Ultralong natural graphene nanoribbons and their electrical conductivity**

Transcription

1 Ultralong natural graphene nanoribbons and their electrical conductivity** By M. Moreno-Moreno, A. Castellanos-Gomez, G. Rubio-Bollinger, J. Gomez-Herrero *, N. Agraït *. [*] Prof. J. Gómez-Herrero, Prof. N. Agraït, Prof. G. Rubio-Bollinger, Mr. A. Castellanos- Gomez, Ms. Moreno-Moreno Departamento de Física de la Materia Condensada (C III), Universidad Autónoma de Madrid, Campus de Cantoblanco, Madrid, Spain. An atomic force microscopy topograph with a long and narrow ( 24 µm 37 nm) graphene nanoribbon emerging from a graphite flake laying on a silicon oxide substrate is shown in the image. The sample is prepared by microcleavage using silicone stamps instead of cellophane tape. These nanoribbons present well defined edges along the graphite crystallographic directions. [**] We would like to thank M. Vélez for introducing us the PDMS stamps fabrication technique. And we also thank B.J. Basilio for the oxidation of our silicon wafers. This work was supported by MEC (Spain) (MAT C02-01/, MAT and NAN C10 05/06), MICINN (Spain) (MAT and CONSOLIDER-INGENIO-2010 CSD ) and, Comunidad de Madrid (Spain) through programs NanoObjects and Citecnomik (S 0505/MAT/0303, S_0505/ESP/0337). Keywords: Graphene, nanoribbons Graphene is a 2D material with many possible applications in material science 1, nanomechanics 2 and nanoelectronics 3. While high-yield production methods for graphene sheets are a hot research topic 4,5,6,7 so far most of the experiments are performed on graphene flakes obtained by micromechanical cleavage 8. As reported by a number of recent theoretical works 9,10,11,12 (just to cite a few) graphene nanoribbons (GNRs) are expected to have new properties respect to graphene flakes. However, experimental work related to GNRs is not so abundant. GNRs have been produced by lithographic technique 13, 14 or by chemical procedures 15,16 but to our knowledge they have been never

2 been obtained by micro mechanical cleavage. This method present some advantages respect to the other two, for instance: e-beam lithography may produce ill-defined edges and chemical synthesis involves a quite sophisticated preparation. We introduce a simple method for graphene flake deposition based on silicone stamps. Using optical and atomic force microscopy (AFM), we characterize the flakes finding ultralong GNRs with minimum width below 20 nm that are electrically characterized by conductance AFM. As the ribbons are consequence of the cleaving process (natural GNRs), we expect clean edges along the crystallographic graphene directions and low defect density. Graphene is a single layer of graphite; consequently, it can be considered as an almost ideal 2- dimensional material. Many of the exotic properties measured in graphene 17,18,19,20 are a direct consequence of this low dimensionality and the relativistic-like behaviour of its charge carriers. This introduces a new kind of properties that cannot be found in other materials. When a graphene flake is reduced to a long nm wide ribbon, additional properties emerge in the already rich scenario of graphene. In particular, lateral confinement of the charge carriers causes the opening of an energy gap, which depends on the width of the nanoribbon 21,13,14. Crystallographic orientation has been predicted to play a key role, as in the case of carbon nanotubes, in the electrical properties of GNRs. Graphene flakes obtained by micromechanical cleavage, show well defined edges naturally produced during the cleaving process. For the case of GNRs obtained by lithography technique, this is unclear and one may expect to have ill-defined edges. State of the art lithography has a resolution of about 10 nm [REF?] and this imposes a limit for defining the GNRs edges. Further complications could arise from the fact that lithography techniques may contaminate the sample and in particular the edges of the GNR 22,23. A new procedure to obtain GNRs was recently presented by Xiaolin Li et al. 15. The authors report submicron length nanoribbons with minimum width below 10 nm. The ribbons are obtained through a complicated chemical process that can be also used (with small variations) to produce graphene flakes. Although the graphene flakes produced by the chemical route are of good quality they are still far from the ones prepared by mechanical cleavage. This may also happen for chemically obtained GNRs. Our samples are prepared by a variation of the transfer printing technique 24. Poly (dimethyl)- siloxane (PDMS) stamps, commonly known as silicone stamps, are used to cleave highly oriented pyrolytic graphite (HOPG). Cleaved graphite is then transferred to an insulating surface to make possible electrical transport measurements. The substrate was previously cleaned by sonication in acetone (5 min.) followed by a short heating in air to remove completely the acetone from the surface. We have chosen a 285 nm thick layer of thermally grown silicon oxide as isolator to facilitate optical characterization of graphene 25. A highly p-type doped silicon substrate allows electrical characterization of the sample even at low temperatures. The use of PDMS stamps, instead of the commonly used Scotch tape, avoids traces of glue over the surface during the graphite transfer process. The essential step in our fabrication procedure is the use of a second clean PDMS stamp to cleave again the graphite flakes previously transferred on the SiO 2 surface. Most of the graphite is removed in this way leaving only thin graphite flakes on the surface. Figure 1 shows an example of a thin graphite flake obtained using this method. Two features can be seen at first glance: first, the well defined crystallographic directions expected for crystalline graphite that is also characteristic of conventional micromechanical cleavage technique using Scotch tape. Second, the flake resembles a frayed fabric. This is particularly clear in the upper side of the flake where many thin ribbons protruding from the mother flake can be seen. For instance, the wide ribbon marked with a green arrow in the optical microscope image has a width, according to the AFM image, of about 1.1 µm.

