HRTEM surface profile imaging of solids

Current Opinion in Solid State and Materials Science 5 (2001) 75–83

HRTEM surface profile imaging of solids Wuzong Zhou a , *, John M. Thomas b b

a School of Chemistry, University of St. Andrews, St. Andrews, Fife, Scotland KY16 9 ST, UK Davy-Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1 X 4 BS, UK

Abstract As an important supplementary technique in surface science, surface profile imaging by high resolution transmission electron microscopy provides surface structural information as well as subterranean structures of solids. The surface-related properties of materials can therefore be better understood. In particular, the application of this technique to the characterisation of nano-scale materials has increased significantly in recent years.  2001 Elsevier Science Ltd. All rights reserved.

1. Introduction Since the method of surface profile imaging by high resolution transmission electron microscopy (HRTEM) was first introduced in the 1980s [1–3], not many microscopists had been attracted into this field, mainly because no major attraction in its theoretical exploitation. However, the demands of this technique from materials scientists have quickened in recent years for two reasons. Firstly, HRTEM is a unique technique for providing a direct atomic image of the surface as well as of the underlying structures of solids, and hence of providing information about their interdependence [4]. Secondly, in the development of nano-scale materials there are difficulties in surface characterisation using many of the traditional surface techniques, such as scanning tunnelling microscopy (STM), scanning electron microscopy (SEM), low energy electron diffraction, etc. By contrast, electron microscopy requires no special treatment of specimen and can be used to investigate the surfaces of small particles of almost any size and any morphology, and the surfaces studied include many facets of the same particle. Transmission electron microscopy usually suffers from the consequences of the strong interaction between electrons and matter, leading to multiple scattering, which complicates the relation between the object structure and the electron wave function at the exit plane of the object. However, for open surfaces of solids, which is the main topic of the present review, the specimen areas of interest *Corresponding author. Tel.: 144-1334-467-276; fax: 144-1334-463808. E-mail addresses: [email protected] (W. Zhou), [email protected] (J.M. Thomas).

are normally very thin and can legitimately be treated as ‘weak phase’ objects, where the image intensity indicates the projected electrostatic potential. Consequently, the explanation of the surface profile images is relatively simple.

2. Bulk specimens Most reports of HRTEM surface profile imaging have focused on materials of practical importance, especially on those having surface sensitive properties. One such example is high temperature superconducting oxides, most of which are polycrystalline (Fig. 1). Their physical properties, such as critical current density, surface conductivity (a key parameter in the development of microwave devices), depend more or less on the surface structures of the crystallites. It has often been observed that as-synthesised superconducting oxides have an amorphous coating layer (2 to 5 nm in thickness) on the crystallite surfaces, arising from surface decomposition [5,6]. Under electron-beam irradiation, the outer layers can recrystallise into the original structure if the compositions remain unchanged during the irradiation [5]. Detecting the chemical compositions of the surface layers is conveniently achieved by energy dispersive X-ray spectroscopy (EDS). Attached to the microscope is an appropriate solid-state detector, and a very small primary electron beam (e.g. 2 nm in diameter) is used [6] (Fig. 2). Such work yields important information concerning the different magnetic properties of the material at low temperature after different specimen treatments [7]. Knowledge of the surface structures of support materials for heterogeneous catalysts is of immense interest. One of

1359-0286 / 01 / $ – see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S1359-0286( 00 )00037-1

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Nomenclature HRTEM STM SEM EDS UHV TEM

High-resolution transmission electron microscopy Scanning tunnelling microscopy Scanning electron microscopy Energy dispersive X-ray spectroscopy Ultra-high vacuum transmission electron microscopy

the best reports on recent HRTEM surface profile imaging of bulk specimens was presented by Jacobsen et al. [**8], in which extremely sharp microfaceting on the surface of thin CeO 2 films was observed in atomic resolution. This work suggested that nominally designated crystallographic surfaces might not coincide with the as-grown surfaces.

