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F E AT U R E A R T I C L E Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576. E-mail: [email protected] b Data Storage Institute, 5 Engineering Drive 1, Singapore 117608 c Department of Physics, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Journal of a Materials Chemistry Yihong Wu,*a,b Bingjun Yang,b,c Baoyu Zong,b Han Sun,c Zexiang Shenc and Yuanping Fengc www.rsc.org/materials Carbon nanowalls and related materials Received 23rd September 2003, Accepted 15th December 2003 First published as an Advance Article on the web 22nd January 2004 Size, dimensionality, and shape play important roles in determining the properties of nanomaterials. So far, most of the nanomaterial researches have been focused on zero-dimensional nanoparticles/nanodots and onedimensional nanowires/nanorods/nanotubes, but very few studies have been carried out on two-dimensional nanosheets. Starting from carbon, recently we have succeeded in growing a class of nanostructured two-dimensional materials either in the pure forms or in the form of composites with carbon. In this paper, we will first briefly discuss various types of two-dimensional systems and then focus on the formation mechanism of carbon nanowalls and their field-emission and electron transport properties. The use of carbon nanowalls as templates for the formation of other types of nanomaterials will also be discussed. 1 Introduction DOI: 10.1039/b311682d The property of a bulk material is largely determined by the types of the constituent chemical elements and the nature of the chemical bonds that ‘hold’ the atoms and molecules together to form the material. However, this ‘conventional wisdom’ no longer holds in the nanometer regime in which, in addition to the chemical bonds, the size, dimensionality, and shape also play important roles in determining the properties of the Yihong Wu received his PhD from Kyoto University, Japan, in 1991 for his work on low-dimensional wide-bandgap semiconductors and their applications in blue-green lasers. From 1991 to 1996, he had been a research scientist with the Center for Optoelectronics, National University of Singapore, a Senior Research Engineer and deputy manager at the Panasonic Singapore Laboratories, and a faculty member at Tohoku University, Sendai, Japan. He re-joined the National University of Singapore in 1996 as a lecturer and was promoted to a senior lecturer in 1998 and an associate professor in 1999. He initiated a Nano Spintronics group at the Data Storage Institute in 1998 and since then he has also been serving as the group manager. His current research interests include spintronic sensors, magnetic random access memory, Yihong Wu and nanomaterials and devices. materials, especially the electronic, magnetic, and optical properties. The size matters when it becomes comparable to the length scale of a certain physical phenomenon that is concerned. Depending on their relative sizes in different spatial directions, materials can be divided into categories of different dimensionality ranging from three-dimensional (3D) to twodimensional (2D), one-dimensional (1D) and zero-dimensional (0D). The above classification is introduced for materials with simple shapes such as slabs/sheets (2D), wires/tubes/rods (1D), and cubes/spheres (0D). With the rapid advance in nanotechnology, it is now also possible to produce materials with a variety of shapes and asymmetry; this will add more degrees of freedom to the nanomaterials that can be used to alter their properties. Although the recent work on nanomaterials has been focused on the 0D and 1D systems,1–8 it is in the 2D system where the top-down approach of nanotechnology has been developed.9–16 This has led to the discovery of the quantum Hall effect17,18 and the creation of new devices such as high electron mobility transistors,19 intersubband infrared detectors and quantum cascade lasers in semiconductor systems,20 and the discovery of giant magnetoresistance (GMR)21,22 and invention of spin-valves in metallic systems.23 The work on the 2D systems also became the foundation for subsequent work on 1D and 0D systems.24,25 The above-mentioned 2D systems are obtained by the so-called top-down appraoch. Most of these 2D systems were realized in laminar structures of semiconductors, insulators, and metals. In addition to these artificially structured 2D systems, there are also many naturally formed 2D systems such as graphite, MgB2, transition metal dichalcogenides, intercalation compounds of graphite, high-T c superconductors, and many others.26–29 The common feature of these materials is that the electrical conduction is highly anisotropic, with a low reistance in the layer plane but a high resistance or being insulating perpendicular to the layers. It is worth noting that most of the layered compounds are also good superconductors. Regardless of