3 Ribbons as narrow as 200 nm can be found in about one fourth of the samples characterized by optical microscopy 25. A more detailed inspection in the vicinity of these ribbons using AFM and scanning electron microscopy reveals that in one half of the cases there are also narrower nanoribbons whose width range between 20 nm ~ 120 nm. The use of an AFM allows us to characterize the thickness of these flakes and nanoribbons that can be as thin as one monolayer. A relevant example of an isolated nanoribbon is shown in figure 1b. The ribbon that starts in a relatively small flake, has a length of about 25 µm, with an almost uniform width of 37 nm and a thickness ranging from 2 to 1 nm. Fig 1. (a) Phase contrast optical microscopy image of a graphene flake and several ribbons following its crystallographic directions. The inset is a 10x10µm 2 AFM topography of the region enclosed by the square. The marked ribbon in the inset is 1.1µm wide and the thinner one on the left is 150nm wide. The former is also visible in the optical microscopy image (marked with an arrow) while the latter is hardly noticeable. As the optical microscope image shows, the sample surface is quite clean. (b) AFM topography of a 37nm wide ribbon emerging from a graphite flake. This ribbon has a length of ~24µm and a thickness ranging from 1 to 2nm. The upper inset is a profile taken along the green upper line showing the dimensions of the nanoribbon. The lower inset is a phase contrast optical microscopy image showing the mother flake. To guide the eye the position of the AFM image has been marked with a rectangle. Inside the rectangle a dashed line shows the position of the nanoribbon. After sample preparation, a 20 nm thick gold electrode is thermally evaporated by means of a glass shadow mask for further electrical characterization. This mask covers the selected nanoribbons and partially the mother flake. Therefore, the nanoribbons are electrically connected to the gold electrode through the mother flake. In this way graphene is never exposed to chemicals as during a lithography process which can alter its electronic properties 22,23. Figure 2a is a composition of 12 AFM topography images showing the edge of a flake (left side of the image) and 3 narrow ribbons protruding from this flake. The total length of the image is about 50 µm. Figure 2b is a zoom of the previous image where the 3 ribbons can be seen in more detail. The profile measured along the upper green line (figure 2c) shows ribbon heights ranging from 0.8 to 1 nm compatible with a thickness of 1 or 2 graphene layers 17. We have taken higher

