SEM images of the heterostructure of 2D carbon and 2D selenium: (a) top view; and (b) cross-sectional view. Scale bars: 1 m m. 

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... 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 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. 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 carbon nanowalls have also been employed as templates for depositing a variety of oxides which include ZnO, TiO 2 , SiO x , and AlO . 33 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 nano- sheets were found to form from electrodeposited iron and cobalt in the temperature range of 350–500 u C. X-Ray diffrac- tion measurements have confirmed that the 2D nano-sheets are Fe 2 O 3 . 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 3CdSO 4 :8H 2 O (61.6 g), H 2 SO 4 (19.6 g), SeO 2 (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 cm 2 1 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 of nanowalls is attributed to the formation of network-like catalytic structures on the substrate through controlling the thickness of the catalyst. Although the artificially structured laminar-type of 2D systems and the naturally formed layered materials have been studied intensively for both fundamental interests and applications, the work on ideal 2D systems formed from the bottom-up technique is still very limited. In this paper, we have briefly reviewed the different types of 2D systems and introduced our recent work on 2D nanocarbons. Further development in this field is expected due to their rich physics and potential ...