whether it is an artificial laminar structure or a naturally formed layered coumpound, the 2D system formed in such a fashion is not a perfect 2D system in the sense that each 2D layer still has to interact with the adjacent layers either chemically or electronically. An ideal 2D system would be such that it consists of only a single nano-sheet without any electronic or chemical interactions with other types of materials or layers. Fig. 1 illustrates the different types of 2D systems. The 2D nano-sheets would be very useful not only for fundamental physics studies but also for practical applications due to their large surface-to-volume ratio. The nanobelts or nanoribbons might be considered as one type of such material This journal is ß The Royal Society of Chemistry 2004 J. Mater. Chem., 2004, 14, 469–477 469 form the tubular structure. This makes the nanotubes very attractive for future applications in electronics,40–42 optoelectronics,43 and sensors.44,45 The large strength of the nanotubes also makes them an ideal building block for nanomechanical devices.46 In contrast to the closed boundary structure of the fullerenes and nanotubes, the 2D graphite sheets are characterized by an open boundary. Theoretical studies have shown that this may bring about nanocarbons’ unique transport and magnetic properties.47–51 Of particular interest is the theoretical predication of ferromagnetism and superconducting instability in 2D graphite sheets and possible existence of unique transport properties. In addition, the 2D nano-sheets also have a surface area which is theoretically twice that of the closed boundary structures, making them more attractive for chemical and biosensor applications. The sharp edges are also promising for field-emission applications. 2.2 Carbon nanowalls Fig. 1 Schematic illustration of materials of different dimensionality ranging from 0D to 3D and the different types of 2D systems. system, depending upon the width-to-thickness ratio and the physcal length scale.30,31 However, considering the structual characteristics of layered compounds, the ideal 2D system would be the one that is formed by ‘peeling’ off the material from a layered compound in a layer-by-layer fashion. Such kinds of materials might have already existed in nature from ancient times, but most of them are yet to be discovered and explored with well-defined objectives. In fact, most of the nanometersized 2D sheets will be more stable in tubular shapes than in flat sheets. However, the 2D nano-sheets may be stable when they form self-supported network structures. Recently we have succeeded in growing such kinds of materials in the carbon system while we were working on the carbon nanotubes.32 After the sucessful growth of 2D carbons, we have extended the technique to the growth of other types of 2D materials, either in the pure form or in the form of composites with carbon.33 As expected, most of them are in network structures. In the following sections, we will discuss the growth mechanism, fieldemission and electron transport properties of the 2D nanocarbons. The use of the carbon nanowalls as templates to grow other types of 2D materials will also be discussed. In spite of the unique properties of 2D nanocarbons predicted by theory, they are yet to be verified experimentally. This is mainly due to the unavailability of such kinds of samples. One of the possible reasons is that nanometer-sized 2D carbons are not stable and tend to form tubular or cage structures. Recently we have succeeded in the growth of well-aligned 2D carbons (dubbed carbon nanowalls) on various substrates using microwave plasma enhanced chemical vapor deposition (MPECVD). The 2D carbons form a self-supported network structure which enhances their stability. As has been discussed elsewhere, the growth of carbon nanowalls was found during the growth of carbon nanotubes.32 Fig. 2(a) shows a typical SEM image of the carbon nanowalls. The distribution of the nanowalls is remarkably uniform over the whole substrate surface area that is typically 1 cm 6 1 cm. Fig. 2(b) shows some of the nanowalls peeled off from the substrate and laid down on top of the nanowall samples. The nanowalls grow very fast within the first 1–2 minutes and nearly stop growing after they reach a height of about 2 mm. The width is in the range of 0.1–2 mm; it increases with decreasing the nanowall density. The thickness of the nanowalls is typically in the range of one to several nanometers, as shown by the HRTEM images in Fig. 2(c) and (d). Note that the HRTEM image at the center portion of Fig. 2(c) was taken from a pile of carbon nano-sheets. The thickness of the nanowall can be estimated from the HRTEM image of a single piece of nano-sheet, as indicated by the arrow 2 Growth of carbon nanowalls 2.1 2D nano-sheets versus 1D nanotubes With the discovery of fullerenes and carbon nanotubes,34,35 a great deal of effort has