Direct characterisation of the outer-most atomic layers of crystals using HRTEM is possible, and the fast development of computer systems enables us to simulate largearea surface profile images. Therefore, it is not only the basic structure, but also the defects and dislocations in the surface layer that can be calculated in order to match the

Fig. 1. HRTEM surface profile image from a crystal of La 2 CuO 4 , showing the (001) surface edge with a schematic drawing of the cation arrangement. The La atoms in the C–La 2 O 3 surface layer are shown by the filled circles. A computer simulation of the image contrast is shown against the experimental micrograph [4].

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Fig. 2. HRTEM surface profile image of superconducting HgBa 2 CuO 41d . The view direction is [010]. The original surface layer (top left corner) of about 2 nm in thickness was amorphous. The inset shows EDS results from the bulk specimen (top) and the surface layer (bottom), indicating Hg loss in the surface. Under the electron beam irradiation, the surface recrystallised into some barium copper oxide microcrystallites. The physical properties of the surface and the bulk are therefore different.

experimental images [4,9–11] (Fig. 1). If the dislocation direction is perpendicular to the viewing direction, the reliability of the determination of the atom position can be much higher than the point resolution of microscope [12]. It is obvious that, in order to observe an atomic image of a clean surface, the surface in question should have essentially zero contamination. Ultra-high vacuum (UHV) HRTEM is therefore a favoured tool to investigate surface structures of solids and a recent example is the in situ direct observation of STM operations. Here the target surface, tip structure, tip–surface gap and changes of the tip geometry can all be monitored simultaneously [*13,**14] (Fig. 3). It was believed that the information obtained was crucial to a reliable explanation of the STM images. Deposition of small particles on surface of bulk solids yields an important type of material, for example supported catalysts. In a review of application of HRTEM to zeolites by Terasaki et al. 3 years ago [15], it was shown that Pt clusters on the surface of zeolite with 1.0 to 1.5 nm in diameter were easily detectable. This method has also been used to study even smaller clusters, Ru 6 , loaded on to

the inner surfaces of mesoporous silica. It was shown [*16] that Ru 6 could be deposited either on the outer surface only or inside the mesopores after passivating the external surface with R 2 SiCl 2 [R5(CH 2 ) 3 NH 2 ]. Metal clusters deposited on the surface of relatively larger nanoparticles are even more interesting in catalysis since a large specific surface can be achieved [17].

Fig. 3. Illustration of the experimental set-up for in situ HRTEM observation of STM operation. Profile images of both the tip and the specimen surfaces were obtained. Reproduced with permission from Surface Science 1998, [*13].

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3. Nano-scale particles In recent years, the largest application of HRTEM surface profile imaging has been in the field of nano-scale materials science. Morphologies of large particles are traditionally studied by using SEM, but, when the particle size is reduced to the nanometer scale, SEM becomes less valid due to its resolution limit. (See, however, the important work of Boyes [18], who has shown that low voltage SEM is an extremely powerful surface tool both structurally and in chemical analysis) HRTEM is therefore a unique technique to image directly individual particles, including their surfaces in a profile view [19–**25]. TEM characterisation of small particles started many years ago, and in the earlier days, most studies were concerned with the determination of sizes and morphologies of particles. The most recent comprehensive review of HRTEM studies of nanocrystals was published by Wang [**26]; and the most important recent developments in this field are the investigation of surface decorated particles, and the in situ observation of growth and modification of particles. Higher resolution HRTEM is therefore required for revealing more structural details, as is illustrated below. Zarur and Ying used HRTEM to characterise catalytic barium hexaaluminate nanoparticles synthesised using a reverse-microemulsion-mediated method and also observed the nanocrystalline CeO 2 surface coating on these particles after post-treatment [27]. This research is concerned with the development of catalysts suitable for high-temperature industrial applications. Rothschild et al. studied WC particles, one of the hardest known materials and an active catalyst for the conversion of methane and carbon dioxide to synthesis gas (CO1H 2 ). When WC particles were encapsulated within 2H–WS 2 fullerene-like shells, HRTEM profile images showed clearly the detailed structures of both the cores and the shells as well as that of the interface [*28]. Epitaxial growth of WO 32x needles on the WC surface was recorded in situ by York et al., and structural relations between WO 32x and WC were revealed by HRTEM [*29]. In addition, the profile image of nanoparticle chains on a soil surface sampled from a lightning-like discharge environment showing a possible formation mechanism of ball lightning reported by Abrahamson and Dinniss [30] demonstrates a new application of the HRTEM in meteorological science. Carbon nanotubes and related materials belong to another unusual family of nanoparticles, formed by one or more closed or partially closed graphitic layers with a hollow core. HRTEM has therefore been extensively used to observe their shapes, structures, number of wall layers, defects on the graphitic surfaces, etc. [31–**34]. A typical early work was performed by Audier et al. [35,36]. The most recent research in this field is also focused on more detailed structures and decorated surfaces. Zhang et al., not only imaged the single-wall carbon nanotubes, but also the detailed interface structures between nanotubes and

