and C levers, respectively.
The validity of the rectangle shape approximation can be checked for each tip by the free resonance frequency measurements. The theoretical relationships between frequencies of the free flexural modes for rectangle cantilevers with uniform thickness are
abc
Fig. 4. SEM photos of CSG (a), NSG (b) and CSC12 (c) cantilevers
5. Principles of AFAM imaging. There are two aspects of AFAM measurements: imaging and evaluation of elastic modulus. We will discuss them separately. The imaging with AFAM means obtaining the lateral distribution of some quantity, which depends on elastic modulus. AFAM is performed in contact mode AFM on sample excited by ultrasonic transducer. With such setup oscillations of surface induce oscillations of the cantilever. Imaging with AFAM is based on changes of the cantilever contact resonance frequency depending on the elastic modulus of surface (Fig. 5). The contact resonant frequency fres is shifted to the higher frequencies at the places with increased elastic modulus (dashed line). As a result amplitude detected at frequency fd is increased by Aa. Iffd <fres contrast of AFAM picture will be inverted: amplitude of cantilever oscillations will be lower at stiffer places. Amplitude detecting AFAM is as fast as conventional topography imaging with AFM. However, evaluation of elastic modulus with this method is difficult, because no direct information about frequency shift is available. The phase detecting AFAM measures phase shift between generator and cantilever. Signal obtained by either amplitude or phase detecting AFAM depend on both resonant peak shift (elastic properties) and changes of peak shape (dissipation). It means that AFAM contrast can be observed when elastic properties of the sample are the same everywhere, but dissipation is different (Fig. 5,b).
Calculations of elastic modulus require measurements of the resonant frequency fres at each point of scan. Measurements of the resonant frequency can be executed by special electronics with feedback control by frequency or by means software, which measures amplitude-frequency (a-f) dependence at each point of the scan. Our setup uses software measuring cantilever oscillation spectra. This scanning method is much slower than conventional AFM imaging (typical time for 128x128 points measuring is 20-40 min with reasonable accuracy). At the same time obtained data have complete information about tip-sample interaction including distribution of resonant frequency, amplitude at the peak maximum, and quality factor. The last two parameters depend on dissipation.
'k Amplitude, a
'1 Amplitude, a
f„
f
<1 Frequency, f
f
't Frequency, f
a
b
Fig. 5. Principle of AFAM measurements; a) The resonant frequency fres is shifted to the higher frequencies at the places with increased elastic modulus (dashed line); b) different peak shape is possible source of AFAM contrast: fres is the same at two different positions but detected amplitude is different
AFAM imaging of polymer surface is characterized by strong tip-sample interaction, since measurements are performed in contact mode. It is well known that contact mode on polymers must be executed with soft cantilevers, otherwise severe destruction of the surface can be observed. This limits number of commercially available cantilevers suitable for AFAM investigation of polymers.
The example of amplitude detecting AFAM on polymers is presented in Fig. 6. The sample is polyethylene single crystals deposited on mica. CSG10 cantilever was used for this measurements with the first free resonance is 15.6 kHz. The right side of the resonant peak was used for detection atf=107 kHz, i.e. brighter areas are stiffer in Fig. 6: mica is stiffer than polymer crystals (mica is visible in top left corner of Fig. 6,a). AFAM contrast on crystal is not uniform: there is clear grain structure on surface of polyethylene. The high-resolution image shows details with the size less than 20 nm (Fig. 6,d). This example demonstrates ability of AFAM to resolve nanometer-scale structure in polymers.
The right determination of the cantilever resonances is the first important task before measurements. Any cantilever has different types of resonances: flexural, torsional, lateral, extensional. Some additional resonances of unknown nature are detected usually in experiment as well. The theoretical model predicts certain distances between modes. These known distances between modes can be used for identifications of contact flexural resonances suitable for AFAM [2, 9].