been devoted to the development of similar types of nanostructures made up of other materials such as WS2, MoS2, NbS2, BN, BC2N, BC3, NiCl2, and CN, etc.36–39 The common feature of these materials is that they have a layered structure in the bulk form. When the size of these materials in one direction decreases to a few or tens of monolayers, the material becomes a thin sheet which is usually unstable and tends to curve and roll up in either one or more directions; this will lead to the formation of various types of nanostructures such as fullerenes, cages, cones, and tubes. The unique shape and symmetry of these nanostructures give rise to unique mechanical, electronic, magnetic, and optoelectronic properties which their bulk counterparts lack. Take the carbon nanotube as an example: it can be either a metal or a semiconductor, depending on the direction along which the graphene sheet is rolled up to 470 J. Mater. Chem., 2004, 14, 469–477 Fig. 2 SEM [(a) and (b)] and HRTEM [(c) and (d)] images of carbon nanowalls. Scale bars: (a) 100 nm, (b) 1 mm, (c) and (d) 5 nm. (a) was taken at a tilt angle of 25u. in the top right-hand corner of Fig. 2(c). Both the SEM and HRTEM observations show that there are two different types of nanowalls, one an open boundary nanographite sheet [Fig. 2(c)], and the other one more like a flattened tube with an empty interior [Fig. 2(d)]; the former predominates. 2.3 Effect of gas flow rate The hydrogen-to-methane flow rate ratio was found to cause rather drastic changes to the morphology of the nanocarbon films. Fig. 3 shows the morphology of the carbon films grown on Au (ca. 20 nm) coated Si substrates at different H2/CH4 flow rate ratios. The growth pressure was 1 Torr. MPECVD is a well-known technique for growing diamond films at a H2/CH4 flow rate ratio of 100. As the growth temperature in this work is about 650–700 uC, the high H2/CH4 flow rate ratio hardly led to any observable growth of carbon within a short period of 5 min. As the flow rate ratio is decreased to 30, some columnar structure of amorphous carbon forms. A further decrease of the gas flow rate ratio leads to the formation of a mixture of fibers/tubes and 2D nanographite sheets. A pure form of carbon nanowalls forms when the gas flow rate ratio is in the range of 4–8. The amorphous carbon forms again when the ratio is too low. Based on these, in all the experiments to be discussed below, the H2/CH4 flow rate ratio has been fixed at 4. 2.4 Effect of electrical field In the early stages of our work on carbon nanowalls, we found that the key to growing the nanowalls instead of nanotubes was the emergence of a strong lateral field caused by the nonuniform charging of the catalyst islands.32 However, in subsequent experiments, it was found that the nanowalls could also grow on substrates without any catalysts. This seemed to cast some doubt on our initial findings and prompted us to perform more experiments under a controlled environment for investigation of the electric field influence. In order to find unambiguous evidence on the effect of the electrical field, we have performed the following: (i) to use surface plasmons (SPs) Fig. 3 SEM images of carbon grown at different H2/CH4 flow rate ratios: (a) 30, (b) 15, (c) 10, (d) 6, (e) 4, (f) 1. Scale bars: (a), (b), (d), and (f) 1 mm; (c) and (e) 100 nm. to excite localized electrical field, (ii) to use large metal droplets as the catalysts, and (iii) to create sharp features on the substrate surface. In what follows we first discuss the effect of the electrical field generated by the surface plasmon, followed by other techniques. The details on how to create SPs on the substrate surface during the growth of carbon nanowalls have been reported elsewhere.52 The surface plasmon was known to cause large enhancement of local electrical fields53 and thus it was expected that the presence of a localized surface plasmon would affect the growth of carbon nanowalls locally. As expected, the influence of the surface plasmon on the growth of carbon nanowalls was a rather drastic one. The influence of the surface plasmon on the macro-scale has been discussed in detail in ref. 52; here we only focus on the microscopic effects of the plasmon-induced electrical field on the orientation of the carbon nanowalls. Fig. 4 summarizes the unique carbon nanowall patterns that have been observed due to the presence of surface plasmons. The circular region consists of an outer ring with denser nanowalls and a flower-like nanowall structure at the center. The latter consists of one to several poles and the number of poles increases with the density of the nanowalls surrounding the circular region, so is the size of the whole region (see Fig. 4a–h). However, the occupation ratio of the outer ring in the whole circular region decreases when the number of poles increases. Fig. 4i shows an enlarged view of the boundary between