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Imaging the surface of silica microparticles with the atomic force microscope: a novel sample preparation method
surface science letters ELSEVIER
Surface Science Letters 304 (1994) L393-L399
Surface Science Letters
Imaging the surface of silica microparticles with the atomic force microscope: a novel sample preparation method KM. Shakesheff **a,M.C. Davies ‘, D.E. Jackson a, C.J. Roberts a, S.J.B. Tendler a, V.A. Brown b, R.C. Watson b, D.A. Barrett b, P.N. Shaw b iltalwatory ofBiophysics and Surfiice Analysis, Department of Pharmaceutical Sciences, The University of Nottingham, b Ue~~~~i
~n~L~e~~~park, ~ott~~gharn~NC7 2RQ UK of ~~~~~~~e~tiea~ Sciences, The U~iv~r~~~of Notiinghum, ~n~~er~~~Park> Nottingham, NG7 2RD, UK
(Received 13 October 1993; accepted for publication 3 December 1993)
High resolution AFM images of the surface topography of 5 &rn diameter spherical silica particles have been obtained using the technique of partially embedding the particles in a thermoplastic adhesive. This novel sample preparation method overcomes the problems of probe-induced particle movement during scanning and image broadening effects, and could facilitate the imaging by scanning probe microscopy techniques of other particulate or fibrous material.
1. Introduction The atomic force microscope (AFM) has been used in the surface imaging of a wide range of materials [I--3]. Examples of the structural information revealed include the atomic organization of Langmuir-Blodgett films [4], the chain folding of polymer molecules [5] and the conformation of biological macromolecules [61. The technique has gained a widespread acceptance due to its ability to provide high resofution topographical data of naked, untreated, surfaces in ambient conditions. The imaging mechanism of the AFM, which involves monitoring the forces experienced between a sharp probe and a sample surface when the two
* ~orrespoodin~ ~39-6028/94/$07.~
author. Fax: +44 (602) 515-102.
are in close proximity [73, enabfes topographs to be generated of almost any relatively flat material. These topographs can be recorded in different environmental conditions, allowing the AFM to monitor interactions occu~ing at interfaces, This ability has been used to observe the conversion of fibrinogen to fibrin in clot formation [8] and the budding of a virus particle from a cell [9], The ability to monitor surface interactions makes the AFM an attractive tool for the analysis of particulate and colioida1 materiaks. However, particles present a number of imaging problems which may limit the applicability of the AFM and other scanning probe microscope (SPM) techniques. One such problem is caused by the movement of particles as the probe scans across them IXO].This phenomenon is termed “sweeping” and prevents surface images being obtained. Another problem is caused by the geometrical interaction
0 1994 Elsevier Science B.V. AII rights reserved
&WI 01~~-2~S4~93~EOZ4~-~
K. M. Shakesheff et al. / &face
between the AFM probe and the sample, which causes a broadening of the topographical features [t 11. The AF‘M is optirn~~~y used for the imaging of relatively fiat materials where the majority of the forces tetween the probe and the sample surface are experienced at the apex of the probe. When the AFM generates an image of a rough surface with vertical height gradients of a similar magnitude to the dimensions of the tip, the side of the probe becomes involved in image generation as shown schematically in Fig. 1. The topograph of the particle in Fig. 1 may be divided into three sections. The first section begins when the side of the AFM probe contacts with the particle and the topograph consists of a self-image of the probe. fn the second section, self-imaging ceases when the apex of the probe scans the upper part of the particle surface generating real surface data. Then self-imaging is restored in the third section, as the reverse side of the probe again contacts with the particie, These imaging problems have limited the anaiysis of particles by atomic force microscopy to the obse~ation of two-dimensionally crystallized polymer microspheres 112,131. In these studies “sweeping” is prohibited by the crystal-f&e lattice and little feature broadening can occur because the particles are closely packed allowing imaging by the probe apex onIy. The novel sample preparation described in this Letter overcomes the imaging difficulties by partialiy embe~~ding the particles in a thermoplastic
Scan direction *
Cantilever
Fig. 1. The geometrica interaction between the AFM probe and the particle generating a broadened topograph. Particle surface topography data is only obtained where the apex of the probe is involved in imaging.