4 magnification AFM images to accurately measure the narrowest ribbon dimension (not shown). The profile in figure 2d belongs to one of these images and has been marked with a short green line in figure 2b. Unfortunately, we have not enough resolution to measure the geometry of the edge and therefore it is not possible to correlate the electrical characteristics of the nanoribbons to its structure. The AFM tip-ribbon dilation produces an unrealistic width for the profile. The width increment due to tip dilation can be estimated as W=2 [h (2 R - h)] 1/2, being R and h the tip radius and ribbon thickness 26. For a 20 nm tip apex radius (value obtained by taking images of a reference sample with SWNTs) imaging a 1 nm height flake, the dilated image width will be increased in approximately 12 nm. This is the same as to say that the real width of the ribbon coincides with the flat part of the profile (figure 2d). This indicates that the real minimum width of this ribbon is below 20 nm. Figure 2 (a) Composition of 12 AFM topography images showing 3 nanoribbons. The upper scale shows the length in microns. (b) 4x4 µm 2 AFM topography showing a detailed view of the 3 nanoribbons. (c) Profile along the upper green line showing the height and width of the nanoribbons. (d) Profile along the lower green line showing the minimum width of the nanoribbons. The electrical characterization of the nanoribbons is carried out by means of conductance atomic force microscopy. In order to obtain reliable results we use AFM cantilevers with about 50 nm gold-palladium coating. Our experience demonstrates that to avoid tip contamination it is important to use recently covered (or kept in vacuum) cantilevers. In our experimental setup, the metallized AFM tip is used to measure the current vs. voltage (IV) characteristics (see figure 3c) of the nanoribbons as a function of the distance between the metallic AFM tip, used as mobile electrode, and a fixed macroscopic gold electrode 27. A highly doped silicon substrate allows us to apply a backgate in our measurements (inset figure 3a). Briefly, the experiments are performed as follows: a nanoribbon is first selected (figure 2) and then electrically characterized. From a linear fit of the IV characteristics (figure 3c) at zero bias voltage, the electrical resistance is determined along the nanoribbon at a fixed backgate voltage (figure 3a). The experimental setup enables us to increase the force applied on the nanoribbon by the mobile electrode, until an optimum contact resistance is reached. For single walled carbon nanotubes (SWNTs), the mechanical load applied to measure the IV curves is a key factor to determine the electrical resistance of the nanotube 28. For low mechanical loads a tunnel barrier is present between the tip and the nanotube, and the electrical resistance is quite high. As the tip presses

5 the nanotube, the tunnel barrier and the electrical resistance drop until an optimum contact is reached 28. If the mechanical load is further increased then, the SWNT is deformed changing its band structure that is reflected on a band gap opening. Using conductance AFM optimum resistances of almost 6.5 kω (the minimum possible resistance for a SWNT) have been reported 29. The results obtained using conductance AFM are in excellent agreement with those obtained using more conventional techniques. For instance, the room temperature 1D electrical resistivity obtained by using an array of nanoelectrodes along a metallic SWNTs 30 is ρ 1D =11 kω/ µm, while using conductance AFM we obtained and average 1D resistivity for 8 different SWNTs of ρ 1D =10±2 kω/ µm 31 (see grey dashed line in figure 3a). For the case of the nanoribbons, we observe a contact improvement at low mechanical loads. This suggests, as for nanotubes, the presence of a tunnel barrier. But once the electrical contact is optimized we have never observed any dependence of the electrical resistance with the mechanical load. Besides, our experiments do not show any particular correlation between the flake thickness and the electrical contact resistance that varies from 1.5 kω to 4 kω for thickness ranging from 0.8 to 17 nm and a common backgate of 0 V confirming the low out of plane conductance of graphite. 32. For all the measurements, we first check the tip state by contacting it with the gold electrode; we consider a tip as acceptable when the measured electrical resistance is lower than 2 kω. Figure 3. (a) and (b) show the low voltage resistance and width, respectively, of nanoribbons A (green squares), B (blue circles), and C (red triangles) as a function of distance to the mother flake. The slope of the resistance (or 1D resistivity, ρ 1D =dr/dl) is fairly constant at large distances from the mother flake. The experimental R(L) of 8 different metallic single wall carbon nanotubes 31 has also been plotted for comparison (gray). The inset in (a) illustrates the experimental procedure. At each position of the conducting tip in the nanoribbon, the current vs tip-sample bias (V ts ) voltage characteristic is measured and the low voltage resistance is obtained from the slope at zero bias. Typical IV characteristics are shown in (c). The rectifying character observed in these characteristics suggests the semiconducting behavior of the ribbons. The inset in (b) shows a zoom of the width of the thinnest part of ribbon A. (d) shows σ 1D =1/ρ 1D as a function of width for ribbon A. The observed linear