the circular region and the region surrounding it. For clarity, the sample was tilted by 20u when taking this picture. It shows clearly that the nanowalls orient randomly outside the circular region, while they align well along the circumference direction in the rim region and change the direction by almost 90u when they move further to the central region. It demonstrates clearly how the carbon nanowalls change their orientation in an extremely localized region. This could hardly be possible without the existence of strongly localized electric fields induced by the surface plasmon. The patterns shown in panels (a)–(h) resemble well the electric field distribution of multiple pole SP predicted by Mie’s theory with the number of poles increasing from (a) to (h).54 It is interesting to note that patterns with both odd and even numbers of poles were observed, though Mie’s theory for a metallic sphere only predicts patterns with an even number of poles. This was probably caused by the uneven shape of the Au particles, which has been confirmed by the SEM observation of Au particles formed on bare substrates. The size of the pattern increases with the number of poles: it is ca. 8 mm for the dipole Fig. 4 SEM images of carbon nanowall patterns formed by the electrical field of surface plasmons with different numbers of poles. (i) is the enlarged image of portion (A) in (h). Scale bars: 1 mm. J. Mater. Chem., 2004, 14, 469–477 471 pattern shown in panel (a) and 20 mm for the multiple pole patterns shown in (g) and (h). Assuming that the surface plasmon emission travels at the same speed of light in vacuum, it gives lifetimes of ca. 25 and 70 fs, respectively. These values agree well with the reported lifetimes for surface plasmons reported in the literature. Although the theory for calculating the electrical field distribution of SPs has existed for about a century since the pioneering work of Mie, it has been difficult to observe the field distribution experimentally due to the strong localization and short lifetime of such fields.55–57 In addition to gaining an insight into the growth mechanism of the carbon nanowalls, this work has also successfully ‘fingerprinted’ the electric field of the SP at nanometer-scale accuracy. This is a remarkable result because it is the only technique reported so far which can detect the electrical field instead of the intensity of the SPs. The effect of the electrical field can also be seen clearly in the region where the two surface plasmons interfere constructively. The interaction occurs through the coherent addition of the electric fields of the two SPs. As a result, the electric field distribution of both SPs inside the interaction region was mutually perturbed, while that of the larger SP outside the interaction region remains almost intact. Fig. 5 shows one such example in which a smaller plasmon (B) is contained within a larger plasmon (A). In the case of an isolated SP, the nanowalls just next to the rim region are oriented in the radial direction of the circular pattern. This is still true along line BE in Fig. 5 where the radial directions of the two circular patterns coincide. On the other hand, at the cross point of AD and BC, and AF and BG, the nanowalls are oriented in the direction which almost bisects the two radial directions. This implies that the nanowalls are orientated in the total electrical field direction which is the vector sum of the electrical fields from the two SPs. In the above experiment, we used Au nanoparticles to excite surface plasmons which in turn generate localized fields to alter the growth of carbon nanowalls. Another natural way to create localized field distributions is to put large metallic particles on the substrate surface which would generate unique field distributions surrounding the particles. We have chosen gallium for the metallic particles because of its low meting point. Fig. 6 shows the SEM images of several types of nanowall pattern formed surrounding the gallium droplets. As it is shown in panel (a), the nanowalls started to grow on the surface of a round gallium droplet in almost the radial direction. However, once the surface stress becomes too high due to the temperature rise or weakening of the surface tension due to the incorporation of carbon into the surface layer, the large gallium droplets break from the top to form disk-like shapes like the ones shown in panels (b) and (e). In this case, the orientation of the nanowalls changes from the radial direction in the proximity of the droplet surface to the circumference direction next to it and then finally back to the radial direction. It seems that the dynamic field distribution is not only determined by the topography of the gallium droplet but also by the nanowalls grown on its surfaces. The final orientation of the nanowalls will be Fig. 5 SEM image of the carbon nanowalls grown in the area where two