We use UAFM [12-14] for observation of the cantilever resonances changes during approach. The UAFM is able to observe transition from the free oscillations to the contact resonances of cantilever during approach to the sample surface (Fig. 7). The spectra in Fig. 7 were obtained with CSG10 cantilever on iPP film. Oscillations are induced by the piezodriver under the tip holder. The horizontal cross-section of Fig. 7a is the amplitude-frequency dependence at certain position of the scanner (Fig. 7,b). Y-coordinate in Fig. 7a is the z-position of the scanner; color bar corresponds to amplitude of cantilever vibration in arbitrary units (current of the photodiode, nA). The brighter areas imply higher amplitude. Fig. 7,a was obtained by the movement of the probe attached to the scanner in direction to the sample surface with simultaneous measurements of the a-f curve at each z-position (a-f-z measurements). Drastic change of the spectrum at z=1615 nm are caused by contact with surface. Increasing z after contact leads to the bending of the cantilever and increasing tip-sample interaction. This force acting on sample from tip can be calculated from force-distance curve (F-z): F=kcAz, where ^z is the cantile-
ver bending. Adhesion forces can be quantified from the retracting F-z curve. The total force acting on sample is sum of the applied load and the adhesion force. Both a-f-z and Fz measurements contain complete information needed for AFAM calculations. It is visible in Fig. 9 that frequency of the n-th contact mode is placed in between frequencies of n-th and n+1-th free modes. This is in agreement with theoretical results [2, 9].
Fig. 6. Amplitude detecting AFAM on polyethylene crystals deposited on mica: a), b), d) are AFAM pictures, c) is topography obtained simultaneously with d)
Fig. 7. CSG10 cantilever, iPP sample. a) a-f-z dependence Numbers mark the free and contact flexural oscillation modes; b) horizontal cross-sections of a) at z=1600 nm and 1650 nm (free and contact spectra). First free resonance is 15.6 kHz
Our experiments show that measurements at one point are not reproducible and often give inconsistent result even on homogeneous samples. It means that the a-f-z data taken at different points can be different. One of the reasons for this is the topography influence on a tip-sample interaction. Measurements offres on the area larger than typical topography feature are able to overcome such problem. The results of such measurements on fused silica are shown in Fig. 8. The measurements at 32x32 points give 1024 values for fres. The topography influence is taken into account by measurements on 2 ^m x 2 ^m area. The average fres and root mean square (RMS) are calculated by SPM software. It is seen from color bars in Fig. 8 that fres are higher for increased loading, which means larger k*. The average fres determined from Fig. 8 are 75.6 kHz for Fig. 8a and 77.4 kHz for both Fig. 8b and Fig. 8c, respectively. RMS is 0.14 kHz for all three images. The high reproducibility of the obtained fres is the result of high number of measurements performed at the same conditions. Analysis similar to one described above was performed for several cantilevers and for all test samples.
Fig. 8. Statistical determination offres on fused silica at three different loads: a) 0 nN, b) 6 nN, c)12 nN. Adhesion force is ~15 nN.
6. AFAM study of the test polymer samples
6.1. Cantilever CSG10. The analysis of free and contact resonances of this tip was performed above.
Ratios of free modes of this cantilever are: f2lf1=6.3\, ff1=17.63. These values are very close to the theoretical ratios; it means that shape of cantilever is almost rectangular with uniform cross-section. It follows from a-f-z measurements on PC, PS, iPP and fused silica that all contact modes are pinned already at low load. The Fig. 9 shows behavior of the contact resonances at different load on softest test sample of iPP. Detailed study by measurements of 32x32 values of fres on 2x2 microns area shows almost the same results for the first three modes and all test samples. The loading has only very small influence on resonance. The complete results for test polymers are shown in Table 3. The adhesion forces where 1.5nN, 3.5nN, 5nN for iPP, PS and PC, respectively.
The pinning of resonances for all samples means too high value of k*lkc. The contact resonant frequencies are not sensitive to the surface properties at large ratios k*lkc, corresponding to the theoretical results [2, 9]. It means that cantilever with higher value of force constant is needed for polymers. Because of contact resonance pinning, the contrast in amplitude detecting AFAM images (Fig. 6) can be explained by different dissipation.