Science Letters 304 (1994) L3934399
Pai-tickruitsble
Fig. 2. Schematic representation ded particle.
of the imaging of an embed-
adhesive (see Fig. 2). This physicaily restrains the particles, so avoiding “sweeping”, and the reduced height of the particles overcomes broadening probfems. The diminished height differences across the sample also reduces the potential for tip damage during scanning, which can become prohibitively inconvenient and expensive, particularly when imaging with high aspect ratio probes. In this Letter, relatively large silica particles are imaged by atomic force microscopy to demonstrate the successful application of this novel sample preparation technique. These particles are used as a high performance liquid chromatography (HPLC) stationary phase material 1141, have an average diameter of 5 pm and are produced by the aggregation of smaller colloidal particles (average diameter 10 to 14 nm> with pores forming in the interstices between the aggregates [15,16]. These pores are vital to the function of the HPLC material as they are an impo~ant factor in solute retention and selectivity [17f. Traditio~~ly, studies on silica stationary phases have considered chromatographic properties such as retention time of compounds which interact with specific surface features of the stationary phase to yield qualitative info~ation on its surface chemistry [ 181. More recently, surface characterisation has been performed by X-ray photoelectron spectroscopy (XPS) and quantitative fast atom bombardment spectrometry (FAB-MS) 1191. Previous studies on the structure of sihca particles have used both scanning f2Q] and transmission [21] electron microscopy. Here, we present for the first time, three-dimensional to~graphical information of the naked surface of these particles.
KM. Shakesheff et al. /Surface
2. Materials and methods Films of the thermoplastic adhesive, Tempfix (Agar Scientific Ltd, Stansted, UK) were produced by heating 1 cm X 1 cm squares of mica on a hot-plate to a temperature in excess of 120°C and then depositing the the~oplasti~ onto the surface, causing it to melt. The films were then cooied slowly by reducing the hot-plate temperature to ambient over a period of one hour. The slow cooling process ensured a smooth film morphoIogy was generated. The silica particles (Hypersil, Sha~don Scientific Ltd, Astmoor, Cheshire, UK) were deposited onto the film surface by dusting the powder from a height of 20 cm. The samples were then heated in an oven at a preset temperature above 40°C for period of time to embed the particfes. The optimum temperature and heating time were determined by monitoring the effect of these variables on the embedding process by scanning electron microscopy (SEMI. SEM images were obtained with a Phillips SEM 505 (Phillips Analytical, Eindhoven, Nether-
Science Letters 304 (1994) L393-L399
lands) at a voltage of 12.5 keV with a spot size of 20 nm. Samples for SEM analysis were prepared by the deposition of a surface gold coating in a SC 510 sputter coater (VG Microtech, Uckfield, UK) operated at 0.1 mbar with a sputtering current of 60 mA. Atomic force images were obtained with a Polaron SP300 (VG Microtech, Uckfield, UK). Silicon nitride probes mounted on cantilevers with spring constants of 0.064 and 0.032 N/m (Park Scientific, California, USA) were used at imaging forces between 1.0 and 0.1 nN and with a scan frequency of IO Hz. The images reported herein are presented as either light or grey scale representations. In the grey scale representations the height of a feature is shown by the shade of the pixels, with black pixels representing the lowest points on the topography, white pixeis the highest points and shades of grey representing intermediate heights. In the light mode, the height is only represented visually. The height difference quoted with each image is the height differential between the highest and lowest points on the image.
Fig. 3. SE&f image ~~4580) of 5 partially embedded sitica particles that woufd be suitable for AFM imaging. Scale bar represents 10 ,um.
KM. Shakesheff et al. /Surface 3.