surface plasmons interfere constructively. Scale bar: 1 mm. 472 J. Mater. Chem., 2004, 14, 469–477 Fig. 6 SEM images of carbon nanowalls formed surrounding the Ga droplets. Scale bars: (a)–(c) and (e) 1 mm; (d) 100 nm. determined self-consistently. For small droplets, however, the nanowalls have a spiral structure along the droplet surface [(c) and (d)]. These results suggest again that the electrical field plays an important role in determining the orientation of the carbon nanowalls. In addition to the use of SPs and large metal droplets, we have also tried to modify the morphology of the substrate surface so as to alter the electrical field distributions at localized areas. One of the approaches that we have tried was to use anisotropic etching to create pyramid-like structures on the Si substrate [Fig. 7(a)]. Owing to the sharp features of the pyramids, the electrical field was expected to be enhanced at the locations of the pyramids which would thus affect the growth of the carbon nanowalls. As shown in Fig. 7(b)–(d), the size of the nanowalls grown on top of the pyramids is Fig. 7 SEM images of (a) pyramid structures formed on a Si substrate using anisotropic etching, and (b)–(d) carbon nanowalls grown on the Si substrate with the pyramid structures. Scale bars: (a) and (b) 1 mm; (c) and (d) 100 nm. apparently larger than that of the nanowalls formed in other places. The directionality of the nanowalls grown on top of the pyramids was also improved as compared with those grown in the flat surface regions. 2.5 Possible growth mechanism Although further studies are required to understand the growth mechanism of the carbon nanowalls, the above results suggest that the electrical field is the most important factor, at least in the particular experimental set up used in this study. The electrical field comes from both the DC bias and the nonuniform charging up of the substrate; the latter can be altered by the existence of metallic particles and other types of sharp features. When the metallic particles act as both a catalyst and a field modulator, both nanotubes and nanowalls can be formed depending on the strength of the lateral electrical field and other growth parameters such as the gas flow rate, temperature, and pressure. However, when non-catalytic particles or bare substrates are used, it is more likely that carbon nanowalls will be formed. To shed some light on how the nanowall was formed, we show in Fig. 8 the SEM images of the nanowalls at different growth stages on the Si substrate coated with a 20-nm-thick gold layer. The images were taken from different portions of the same substrate which was placed on the substrate holder in such a way that the growth rate would vary from one side to the other due to the offset from the optimum position. During the pre-heating stage, the Au film becomes isolated nanoparticles. As can been seen from Fig. 8(a), at the very beginning of the growth, carbon forms surrounding the Au particles; but instead of forming a closed tubular structure, they expanded in the lateral directions and finally became connected with one another to form larger walllike structures. As shown in panel (e), at the initial stage, the walls are not single pieces of graphite but are made up of individual half-opened tubules. With the progress of the growth, the individual half-opened tubules expand in the lateral direction to form single pieces of nanowalls [from panel (b) to (d)]. This observation is consistent with our previous observations on the growth of carbon nanowalls using transition metals as the catalysts,32 which implies that the existence of a strong lateral field is the key to growing carbon nanowalls in our case. on 1 cm 6 1 cm Cu substrates. The field-emission measurements were carried out in a vacuum chamber at a base pressure of 1 6 1025 Torr. A polished copper (1 cm 6 1 cm) anode was positioned 20–200 mm away from the nanowall cathode to collect the emitted electrons from the latter. As the base pressure of our measurement chamber is relatively high, the measurements were carried out at different temperatures so as to investigate the effect of absorbates on the nanowall surfaces. The turn-on electrical field usually increases with decreasing the anode–cathode distance. It is typically in the range of 1–1.5 V mm21 at an emission current density of 10 mA cm22, though the value can be much smaller for some ‘best case’ samples. Fig. 9 shows the emission current densities as a function of the applied electric field for one of such samples obtained at temperatures of 20, 200, 300 and 400 uC, and at an anode–cathode distance of 50 mm. The experiments were carried at 20 uC first, and then repeated at different temperatures after the substrate was heated up using a resistive heater and stabilized at each temperature setting point. The current densities were 0.19 and 9.53 mA cm22 at electrical fields of 0.32 and 0.62 V mm21, respectively. The turn-on electric field decreased to 0.26, 0.2 and 0.16 V/mm with the temperature increasing to 200, 300 and 400 uC, respectively. The highest emission current density obtained was 17.6 mA cm22 at an applied electric field of 0.32 V mm21 at 400 uC. A further increase of the applied electric field resulted in arcing, which prevented us from evaluating the highest possible current density. This is in part caused by the high base pressure of our particular measurement chamber. Fig. 9(b) shows the corresponding Fowler–Nordheim (F–N) plots of the field-emission curves shown in Fig. 9(a). The fieldemission characteristics can be analyzed using the F–N equation58 ! 