70 80 kHz
Fig. 9. AFAM by CSG10 probe: a-f-z for the 1st contact mode on iPP
Table 3
Contact resonances of CSG10 probe on test polymers and fused silica. fres is the average from 1024 measurements. For PC and iPP first three modes were
measured
Load, nN fres, kHz (PC) fres, kHz (PS) fres, kHz (iPP) fres, kHz (Fused Si)
76.7 76.1
0 237.7 76.7 233.4 75.6
476.8 444.0
77.3 77.2
6 240.7 77.4 240.6 77.4
479.2 476.4
77.6 77.7
12 242.1 77.8 244.0 77.4
487.7 494.0
77.6 78.0
18 242.1 77.9 244.9
489.5 497
6.2. Cantilever NSG10. This relatively stiff cantilever can be used for polymer measurements only at very small cantilever deflection. Scanning of polymers with this type of tips is difficult because lateral forces damage surface in contact mode. The ratio of the free resonances is f2lf1=6.04, f1=148.7kHz, which is still acceptable for theoretical model. The a-f-z measurements show strong dependence of resonant peak position on z position of scanner (Fig. 10). The results obtained with NSG10 cantilever on test samples by scanning 2x2 microns area (16x16 points) are shown in Table 4. The clear difference between resonances is seen. However, the data for iPP are not in agreement with indentation testing (even intersection with PS data is observed). The adhesion forces for all three samples were in the range 4-7 nN.
Table 4
First contact resonances of NSG10 on test samples. fres is the average from 256 measurements
Load, nN fres, kHz (PC) fres, kHz (PS) fres, kHz (iPP)
45 499.8 522.1 515.3
90 513.1 536.8 542.3
135 529.9 546.3 552.0
J
Fig. 11. Tapping mode after AFAM by NSG10 probe: square in the center is the result of
scanning in contact mode on iPP
The scanning in tapping mode reveals severe destruction of surface after three measurements in contact mode at loads from 45 to 135 nN (Fig. 11). Destruction of surface was also observed after one scanning with the load of 45 nN. It means that NSG10 cantilevers are too rigid for most of polymer samples.
6.3. Cantilever CSC12/Cr/Au. Lever C. These cantilevers have nonuniform thickness as it seen from Fig. 4. The ratio of free resonanses f2/f1=8.9 means that theoretical model used by us is not applicable for such cantilevers. However, these cantilevers are very sensitive to k* at low load. A new theory for such type of levers is needed. The advantage of these cantilevers is large tip radius owing to additional gold layer. As a result scanning of polymers in contact mode becomes easier and no visible destruction was observed after AFAM regime.
6.4. Elastic modulus calculation. The data obtained by NSG10 for PC and PS are most consistent and can be used for elastic modulus calculation. The NSG10 cantilever has approximately rectangle shape that means theoretical model is applicable. The principle of calculation is based on using the one of the samples as a reference to evaluate tip radius by (1). The PC sample was taken as a reference. The equations for complete model (Fig. 3) [9] with average parameters for NSG10 (Table 2) give k*=103 N/m, R=220 nm for Ft=90 nN. The Young's modulus of PS calculated for R=220 nm and Ft=90 nN is 3.9 GPa. For Ft=45 nN and 135 nN the elastic modulus of PS is 3.7 GPa and 3.4 GPa, respectively. Average value from three measurements at different load is 3.67 GPa. The obtained values of elastic modulus demonstrate good agreement between AFAM and nanoindentation. The large value of R can be explained by existence of plastic deformation during measurements by NSG10 probe.
Conclusions:
1. The most important part of AFAM setup is the cantilever, which must be sensitive to surface properties and suitable for theoretical consideration. The commercial cantilevers studied here are not suitable for AFAM because of different reason. The data obtained by NSG10 cantilever for PS and PC are consistent and give proper value of elastic modulus. However, this cantilever is too stiff for scanning. NSG10 type of probes usually doesn't have good contact spectra (only 1st contact mode is detected). CSG10 probes are too soft and are not sensitive to surface properties. The best choice for AFAM imaging is the contact CSC12lCrlAu cantilever. However, calculation of the elastic modulus from data obtained by this cantilever is impossible since thickness of the cantilever is not uniform. A new theory is needed for the data obtained by CSC12 cantilevers. It is necessary to produce special cantilevers for AFAM with well-defined geometry and good sensitivity. A good candidate is recently developed NSG03 probe (NT-MDT Co), which parameters are in between of that of NSG10 and CSG10 probes.