Results a td discussion
Initial sts dies utilized the SEM to obtain an overview of the sample surface and to optimize the sample preparation variables. These studies highlighted the importance of the embedding temperature and time. The SEM image in Fig. 3 shows five I artially embedded silica particles surrounded by he the~oplastic adhesive. This sample was preI ared at 60°C with a heating time of 2 h. Shorter I eating times at this temperature resulted in in tdequate embedding, whilst times in excess of tl lree hours resulted in the particles becoming ir rmersed in the adhesive. Increasing the temper: ture to 90°C accelerated the embedding procesr . An equivalent degree of embedding to that seer in Fig. 3 was achieved with 15 min heating at S0°C. Further increasing the temperature to aba ,e the melting point of the thermoplastic adhc sive (120°C) greatly accelerated the embedding, but the process became less predictable. Fa r the purposes of this study a temperature of 6( “C was chosen to minimise surface damage an i increase the predictability of the embedding.
Fig. 4. ” EM image (~4400) showing heterogeneous
Science Letters 304 (1994) L393-L399
The embedding process did not occur to the same degree over the whole sample surface. In sparsely populated areas the majority of particles achieved partial embedding as seen in Fig. 3. However, embedding was less homogeneous where particles were found in aggregates due to the physical size of these aggregates increasing the degree of embedding required. An example of such an area is shown in Fig. 4 in which the central three particles are partially embedded, but some of the particles around this area appear to be resting on the surface of the adhesive. For the purposes of atomic force microscopy, areas like that seen in Fig. 3 offer the highest probability of successful imaging. Therefore, imaging was performed in sparsely populated areas of the sample, these areas being identified by a charged coupled device (ccd) camera built into the AFM. Fig. 5 shows a 5 pm X 5 km AFM image of an area containing a partially embedded silica particle. A cross section through the particle is shown below the image. The top of the particle sits 200 nm above the adhesive surface and the non-embedded portion of the particle has a diameter of approximately 2 pm. Surface detail on the parti-
embedding of clustered silica particles. Scale bar represents 10 Frn.
K.M. Shakesheff et al. /Surface
Science Letters 304 (1994) L393-L399
cle shows an aggregated structure as would be expected from their method of manufacture which involves the co-agulation of nano-particles. In the top half of the image there are a series of ridges running in the scan direction. These features are generated by the probe as it scans over the adhesive surface. This process ceases in the bottom half of the scan, where the adhesive is seen to have a smooth surface morphology ideal for a substrate material. Having identified a suitably embedded particle on a 5 pm x 5 ym scan it was then possible to generate higher resolution images on the particle surface. The AFM image in Fig. 6 is a 500 nm x Fig. 6. AFM image showing the aggregated surface topography of a silica particle (500 nmX500 nm image, light mode with height difference of 79 nm). Scale bar represents 100 nm.
Fig. 5. AFM image of a partially embedded particle with a cross section of the particle showing the top 200 nm of the particle is above the thermoplastic adhesive surface (5 pm x5 pm image, light mode with height difference of 259 nm). Scale bar represents 1 pm.
500 nm scan of a silica particle surface which displays an aggregated surface structure. The aggregates have diameters of between 20 and 50 nm. This image warrants close examination because it demonstrates a problem in the interpretation of relatively rough AFM topographs. In relatively flat areas of the scan (examples marked F), where particles are clustered together the average diameter of the particles is similar to the estimated diameters of the starting material in manufacture. However in rougher areas (examples marked R), where the side of the probe is involved in imaging, a larger diameter is recorded due to the broadening phenomenon that has been modelled by Thundat et al. [ll]. Further images were recorded at higher magnification to demonstrate the ability of the AFM to obtain fine three-dimensional structural detail from the surface of the embedded particles. The AFM image in Fig. 7 is a 250 nm x 250 nm scan. Below the image, a cross section is shown which provides information on the structure of an interstice between two particles. This sample preparation technique is applicable to other particulate materials and other scanning probe microscopes. The rate of the embedding process will vary between particles, with density and size being the most important variables, requiring the optimisation of the time and
KM. ~~ak~~e~et
al. /Surface Science Letters 304 (1994) L393-L399
of partial embedding in a thermoplastic adhesive as a successful method of minimising probe induced particle movement and large scale broadening. The images generated of the naked surface present the three-dimensional topography of the aggregated structure of the material. This information can be used in the study of the interfacial and pore structures which are important in the chromatographical separation process. Using this technique many particle surfaces that have not been accessible to scanning probe microscopy imaging may now be analysed, creating the possibility of high resolution three-dimensional structural information attainment in non-vacuum conditions. Furthermore, the technique allows the exciting developments in the measurement of tribological characteristics of material on nanometre length scales to be applied to particulate and fibrous materials.