3/2 V2 {Bw , (1) I! 2 exp d bV =d where I is the current in Amps, V is the applied voltage in 3 Properties of carbon nanowalls 3.1 Field-emission The unique morphology and geometric shape of the nanowalls make them promising for field-emission applications. To this end, a series of experiments have been carried out to investigate the field-emission characteristics of the nanowalls. The nanowalls for the field-emission experiments were grown Fig. 8 SEM images of carbon nanowalls grown on a Si substrate at different stages of growth using Au as the catalyst. Scale bars: 100 nm. Fig. 9 (a) Emission current density as a function of the electrical field at different temperatures for carbon nanowalls, and (b) the corresponding F–N plots of the curves in (a). J. Mater. Chem., 2004, 14, 469–477 473 units of Volts, B is a constant given by 6.8 6 107, w is the emitter work function in eV, d is the spacing between the anode and the cathode in units of cm, and b is the geometric enhancement factor. As can be seen in Fig. 9(b), the plots of ln(I/V 2) versus 1/V yielded a straight line which confirmed that the current resulted from field-emission. This is particularly true for the measurements performed at 300 and 400 uC. However, the agreement is not good in the low-temperature curves for which we need to divide the curves into two straight line segments so as to fit the F–N equation. This suggests that there exist two energy barriers with different heights at lower temperatures, which could be caused by the absorbates on the nanowall surfaces. As the absorbate-induced work function change is proportional to the effective field at the absorbate, it is not difficult to understand that a higher barrier height was observed at the higher field portion of the F–N plot. Owing to desorption of the absorbates, the higher barrier portion disappears at higher temperatures. From these plots, we could obtain the slope which is equal to Bw3/2d/b, from where the field enhancement factor b could be estimated. Using the work function w of 5 eV, we calculated that the field enhancement factor b was 31 400, 46 200, 54 800 and 62 900 for the experiments performed at 20, 200, 300 and 400 uC, respectively. The temperature dependence is difficult to be understood because the enhancement factor is supposed to be only dependent upon the geometrical shape of the emitter and distance between the cathode and anode. One may argue that this could be due to the decrease of the work function with the increase of temperature. But, if we extend the straight lines of the F–N plot in Fig. 9(b) to the left side until they intersect with the longitudinal axis, we found that the four lines intersect at the same point. Note that in this case, we used the lower field portions of the curves for temperatures at 20 and 200 uC. As the intersection point is dependent upon the work function only, it suggests that the increase of b is not due to the decrease of w. In addition to the temperature-dependence, the value of b at low temperature is also one order of magnitude higher than those reported for carbon nanotubes at similar value of cathode–anode spacing. Further experiments are needed to find out the underlying mechanism for the large enhancement factor. In addition to the temperature-dependent field-emission measurements, experiments were also carried out to study the effects of N2, H2, O2 and CH4 gases on the field-emission properties of carbon nanowalls at room temperature. The results will be published elsewhere. Although we have in general obtained favorable results for the carbon nanowalls, it is still too early to say that the nanowalls are better than the nanotubes for field-emission applications.59–63 Further comparative experiments need to be carried out under similar conditions so as to find out the real differences in field-emission between the two types of nanocarbon materials.64 dependence of the resistance in the temperature range from 2 to 300 K which is shown in the inset of Fig. 10. As can be seen from the figure, the resistance decreases with decreasing the temperature from 300 to about 106 K, and then shows an upturn at lower temperatures. The temperature (T) dependence of the resistance (R) can