Aligned carbon nanotube films for cold cathode applications

Aligned carbon nanotube films for cold cathode applications A. N. Obraztsov,a) I. Pavlovsky, and A. P. Volkov Department of Physics, Moscow State University, Moscow 119899, Russia

Elena D. Obraztsova General Physics Institute, Russian Academy of Sciences, Moscow 117942, Russia

A. L. Chuvilin and V. L. Kuznetsov Boreskov Institute of Catalysis, Russian Academy of Sciences, Novosibirsk 630090, Russia

共Received 9 July 1999; accepted 27 January 2000兲 Thin film material of oriented multiwall carbon nanotubes was obtained by noncatalytical chemical vapor deposition in a glow-discharge plasma. The film phase composition, surface morphology, and structural features were studied by Raman and electron microscopy techniques. Low-voltage electron field emission of thin film nanotube material was obtained and examined in diode configuration. The I–V curves in Fowler–Nordheim coordinates were linear and the corresponding threshold average field was about 1.5 V/␮m. The emission current density was up to 50 mA/cm2 at the field of 5 V/␮m. The emission site density reached 107 cm⫺2 at the same value of electric field. © 2000 American Vacuum Society. 关S0734-211X共00兲09602-5兴

I. INTRODUCTION A specific shape of carbon nanotubes with a very large aspect ratio is a reason of numerous speculations about their potential application as electron field emitters.1–3 Carbon nanotube 共CNT兲 material can be produced in macroscopic quantities by using graphite evaporation in the course of arc discharge or laser ablation or by using thermal decomposition of hydrocarbons.4,5 Usually, CNT are dispersed and misaligned in the material, which contains also metal catalyst particles and amorphous carbon. This restrict both the investigations of nanotube’s properties and their applications. In case of materials containing oriented carbon nanotubes, the problem of contamination is also kept because all synthesis methods, reported up to now, use catalysts.1,6–9 From the viewpoint of the field emission 共FE兲 applications, there is another imperfection of oriented CNT material — a very dense packing of nanotubes. This prevents local enhancement of the electric field due to a screening effect. In this article, we report on noncatalytical growth of thin film material consisting of oriented carbon nanotubes with a close-to-optimal shape and distribution over the film surface. The investigation of their FE and structural properties has been performed. A possible mechanism explaining some unusual features of FE from the nanostructured carbon has been suggested.

II. EXPERIMENT All films investigated were grown on 25⫻25 mm2 Si substrates in a direct current 共dc兲 discharge plasma assisted chemical vapor deposition 共CVD兲 system described in detail elsewhere.10 The growth parameters were following: the suba兲

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strate temperate was T s ⬇1050 °C, the gas mixture was CH4 :H2⫽10:90, the total gas pressure was P⬇100 Torr, the deposition duration was 90 min. The field-emission characteristics of the films were tested using a diode-configuration system consisting of a cathode 共the film under test兲 and an anode mounted in a turbopumped vacuum chamber with the base pressure of 10⫺6 Torr. As anodes, we used a tungsten 5-mm-diam cylinder with flat surface or a 30-mm-diam glass plate covered with indiumtin-oxide 共ITO兲 and a layer of phosphor 共with a thickness of several microns兲 deposited over ITO. The anode was positioned relatively to the cathode by a precision screw translator with an accuracy of 5 ␮m. The details were described in Ref. 11. The FE current–voltage 共I–V兲 dependencies were obtained by applying a negative dc voltage of up to 1500 V from a PC-controlled power supply, while the emission current was measured automatically as the voltage was ramped. A current limit of 1.5 mA was set in these measurements. The phosphor-coated glass screen was used to characterize a spatial distribution of emission sites over the film surface. To prevent the excessive sputtering of the phosphor by emitted electrons, in these measurements we used a pulsed voltage supply 共pulse duration was about 5 ␮s, repetition rate varied from 30 to 500 Hz, peak voltage was set in the range from 200 to 2000 V兲. The images of the cathodoluminescent screen were captured by a charge coupled device 共CCD兲 camera. Raman spectra measurements were performed in microRaman configuration using a Jobin Ivon triple monochromator S3000. An Ar ion laser operating at 488 nm line was used for spectra excitation. The film surface morphology was studied using a scanning electron microscope 共SEM兲, Cambridge Instruments Stereoscan 240, and a transmission electron microscope 共TEM兲, JEM-4000EX. TEM images of the film surface were obtained for V-shaped chips of the silicon substrate with the CVD film. When such a sample was positioned so that the

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FIG. 3. HRTEM image of a fragment of one tip-like structure on the CVD film surface.

FIG. 1. 共a兲 SEM microraph of a CVD film surface. 共b兲 Cross-section SEM micrograph of CVD film, obtained for sample cleavage.

axis of the electron beam of the microscope was parallel to the substrate plane, the film fragments remaining on the sharp end of the chips were thin enough so that an image of their structure could be obtained ‘‘in transmission.’’ III. RESULTS All the CVD films fabricated at above-mentioned conditions were black and homogenous all over the surface. The film thickness was about 4 ␮m as measured on the cleavage by using SEM. Typical SEM images of the CVD thin film are shown in Fig. 1. It is clearly seen that the film consists of flake-like structures exhibiting a rather complicated surface morphology.