5. Ac~owledgements
Width
(nm)
Fig. 7. 250 nmx250 nm AFM image of interstice between particles with a cross section. (grey scale mode with height difference of 47 nm). Scale bar represents 50 nm.
K.M.S. would like to acknowledge the support of the BRITE Euram programme, C.J.R. the support of the SERC/DTI Protein Engineering Link Project with VG Microtech and Glaxo Group Research and V.A.B. and R.C.W. the support of the SERC Separation Processes Initiative. The authors are grateful to Shandon Scientific Ltd. for the provision of silica microparticles and Phil Williams for assistance in image analysis.
6. References temperature of sample preparation. This method could also be of use in the imaging of other samples in which the vertical height of the material above the substrate needs to be reduced to facilitate AFM imaging, an example of such a material being fibres with diameters in the 10 to 100 ,um range.
4. Conclusions The atomic force microscopy imaging of silica particle surfaces has demonstrated the technique
HI M. Radmacher, R.W. Tillmann, M. Fritz and HE. Gaub, Science 257 (19921 1900.
I21SAC. Gould, B. Drake, C.B. Prater, A.L. Weisenhorn, S. Marine, H.G. Hansma, PK. Hansma, J. Massie, M. Longmire, V. Elings, B. Dixon Northern, B. Mukergee, C.M. Peterson, W. Stockenius, T.R. Albrecht and C.F. Quate, J. Vat. Sci. Technol. A 8 (1990) 369. [31 SM. Lindsay, N.J. Tao, J.A. DeRose, P.I. Oden, Y.L. Lyubchenko, R.E. Warrington and L. Shalyakhtenko, Biological Applications of the Scanning Probe Microscopy, in: Scanning Tunnelling Microscopy: Theory, Techniques and Applications, Ed. D.A. BonneIl (VCH, Weinheim, 1993). [41L.F. Chi, M. Anders, H. Fuchs, R.R. Johnston and H. Ringdorf, Science 259 (19931 213.
KM. Shakesheff et al. /Surface
[51 F. Lin, N.V. Gvozdic and D.J. Meier, Topometrix
Application Note No. 2, October 1992, Topometrix Corporate Headquarters, 1505 Wyatt Drive, Santa Clara, CA 950541524, USA. 161 0. Miiller, H.E. Gaub, M. Barmann and E. Sackmann, Macromolecules 24 (1991) 3111. 171 G. Binning, C.F. Quate and Ch. Gerber, Phys. Rev. Lett. 59 (1986) 930. [81 B. Drake, C.B. Prater, A.L. Weisenhorn, S.A.C. Gould, T.R. Albrecht, C.F. Quate, D.S. Cannell, H.G. Hansma and P.K. Hansma, Science 243 (1989) 1586. 191 W. Haberle, J.K.H. Horber, F. Ohnesorge, D.P.E. Smith and G. Binnig, Ultramicroscopy 42-44 (1992) 1161. DO1 C.J. Roberts, M.J. Wilkins, G. Beamson, M.C. Davies, D.E. Jackson, P.D. Scholes, S.J.B. Tendler and P.M. Williams, Nanotechnology 3 (1992) 98. 1111T. Thundat, X.Y. Zheng, S.L. Sharp, D.P. Allison, R.J. Warnack, P.I. Joy and T.L. Ferrell, Scanning Microsc. 6 (19921 903. [121 Y. Li and S.M. Lindsay, Rev. Sci. Instrum. 62 (1991) 11.