be fitted very well using the following equation:65 {127 15:1 R~0:0018Tz2:5 exp z10:8 exp (2) T Tz34:5 in which the first term represents the metallic contribution, the second term is due to the quasi-1D characteristic of the network structure, and the last term accounts for the hopping/ tunnelling resistance of the junctions. The units of R and T are in V and K, respectively. The junction resistance dominates the total resistance in the entire temperature range, in particular, at low temperatures. Hopping is thermally activated at high temperature, but at low temperature tunnelling is dominant. Also shown in the inset of Fig. 10 is the rate of change of resistance with temperature (dR/dT). The resistance continues to increase with decreasing temperature, reaching the first local maximum at about 4.2 K; after a local minimum is reached at about 3.6 K, it increases again until the temperature decreases to 3 K below which the resistance decreases monotonically until reaching 2 K, which is the lowest temperature of the SQUID setup used in this experiment. When a magnetic field is applied in the direction perpendicular to the substrate surface, i.e. almost along the surface of the carbon nano-sheets, both local maxima of the curve shift to lower temperatures. The resistance, in general, also increases with the magnetic field. It is interesting to note that the resistance is not affected by the applied field at temperatures higher than 7 K. The result may indicate that superconducting instability is developed below 7 K in the 2D carbon nano-sheets, though we don’t exclude other possibilities. The detailed mechanism responsible for the temperature-dependent behaviour of the resistance will be discussed elsewhere in combination with the magnetic measurement data. Fig. 11(a) shows the magnetoresistance curves at different temperatures in which the average background signals have been removed by first fitting the curves with polynomials and 3.2 Electron transport properties The ideal graphite is a semimetal because of its zero bandgap and vanished density of states at the Fermi level. However, the situation changes for nanometer-sized graphene sheets due to the edge and surface effect. This may bring about the nanocarbons’ unique magnetic and transport properties. In spite of the theoretical efforts, to the best of our knowledge there has been no report on the experimental study of the transport and magnetic properties of 2D nanographite sheets, probably due to the unavailability of such kinds of materials. The successful growth of the carbon nanowalls without any catalysts now allows us to carry out transport and magnetic property studies of 2D carbons. Owing to the space limitations, here we only present briefly the results on electron transport. Fig. 10 shows the temperature dependence of the resistance in the temperature range from 2 to 10 K at applied magnetic fields of 0 and 400 Oe. We first discuss the temperature 474 J. Mater. Chem., 2004, 14, 469–477 Fig. 10 Temperature dependence of the resistance of the carbon nanowalls at low temperature at zero-field and a field of 400 Oe. Inset is the temperature dependence of the resistance over a wider temperature range. Also shown is the first derivative of the resistance with respect to the temperature. Fig. 12 SEM images of Au formed on the carbon nanowall templates at different nominal thicknesses: (a) and (b) 20 nm; (c) and (d) 30 nm; (e) and (f) 100 nm. Scale bars: (a)–(d) and (f) 100 nm; (e) 1 mm. Fig. 11 Magnetoresistance curves of the carbon nanowalls measured at different temperatures (a), and enlarged portion of the curve at 4.31 K (b). The inset of (b) is the Fourier transform spectrum of the entire curve at 4.31 K shown in (a). then subtracting the fitted data from the original data. For the sake of clarity, the curves are displaced along the vertical axis. The resistance oscillates strongly with the external field which is applied perpendicular to the substrate surface. The oscillation sets in at about 7 K and its amplitude increases by more than three orders of magnitude when the temperature is decreased from 6 to 2 K. The oscillations are, in general, quasi-periodic; however, the periodicity improves with temperature. To illustrate this trend, a portion of the magnetoresistance curve at 4.31 K is shown in Fig. 11(b) with the inset being the Fourier transform spectrum in which three peaks corresponding to different periodicities have been observed. Detailed studies are being carried out to reveal the physics behind the oscillatory phenomenon. 