nanorods or particles of silicon carbide and transition metal carbides [**34]. From surface profile images of so-called bamboo-like carbon nanofibres, it has been found that their surface is no longer a cylindrical graphitic sheet, but terminated with many open ends of graphitic sheets [37] (Fig. 4a). Large amounts of aligned such nanofibres, synthesised using microwave plasma-assisted chemicalvapour deposition, showed excellent field emission properties, and a side-emission mechanism was proposed based on the HRTEM images. When B and N were introduced into the carbon nanofibres, the HRTEM profile images showed a cactus-like structure with many small graphitic spines standing on the surface of the fibres [38] (Fig. 4b). Work by Chen et al. showed that carbon nanotubes can be coated by polypyrrole to form a new composite material, and their HRTEM images revealed that the polypyrrole coating layers form bridges between the nanotubes to give dense composite films [39]. It has become increasingly apparent that nanotubes and fullerenes [40–42] may be made from materials other than carbon. Characterisation of MoS 2 fullerenes by HRTEM reported by Parilla et al. relied on a profile view where two or three layers of MoS 2 network with a hollow centre can be observed, similar to the observation of the carbon nanotubes [40]. HRTEM images of vanadium oxide nanotubes viewed down the direction either perpendicular or parallel to the tube axis revealed the detailed structures of these new materials [42]. Particles embedded in a solid shell (core-shell particles) [43–46] is an interesting form of material used for the controlled release and targeting of drugs as well as for the protection of sensitive agents, such as enzymes and proteins. Donath et al. presented an outstanding HRTEM image of nine-layer (ca. 20 nm thick) polyelectrolyte shells [43]. Marinakos et al. synthesised polymer coated Au nanoparticles followed by removing the Au particles to obtain hollow nanoscale conductive polymer capsules [44], and the corresponding profile images were used to distinguish these two materials. If a coating layer of a nanoscale core is well ordered and gives significantly strong image contrast – in excess of that from the core material – it can be readily revealed by HRTEM as a plan-view. This is so for a nanocrystalline CeO 2 coating on barium hexaaluminate nanoparticles mentioned above [27]. Further applications of this plan-view images to biological specimens are discussed below. It is noteworthy that in situ studies of the growth of small particles have become an important topic. For example, in situ observations of growth of silicon particles revealed the kinetical stabilities of the crystal facets, one of the interesting conclusions being that the (001) face is an atypical low index surface that becomes rounded as soon as the growth stopped [47,48]. In addition, in situ observations of phase transformation in small particles of various sizes were demonstrated by Tanaka et al. [*49]. In situ observation of coalescence of single-walled carbon

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Fig. 4. HRTEM images of some peculiar surfaces of carbon nanofibres. (a) The surface of the bamboo-like nanofibres are wavy along the fibre axis (low magnification) and it terminates with many open ends of graphitic sheets. These open ends may act as active centres in chemical or physical processes. (b) B- and N-doped carbon nanotubes showing a cactus-like structure with many small graphitic spines standing on the surface of the fibres.

Fig. 5. HRTEM surface profile image of nickel nanocrystals encapsulated by onion-like graphitic shells. The arrangement forms under electron irradiation at temperatures above 700 K. It was found that nickel atoms migrate into the graphite layers to form crystalline C–Ni phases (left side). Reproduced from Ref. [**51].