Science Letters 304 (1994) L393-L399 [13] M.C. Goh, D. Juhue, 0. Leung, Y. Wang and M.A. Winnik, Langmuir 9 (1993) 1319. [14] A Berthod, J. Chromat. 549 (19911 1. [15] K. Unger, Porous Silica, Its Properties and Use as a support in Column Liquid Chomatography (Elsevier, Amsterdam, 1977). [16] R.K. Iler, The Chemistry of Silica: Soiubility, Polymerization, Colloid and Surface Properties, and Biochemistry (Wiley, New York, 1979). 1171 J. Kiihler, D.B. Chase, R.D. Farlee, A.J. Vega and J.J. Kirkland, J. Chromat. 352 (1986) 275. 1181 L.C. Sander, J. Chromat. Sci. 26 (1988) 380. [19] V.A. Brown, D.A. Barrett, P.N. Shaw, M.C. Davies, H.J. Ritchie, P. Ross, A.J. Paul and J.F. Watts, Surf. Interface Anal., in press. [20] T.D. Wilson and D.M. Simmons, Chromatographia 45 (1993) 295. 1211 N. Tanaka, K. Hashidzume, M. Araki, H. Tsuchiya, A. Okuno, K. Iwaguchi, S. Ohnishi and N. Takai, J. Chromat. 448 (1988) 95.
Surface Science Letters 304 (1994) L393-L399
Surface Science Letters
Imaging the surface of silica microparticles with the atomic force microscope: a novel sample preparation method KM. Shakesheff **a,M.C. Davies ‘, D.E. Jackson a, C.J. Roberts a, S.J.B. Tendler a, V.A. Brown b, R.C. Watson b, D.A. Barrett b, P.N. Shaw b iltalwatory ofBiophysics and Surfiice Analysis, Department of Pharmaceutical Sciences, The University of Nottingham, b Ue~~~~i
~n~L~e~~~park, ~ott~~gharn~NC7 2RQ UK of ~~~~~~~e~tiea~ Sciences, The U~iv~r~~~of Notiinghum, ~n~~er~~~Park> Nottingham, NG7 2RD, UK
(Received 13 October 1993; accepted for publication 3 December 1993)
High resolution AFM images of the surface topography of 5 &rn diameter spherical silica particles have been obtained using the technique of partially embedding the particles in a thermoplastic adhesive. This novel sample preparation method overcomes the problems of probe-induced particle movement during scanning and image broadening effects, and could facilitate the imaging by scanning probe microscopy techniques of other particulate or fibrous material.
1. Introduction The atomic force microscope (AFM) has been used in the surface imaging of a wide range of materials [I--3]. Examples of the structural information revealed include the atomic organization of Langmuir-Blodgett films [4], the chain folding of polymer molecules [5] and the conformation of biological macromolecules [61. The technique has gained a widespread acceptance due to its ability to provide high resofution topographical data of naked, untreated, surfaces in ambient conditions. The imaging mechanism of the AFM, which involves monitoring the forces experienced between a sharp probe and a sample surface when the two
* ~orrespoodin~ ~39-6028/94/$07.~
author. Fax: +44 (602) 515-102.