6 dependence is to be expected for a material with a constant sheet resistivity ρ 2D, since σ 1D = w/ ρ 2D. From the inverse of the slope we obtain a sheet resistivity ρ 2D = 500 Ω. Figure 3a shows the low voltage resistance for the 3 nanoribbons as a function of their length. Below, sharing the same horizontal axis, the width of the 3 ribbons has been plotted. The 1D resistivity, ρ 1D =dr/dl, of ribbon RC is almost constant and very low (<1.5 kω/ µm), the same magnitude for the 2 other ribbons grows to hundreds of kω/ µm in the narrowest part. The correlation between 1D resistivity and width w is evidenced in figure 3d, which shows the inverse of ρ 1D vs w for RA. The observed linear dependence is expected for a material with a constant sheet resistivity ρ 2D, since the contribution of an element of length L to the resistance of the nanoribbon would be R= ρ 2D /w L, and consequently 1/ρ 1D = w/ρ 2D. The value obtained for the sheet resistivity ρ 2D = 500 Ω, far from the maximum resistivity value (Dirac point) 33, is consistent with that observed by Kim 13. Our results are also compatible with the existence of an inactive width of several nanometers, although the dispersion in the data and the experimental uncertainty in the GNR width makes difficult a precise determination of the resistivity value. Chemically fabricated nanoribbons show also a very similar value for the resistivity 16. We have estimated the mobility of a 80 nm wide and 22 µm long GNR using a simple electrostatic model 17. We obtain a value of 800 cm 2 /Vs which is of the same order of magnitude of the mobility reported in references 14 and 16. This work is the result of an intensive but not extensive effort for the fabrication and characterization of narrow nanoribbons. We are sure that a more thoughtful search will certainly lead to even narrower nanoribbons where the effects of one dimensional quantum confinement shall be more accused. Other interesting effects have also been predicted for narrow (less than about 6 nm) GNRs with well defined edges. For instance, spin-separation and half-metallicity in the presence of a transverse electric field with possible applications in spintronics have been predicted for zig-zag edged GNRs Stankovich, S. et al. Graphene-based composite materials. Nature 442, (2006) 2 Bunch JS et al. Electromechanical resonators from graphene sheets. Science (2007) 3 A. K. Geim and K. S. Novoselov. The rise of graphene. Nature Materials 6, (2007) 4 C. Berger, et al. Ultrathin Epitaxial Graphite: 2D Electron Gas Properties and a Route toward Graphene-based Nanoelectronics. J. Phys. Chem. B, 108, 19912, (2004). 5 Cristina Gómez-Navarro et al. Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets. Nano Lett., 7, , (2007) 6 Xiaolin Li et al. Highly conducting graphene sheets and Langmuir Blodgett Films. Nature Nanotechnology. 3, 538, (2008). 7 Hernandez, Y. et al. High-yield production of graphene by liquid-phase exfoliation of graphite Nature Nanotech. 3, (2008). 8 Novoselov, K. S. et al. Two dimensional atomic crystals. Proc. Natl Acad. Sci. USA 102, 10451, (2005). 9 Tobias Wassmann et al. Structure, Stability, Edge States, and Aromaticity of Graphene Ribbons. Phys. Rev. Lett. 101, (2008) 10 A. Mañanes, F. Duque, A. Ayuela, M. J. López, and J. A. Alonso, Half-metallic finite zigzag single-walled carbon nanotubes from first principles.phys. Rev. B 78, (2008) 11 S. Adam, S. Cho, M. S. Fuhrer, and S. Das Sarma. Density Inhomogeneity Driven Percolation Metal-Insulator Transition and Dimensional Crossover in Graphene Nanoribbons. Phys. Rev. Lett. 101, (2008) 12 Norbert Nemec, David Tománek, and Gianaurelio Cuniberti. Modeling extended contacts for nanotube and graphene devices. Phys. Rev. B 77, (2008) 13 Han M., et al. Energy band gap engineering of graphene nanoribbons. Phys. Rev. Lett. 98, (2007). 14 Chen, Z. et al. Graphene nano-ribbon electronics. Physica E 40, 228, (2007).

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