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nanotubes under electron irradiation is one of the most spectacular pieces of research. Terrones et al. recorded a series of HRTEM images of a bundle of single walled nanotubes viewed down the tube axis to show a group of rings. They observed two of the tubes combining to form a larger one [**50]. Earlier this year, Banhart et al. reported several HRTEM profile images to demonstrate an in situ observation of formation of nickel nanocrystals encapsulated by onion-like graphitic shells, and also the migration of nickel atoms into the graphite layers to form crystalline C–Ni phases [**51] (Fig. 5). HRTEM was also the main characterisation method for monitoring the formation of 1D self-organised Au nanoparticles on carbon thin films

and these particles were encapsulated in carbon nanocapsules. The amorphous carbon coating layers were graphitized into two or three graphite sheets. The particles grew further to form Au nanowires encapsulated in carbon nanotubes [52]. HRTEM has also been used to characterise nano-scale aggregates of polymers [53], copolymer nanotubes [54], supramolecules [*55] etc. The presence of heavy metal clusters in nano-scale single molecule [DAB-dendrhN(CH 2 PPh 2 ) 2 j 16 (m 3 : h1 h1 h1 -Ru 5 C(CO) 12 ) 16 ] made it possible for them to be directly imaged with very high image contrast. On a carbon film, the molecules showed a square shape in projection, and this moved around under

Fig. 6. Electron micrographs of carbon nanotubes coated with streptavidin molecules (a water-soluble protein). It shows helical crystallisation of proteins on carbon nanotubes. Reproduced with permission from Wiley-VCH Verlag GmbH.  1999, [**59].

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the electron beam like a dice before collapsing in a few seconds – just long enough for recording the images [*55].

scale inorganic, organic and biological materials will undoubtedly increase rapidly in the near-term future.

4. Biological specimens

References

Further extension of the application of the HRTEM surface profile imaging to biological specimens is another interesting development [56–60]. In their research of the eukaryotic community of modern analogue, the Santa Barbara Basin, Bernhard et al. used TEM to show crosssection views of aligned bacteria under the nematode’s aunulate cutile, and a dense layer of rod-shaped bacteria completely vesting the cuticle [56]. The magnification used was low, therefore, only the sizes, morphologies and the locations of the bacteria were detected. Chirality is a common phenomena at every level of biological structures. Since the HRTEM image shows a 2D projection of specimens, including information of surface as well as inner structure, characteristic image contrast allows us to distinguish left-handed from right-handed helices of large molecules [57,58]. When the specimen size reduces to nano-scale, the advantage of using HRTEM seems to be obvious. Helical crystallisation of proteins on carbon nanotubes is a significant advance towards developing new biosensors and bioelectronic nanomaterials. Since the layer contained ordered protein molecules – contributing strong diffraction contrast to the images unlike the contribution of the carbon nanotube to the image contrast which is relatively small – the surfaces of such protein-decorated nanotubes appeared in HRTEM images not only as a profile projection but also as a plan-view projection [**59] (Fig. 6). In Storhoff and Mirkin’s review article about synthesis of nano-scale materials using DNA as template, HRTEM images showed two-dimensional aggregates of DNA-linked 13 nm Au nanoparticles in an ordered arrangement and nanoparticle ‘satellite structures’ comprised of a 31 nm Au nanoparticle linked through DNA hybridization to several 8 nm Au particles [60]. Another review paper by Sleytr et al. presents many HRTEM images of crystalline bacteria cell surface layers either in a plan-view or in a profile-view [61].

Papers of particular interest, published within the annual period of review, have been highlighted as: * of special interest; ** of outstanding interest.

5. Conclusions HRTEM has hitherto never been regarded as a significant technique for surface science, e.g. it is hardly perceived as a relevant introduction in any surface science textbook. However, HRTEM now plays an important role in providing information about the surfaces of solids, especially those of nano-scale materials. These materials are the key components in nanotechnology. It is quite certain that HRTEM is a most powerful technique for local surface structural investigation. Its application to nano-

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