2. The statistical determination of fres from scanning of homogeneous samples gives more reproducible and reliable data than measurements at one point.
3. The desirable procedure of AFAM measurements on polymer sample can be described as follow. From a-f-z measurements the dependence fres-z is extracted. This measurement must be repeated at each point of scan giving the lateral distribution of fres-z. Movement of tip from point to point must be performed in tapping mode or contact mode with lowest possible load. The cross-section of obtained data at certain z consists of lateral distribution of fres, which can be used for evaluation of the Young's modulus.
Acknowledgements:
This work was supported by COST P12 and Nazarbayev University-LBNL cooperation program. The authors are grateful to Prof. W. Arnold and Dr. U. Rabe (IZFP, Saarbrucken, Germany) for AFAM training and providing software for AFAM calculations, Dr. D. Tranchida (University of Palermo, Italy) for providing test samples and help with nanoindentation measurements. The authors also thank Niek Lousberg (TUle, The Netherlands) for his help with SEM measurements.
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REFERENCES
1. Rabe U., Arnold W. Acoustic microscopy by atomic force microscopy, Appl. Phys. Lett., 1994, Vol. 64, pp. 1493.
2. Rabe U., Janser K., Arnold W. Vibrations of free and surface-coupled atomic force microscope cantilevers: Theory and experiment, Rev. Sci. Instrum, 1996, Vol. 67, pp. 3281.
3. Rabe U., Scherer V., Hirsekorn S., Arnold W. Nanomechanical surface characterization by atomic force acoustic microscopy, J. Vac. Sci. Technol. B, 1997, Vol. 15, pp. 1506.
4. Turner J.A., Hirsekorn S., Rabe U., Arnold W. High-frequency response of atomic-force microscope cantilevers, J. Appl. Phys., 1997, Vol. 82, pp. 966.
5. Kester E., Rabe U., Presmanes L., Tailhades Ph., Arnold W. Measurement of Young's modulus of nanocrystalline ferrites with spinel structures by atomic force acoustic microscopy, J. Phys. Chem. Sol, 2000, Vol. 61, pp. 1275.
6. Amelio S., Goldade A. V., Rabe U., Scherer V., Bhushan B., Arnold W. Measurements of elastic properties of ultra-thin diamond-like carbon coatings using atomic force acoustic microscopy, Thin Solid Films, 2001, Vol. 392, pp. 75.
7. Prasad M., Kopycinska M., Rabe U., Arnold W. Measurement of Young's modulus of clay minerals using atomic force acoustic microscopy, Geophys. Res. Lett., 2002, Vol. 29, pp. 13-1-13-4.
8. Rabe U., Kopycinska M., Hirsekorn S., Arnold, W. Evaluation of the contact resonance frequencies in atomic force microscopy as a method for surface characterization, Ultrasonics, 2002, Vol. 40, pp. 49-54.
9. Kopycinska-Muller M. On the elastic properties of nanocrystallyne materials and the determination of elastic properties on a nanoscale using the atomic force acoustic microscopy technique, Ph. D. Thesis, Saabrucken, 2005.
10. Oliver W.C., Pharr G.M. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res., 1992, Vol. 7, pp. 1564-1583.
11. Oliver W.C., Pharr G.M. Measurement of hardness and elastic modulus by instrumented indentation: Advances in understanding and refinements to methodology, J. Mater. Res., 2004, Vol. 19, pp. 3-20.
12. Yamanaka K., Ogiso H., Kolosov O. Ultrasonic force microscopy for nanometer resolution subsurface imaging, Appl. Phys. Lett, 1994, Vol. 64, pp. 178-180.
13. Yamanaka K., Nakano S. Ultrasonic atomic force microscope with overtone excitation of cantilever, Jpn. J. Appl. Phys., 1996, Vol. 35, pp. 93.
14. Yamanaka K., Tsuji T., Noguchi A., Koike T. Nanoscale elasticity measurement with in situ tip shape estimation in atomic force microscopy, Rev. Sci. Instrum., 2000, Vol. 71, pp. 2403.