are in close proximity [73, enabfes topographs to be generated of almost any relatively flat material. These topographs can be recorded in different environmental conditions, allowing the AFM to monitor interactions occu~ing at interfaces, This ability has been used to observe the conversion of fibrinogen to fibrin in clot formation [8] and the budding of a virus particle from a cell [9], The ability to monitor surface interactions makes the AFM an attractive tool for the analysis of particulate and colioida1 materiaks. However, particles present a number of imaging problems which may limit the applicability of the AFM and other scanning probe microscope (SPM) techniques. One such problem is caused by the movement of particles as the probe scans across them IXO].This phenomenon is termed “sweeping” and prevents surface images being obtained. Another problem is caused by the geometrical interaction
0 1994 Elsevier Science B.V. AII rights reserved
&WI 01~~-2~S4~93~EOZ4~-~
K. M. Shakesheff et al. / &face
between the AFM probe and the sample, which causes a broadening of the topographical features [t 11. The AF‘M is optirn~~~y used for the imaging of relatively fiat materials where the majority of the forces tetween the probe and the sample surface are experienced at the apex of the probe. When the AFM generates an image of a rough surface with vertical height gradients of a similar magnitude to the dimensions of the tip, the side of the probe becomes involved in image generation as shown schematically in Fig. 1. The topograph of the particle in Fig. 1 may be divided into three sections. The first section begins when the side of the AFM probe contacts with the particle and the topograph consists of a self-image of the probe. fn the second section, self-imaging ceases when the apex of the probe scans the upper part of the particle surface generating real surface data. Then self-imaging is restored in the third section, as the reverse side of the probe again contacts with the particie, These imaging problems have limited the anaiysis of particles by atomic force microscopy to the obse~ation of two-dimensionally crystallized polymer microspheres 112,131. In these studies “sweeping” is prohibited by the crystal-f&e lattice and little feature broadening can occur because the particles are closely packed allowing imaging by the probe apex onIy. The novel sample preparation described in this Letter overcomes the imaging difficulties by partialiy embe~~ding the particles in a thermoplastic
Scan direction *
Cantilever
Fig. 1. The geometrica interaction between the AFM probe and the particle generating a broadened topograph. Particle surface topography data is only obtained where the apex of the probe is involved in imaging.
Science Letters 304 (1994) L3934399
Pai-tickruitsble
Fig. 2. Schematic representation ded particle.
of the imaging of an embed-
adhesive (see Fig. 2). This physicaily restrains the particles, so avoiding “sweeping”, and the reduced height of the particles overcomes broadening probfems. The diminished height differences across the sample also reduces the potential for tip damage during scanning, which can become prohibitively inconvenient and expensive, particularly when imaging with high aspect ratio probes. In this Letter, relatively large silica particles are imaged by atomic force microscopy to demonstrate the successful application of this novel sample preparation technique. These particles are used as a high performance liquid chromatography (HPLC) stationary phase material 1141, have an average diameter of 5 pm and are produced by the aggregation of smaller colloidal particles (average diameter 10 to 14 nm> with pores forming in the interstices between the aggregates [15,16]. These pores are vital to the function of the HPLC material as they are an impo~ant factor in solute retention and selectivity [17f. Traditio~~ly, studies on silica stationary phases have considered chromatographic properties such as retention time of compounds which interact with specific surface features of the stationary phase to yield qualitative info~ation on its surface chemistry [ 181. More recently, surface characterisation has been performed by X-ray photoelectron spectroscopy (XPS) and quantitative fast atom bombardment spectrometry (FAB-MS) 1191. Previous studies on the structure of sihca particles have used both scanning f2Q] and transmission [21] electron microscopy. Here, we present for the first time, three-dimensional to~graphical information of the naked surface of these particles.
KM. Shakesheff et al. /Surface
2. Materials and methods Films of the thermoplastic adhesive, Tempfix (Agar Scientific Ltd, Stansted, UK) were produced by heating 1 cm X 1 cm squares of mica on a hot-plate to a temperature in excess of 120°C and then depositing the the~oplasti~ onto the surface, causing it to melt. The films were then cooied slowly by reducing the hot-plate temperature to ambient over a period of one hour. The slow cooling process ensured a smooth film morphoIogy was generated. The silica particles (Hypersil, Sha~don Scientific Ltd, Astmoor, Cheshire, UK) were deposited onto the film surface by dusting the powder from a height of 20 cm. The samples were then heated in an oven at a preset temperature above 40°C for period of time to embed the particfes. The optimum temperature and heating time were determined by monitoring the effect of these variables on the embedding process by scanning electron microscopy (SEMI. SEM images were obtained with a Phillips SEM 505 (Phillips Analytical, Eindhoven, Nether-
Science Letters 304 (1994) L393-L399
lands) at a voltage of 12.5 keV with a spot size of 20 nm. Samples for SEM analysis were prepared by the deposition of a surface gold coating in a SC 510 sputter coater (VG Microtech, Uckfield, UK) operated at 0.1 mbar with a sputtering current of 60 mA. Atomic force images were obtained with a Polaron SP300 (VG Microtech, Uckfield, UK). Silicon nitride probes mounted on cantilevers with spring constants of 0.064 and 0.032 N/m (Park Scientific, California, USA) were used at imaging forces between 1.0 and 0.1 nN and with a scan frequency of IO Hz. The images reported herein are presented as either light or grey scale representations. In the grey scale representations the height of a feature is shown by the shade of the pixels, with black pixels representing the lowest points on the topography, white pixeis the highest points and shades of grey representing intermediate heights. In the light mode, the height is only represented visually. The height difference quoted with each image is the height differential between the highest and lowest points on the image.