4 Carbon nanowalls as a template for growing other types of materials The novel surface morphology of the carbon nanowalls makes it an ideal template for synthesizing mesoporous materials with high surface areas. One of the possible applications of the carbon nanowalls is in batteries. To this end, we have tried to fabricate composites comprised of carbon and magnetic nanoparticles. This has been done from two different approaches. In the first approach, the nanowalls were used as the adhesive bases to absorb nanoparticles available commercially, whereas in the second approach the nanowalls were used as templates to deposit the nanoparticles using electrochemistry. The details can be found in refs. 33 and 66. In addition to electroplating, we have also employed evaporation to deposit Au and Cu on the carbon nanowalls. Fig. 12 shows the typical SEM images of Au films formed on the nanowall surfaces: (a) and (b), for a nominal thickness of 20 nm; (c) and (d), for a nominal thickness of 30 nm; and (e) and (f) for a nominal thickness of 100 nm. The Au film is in a particulate form when the nominal thickness is 20 and 30 nm, while it becomes a continuous layer when the nominal thickness reaches 100 nm. It is interesting to see from panel (a) that the nanoparticles formed on the carbon nanowalls are smaller than those formed on the bare substrate just next to the nanowalls. Fig. 13 shows the SEM images of Cu formed on the nanowalls with a nominal thickness of 30 nm. Comparing with Au, the Cu particles are even smaller on the side walls. The other feature is that continuous Cu wires are formed on the top edges of the nanowalls. The above results demonstrate that, compared with the nanotubes, the nanowalls are more suitable for functionalizing purposes which makes them promising for chemical and biological applications. The nanowalls were also found to be good templates for fabricating nanocrystals of certain metals such as zinc. Fig. 14 shows the SEM images of Zn nanocrystals formed by molecular beam epitaxy at room temperature [(a) and (b)] and by electrodeposition (c), though the similar types of nanocrystals could not be grown on a flat substrate under similar conditions. The Fig. 13 SEM images of Cu deposited on the carbon nanowall templates at a nominal thickness of 30 nm. Scale bars: 100 nm. J. Mater. Chem., 2004, 14, 469–477 475 Fig. 14 SEM images of the Zn nanocrystals grown on the carbon nanowall templates by: (a) and (b) molecular beam epitaxy; and (c) electroplating. Scale bars: 100 nm. carbon nanowalls have also been employed as templates for depositing a variety of oxides which include ZnO, TiO2, SiOx, and AlOx.33 5 Other kinds of 2D nanomaterials In addition to carbon, we have also tried to form 2D oxides of transition metals. The general technique that we have used was to deposit metal films of appropriate thickness by electrodeposition and subsequently to anneal the sample in air for several hours. The end product ranges from 0D nanoparticles to 1D nanowires and 2D nano-sheets, depending on the starting materials and the anneal temperature. The 2D nanosheets were found to form from electrodeposited iron and cobalt in the temperature range of 350–500 uC. X-Ray diffraction measurements have confirmed that the 2D nano-sheets are Fe2O3. In addition to the growth of nanoparticles and films on the carbon nanowalls, it would also be good if one can grow other types of 2D nanostructures on top of the edges of the carbon nanowalls. Considering the sharp edges of the carbon nanowalls, a natural way to do this is to use electroplating because of the high current density at the edges of the nanowalls. However, as discussed above, the deposition of magnetic materials does not necessarily only originate from the edges but also from other high current density points on the nanowalls, and in most cases nanoparticles or continuous films are formed. Is it possible to form 2D materials following the morphology of the carbon nanowalls? We believe that there is a possibility there and it is only a matter of how to identify the right materials. As one of the examples, Fig. 15 shows 2D selenium grown on top of the edges of carbon nanowalls using pulsed electrodeposition. The solution used is 3CdSO4:8H2O (61.6 g), H2SO4 (19.6 g), SeO2 (0.062 g), and water (800 ml). Fig. 15(a) shows the top view, and Fig. 15(b) shows the cross-sectional view. Although some small particles are formed inside the carbon nanowalls, most of the selenium is deposited as 2D nano-sheets on top of the carbon nanowall edges. The Raman spectrum is dominated by a sharp peak at 256.92 cm21 which suggests that the as-deposited material is amorphous selenium. These types of 2D heterostructures could be useful in increasing the electron–hole separation efficiency when being applied to solar cells. To this end, further studies are needed to identify the right materials so as to form pn junctions. Recently Ng et al.67 have succeeded in the growth of ZnO nanowalls based on the vapor–liquid–solid mechanism. In this case, the growth 476 J. Mater. Chem., 2004, 14, 469–477 Fig. 15 SEM images of the heterostructure of 2D carbon and 2D selenium: (a) top view; and (b) cross-sectional view. 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