Статью рекомендовал к опубликованию д.ф.-м.н., профессор А.А. Лаврентьев.
Алексеев Александр Михайлович - NURIS, Университет Назарбаева, Лаборатория Микроскопии; e-mail: Alexander.alekseev@nu.edu.kz; 010000, Астана, Казахстан; тел.:
+77172706117; к.ф.-м.н.; старший научный сотрудник
Йоахим Лоос - DSM, Нидерланды; e-mail: j.loos@tue.nl; старший научный сотрудник.
Alekseev Alexander Mikhaylovich - NURIS, Nazarbayev University, Laboratory of Microscopy; e-mail: Alexander.alekseev@nu.edu.kz; 010000, Astana, Kazakhstan; phone: +77172706117; cand. of phis.-math. sc.; senior scientist.
Joachim Loos - DSM, The Netherlands; e-mail: j.loos@tue.nl; senior scientist.
УДК 621.38-022.532
О.А. Агеев, О.И. Ильин, В.С. Климин, М.В. Рубашкина, В.А. Смирнов, Ю.В. Сюрик, О.Г. Цуканова
ИССЛЕДОВАНИЕ ВОЗМОЖНОСТИ СОЗДАНИЯ БИОМИМИЧЕСКИХ АДГЕЗИОННЫХ ПОКРЫТИЙ НА ОСНОВЕ МАССИВА ВЕРТИКАЛЬНО ОРИЕНТИРОВАННЫХ УГЛЕРОДНЫХ НАНОТРУБОК*
Проведены экспериментальные исследования адгезии массива вертикально ориентированных углеродных нанотрубок (ВОУНТ) методом атомно-силовой микроскопии (АСМ) с использованием зондов радиусами 35 и 220 нм. Показано, что величина локальной адгезии массива ВОУНТ, рассчитанная по результатам исследования зондом с радиусом 35 нм, значительно превышает теоретическую из-за малой площади контакта поверхности массива с зондом АСМ. Использование зонда радиусом 220 нм позволило оценить макроадгезию массива ВОУНТ, которая составила 38,35±2,55 Н/см2, что хорошо согласуется с имеющимися литературными данными. Полученные результаты могут быть использованы при разработке экспресс-методики определения адгезии массива ВОУНТ методом атомно-силовой микроскопии, а также для создания биомимических адгезионных структур на основе вертикально ориентированных углеродных нанотрубок.
Нанотехнологии; наноматериалы; биомиметика; вертикально ориентированные углеродные нанотрубки; адгезия; атомно-силовая микроскопия; фокусированный ионный пучок; кантилевер.
O.A. Ageev, O.I. Ilin, V.S. Klimin, M.V. Rubashkina, V.A. Smirnov, J. Syurik,
O.G. Tsukanova
THE RESEARCH OF POSSIBILITY OF BIOMIMIC ADHESIVE COVERINGS CREATION BASED ON THE VERTICALLY FOCUSED CARBON NANOTUBES MASSIF
The experimental studies of the adhesion of vertically aligned carbon nanotubes (VACNT) array were carried by atomic force microscopy (AFM) using two probes with radiuses of 35 and 220 nm. It was shown that the dimension of the local adhesion (VACNT) array calculated according to the survey probe with a radius of 35 nm is significantly exceeds the theoretical one because of the small contact area of the array surface with the AFM probe. Using the probe with a 220 nm radius allowed us to estimate the macro adhesion of the VACNT array which amounted 38,35 ± 2,55 N/cm2 and that is well coordinated with the available literary data. The received results can be used in developing a rapid method of determining the adhesion of the VACNT array by atomic force microscopy, as well as to create biomimic adhesive structures on the basis of vertically aligned carbon nanotubes.
Nanotechnology; nanomaterials; biomimetics, vertically aligned carbon nanotubes; adhesion; atomic force microscopy; focused ion beams; cantilever.
* Исследование выполнено при финансовой поддержке РФФИ (проекты: № 14-07-31162 мол_а, № 14-07-31322), а также в рамках базовой части государственного задания Минобрнауки России проекта № 2014/174 и при поддержке Минобрнауки РФ, проект №14.575.21.0045.