Fig. 3. SE&f image ~~4580) of 5 partially embedded sitica particles that woufd be suitable for AFM imaging. Scale bar represents 10 ,um.
KM. Shakesheff et al. /Surface 3.
Results a td discussion
Initial sts dies utilized the SEM to obtain an overview of the sample surface and to optimize the sample preparation variables. These studies highlighted the importance of the embedding temperature and time. The SEM image in Fig. 3 shows five I artially embedded silica particles surrounded by he the~oplastic adhesive. This sample was preI ared at 60°C with a heating time of 2 h. Shorter I eating times at this temperature resulted in in tdequate embedding, whilst times in excess of tl lree hours resulted in the particles becoming ir rmersed in the adhesive. Increasing the temper: ture to 90°C accelerated the embedding procesr . An equivalent degree of embedding to that seer in Fig. 3 was achieved with 15 min heating at S0°C. Further increasing the temperature to aba ,e the melting point of the thermoplastic adhc sive (120°C) greatly accelerated the embedding, but the process became less predictable. Fa r the purposes of this study a temperature of 6( “C was chosen to minimise surface damage an i increase the predictability of the embedding.
Fig. 4. ” EM image (~4400) showing heterogeneous
Science Letters 304 (1994) L393-L399
The embedding process did not occur to the same degree over the whole sample surface. In sparsely populated areas the majority of particles achieved partial embedding as seen in Fig. 3. However, embedding was less homogeneous where particles were found in aggregates due to the physical size of these aggregates increasing the degree of embedding required. An example of such an area is shown in Fig. 4 in which the central three particles are partially embedded, but some of the particles around this area appear to be resting on the surface of the adhesive. For the purposes of atomic force microscopy, areas like that seen in Fig. 3 offer the highest probability of successful imaging. Therefore, imaging was performed in sparsely populated areas of the sample, these areas being identified by a charged coupled device (ccd) camera built into the AFM. Fig. 5 shows a 5 pm X 5 km AFM image of an area containing a partially embedded silica particle. A cross section through the particle is shown below the image. The top of the particle sits 200 nm above the adhesive surface and the non-embedded portion of the particle has a diameter of approximately 2 pm. Surface detail on the parti-
embedding of clustered silica particles. Scale bar represents 10 Frn.
K.M. Shakesheff et al. /Surface
Science Letters 304 (1994) L393-L399
cle shows an aggregated structure as would be expected from their method of manufacture which involves the co-agulation of nano-particles. In the top half of the image there are a series of ridges running in the scan direction. These features are generated by the probe as it scans over the adhesive surface. This process ceases in the bottom half of the scan, where the adhesive is seen to have a smooth surface morphology ideal for a substrate material. Having identified a suitably embedded particle on a 5 pm x 5 ym scan it was then possible to generate higher resolution images on the particle surface. The AFM image in Fig. 6 is a 500 nm x Fig. 6. AFM image showing the aggregated surface topography of a silica particle (500 nmX500 nm image, light mode with height difference of 79 nm). Scale bar represents 100 nm.
Fig. 5. AFM image of a partially embedded particle with a cross section of the particle showing the top 200 nm of the particle is above the thermoplastic adhesive surface (5 pm x5 pm image, light mode with height difference of 259 nm). Scale bar represents 1 pm.
500 nm scan of a silica particle surface which displays an aggregated surface structure. The aggregates have diameters of between 20 and 50 nm. This image warrants close examination because it demonstrates a problem in the interpretation of relatively rough AFM topographs. In relatively flat areas of the scan (examples marked F), where particles are clustered together the average diameter of the particles is similar to the estimated diameters of the starting material in manufacture. However in rougher areas (examples marked R), where the side of the probe is involved in imaging, a larger diameter is recorded due to the broadening phenomenon that has been modelled by Thundat et al. [ll]. Further images were recorded at higher magnification to demonstrate the ability of the AFM to obtain fine three-dimensional structural detail from the surface of the embedded particles. The AFM image in Fig. 7 is a 250 nm x 250 nm scan. Below the image, a cross section is shown which provides information on the structure of an interstice between two particles. This sample preparation technique is applicable to other particulate materials and other scanning probe microscopes. The rate of the embedding process will vary between particles, with density and size being the most important variables, requiring the optimisation of the time and
KM. ~~ak~~e~et
al. /Surface Science Letters 304 (1994) L393-L399
of partial embedding in a thermoplastic adhesive as a successful method of minimising probe induced particle movement and large scale broadening. The images generated of the naked surface present the three-dimensional topography of the aggregated structure of the material. This information can be used in the study of the interfacial and pore structures which are important in the chromatographical separation process. Using this technique many particle surfaces that have not been accessible to scanning probe microscopy imaging may now be analysed, creating the possibility of high resolution three-dimensional structural information attainment in non-vacuum conditions. Furthermore, the technique allows the exciting developments in the measurement of tribological characteristics of material on nanometre length scales to be applied to particulate and fibrous materials.
5. Ac~owledgements
Width
(nm)
Fig. 7. 250 nmx250 nm AFM image of interstice between particles with a cross section. (grey scale mode with height difference of 47 nm). Scale bar represents 50 nm.
K.M.S. would like to acknowledge the support of the BRITE Euram programme, C.J.R. the support of the SERC/DTI Protein Engineering Link Project with VG Microtech and Glaxo Group Research and V.A.B. and R.C.W. the support of the SERC Separation Processes Initiative. The authors are grateful to Shandon Scientific Ltd. for the provision of silica microparticles and Phil Williams for assistance in image analysis.
6. References temperature of sample preparation. This method could also be of use in the imaging of other samples in which the vertical height of the material above the substrate needs to be reduced to facilitate AFM imaging, an example of such a material being fibres with diameters in the 10 to 100 ,um range.
4. Conclusions The atomic force microscopy imaging of silica particle surfaces has demonstrated the technique
HI M. Radmacher, R.W. Tillmann, M. Fritz and HE. Gaub, Science 257 (19921 1900.
I21SAC. Gould, B. Drake, C.B. Prater, A.L. Weisenhorn, S. Marine, H.G. Hansma, PK. Hansma, J. Massie, M. Longmire, V. Elings, B. Dixon Northern, B. Mukergee, C.M. Peterson, W. Stockenius, T.R. Albrecht and C.F. Quate, J. Vat. Sci. Technol. A 8 (1990) 369. [31 SM. Lindsay, N.J. Tao, J.A. DeRose, P.I. Oden, Y.L. Lyubchenko, R.E. Warrington and L. Shalyakhtenko, Biological Applications of the Scanning Probe Microscopy, in: Scanning Tunnelling Microscopy: Theory, Techniques and Applications, Ed. D.A. BonneIl (VCH, Weinheim, 1993). [41L.F. Chi, M. Anders, H. Fuchs, R.R. Johnston and H. Ringdorf, Science 259 (19931 213.
KM. Shakesheff et al. /Surface
[51 F. Lin, N.V. Gvozdic and D.J. Meier, Topometrix
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