FIG. 2. Typcial TEM image of the CVD film surface. J. Vac. Sci. Technol. B, Vol. 18, No. 2, MarÕApr 2000

TEM investigations show that the film surface above the flakes is covered by another type of structures with a tip-like shape 共see Fig. 2兲. The channels can be seen inside a majority of these tips. Usually, the tips are oriented perpendicularly to the film surface. This orientation coincides with the direction of an electric field applied during the film growth in a course of CVD process.10 Typically, the tips’ diameter is of 10–50 nm and their height is of 1–2.5 ␮m. A distance between tips is of 0.5–2 ␮m. High-resolution transmission electron microscopy 共HRTEM兲 shows that the tips consist of coaxial cylindrical atomic layers 共of 10–20兲 and the inner channel diameter is about 5 nm 共see Fig. 3兲. The distance between layers is about 0.34 nm. This corresponds to interlayer distance in graphite 共see, e.g., Ref. 4兲. The graphitic type of atomic structure was confirmed by our Raman scattering 共RS兲 observations. A typical Raman spectrum is shown in Fig. 4. The characteristics features are: a high intensity and a narrowness 共14 cm⫺1) of ‘‘G’’ 共graphite兲 peak at 1582 cm⫺1 which is a high-frequency E 2g firstorder mode that being characteristic for well-crystallized graphite 共see, for example, Ref. 12兲; a low contribution of a disorder-induced ‘‘D’’ peak at 1350 cm⫺1; and intense narrow band of two-phonon scattering at 2714 cm⫺1 and even a presence of a weak signal of three-phonon scattering. Comparing the Raman spectra of our CNT film material and the

FIG. 4. Typical Raman spectra of the CVD film. The peaks labeled by asterisks are due to the gas discharge in laser tube.

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FIG. 6. Image of 600⫻400 ␮m2 area of phosphor screen obtained with use of optical microscope.

FIG. 5. Fowler–Nordheim plot for typical I–V dependence obtained for diode with anode-to-cathode distance d⫽150 ␮m. Inset shows the same dependence in normal coordinates.

powder-like pure multiwall carbon nanotube 共MWCNT兲 material, we have established that these spectra are very similar to each other. But they remind also the spectra of nanocrystalline graphite.12 We can conclude that the above-mentioned spectral features are necessary but not sufficient for an undoubted identification of MWCNT by RS measurements. These results have been obtained with a traditional back scattering scheme. Trying to separate Raman signals from the upper layer of CVD film 共with the tip-like features兲 and the lower layer 共adjacent to Si substrate兲, we have examined the cross sections of the V-shaped materials chips, which have been investigated initially by HRTEM 共see Fig. 2兲. Despite the difference in the film structure, we were not able to observe any change in the Raman signal for areas located at various distance from Si substrate. The laser beam spot size in this experiments was about 2 ␮m. Integration of the Raman signal over the whole film thickness allows to conclude only that in average our CVD films contain rather wellgraphitized carbon material with a very small admixture of amorphous carbon. The results of the FE properties investigations were similar to those for other nanostructured carbon films, reported in our recent papers.13,14 Figure 5 presents a typical I–V dependence of a vacuum diode in which the cathode is a CNT film grown on a silicon substrate and the anode is a flat tungsten plate. The diode current 共I兲 versus applied voltage 共V兲, obtained at a cathode-to-anode distance d⫽150 ␮m is shown in inset of Fig. 5 where the Fowler–Nordheim 共FN兲 plot of log(I/V2) vs 1000/V are depicted also. Usually, the linear behavior of such dependencies is considered as a confirmation of a field emissive origin of the electron current.1–3 From Fig. 5, the linear behavior of the FN plot for the cathodes investigated is observed only for FE currents values JVST B - Microelectronics and Nanometer Structures

ranging from 10⫺9 to 10⫺5 A. As a rule, the I–V curves obtained in these measurements were stable and reproducible for several times cycling of the voltage up and down, if the current density does not exceed 5 mA/cm2 in the dc regime of the measurements. At the same time, in a pulsed regime, used for the observation of distribution of the emission sites, the peak value of emission current can reach 100 mA/cm2 without any degradation of cathode emissivity. The phosphor screens luminosity under the action of electrons emitted was quite homogenous in macroscopic scale, over surface of the anode screen. It exhibited a granular structure, clearly seen in Fig. 6, which shows an image of the screen area of 600⫻400 ␮m2 obtained in an optical microscope. It should be noted that no completely dark spots were detected on the screen surface. The same screens were tested with usual electron sources, like an electron gun, and exhibited microscopically homogenous luminosity. Taking into account that the phosphor layer consists of the particles of about 1 ␮m in size 共the layer thickness is about 7 ␮m兲, the FE site density can be estimated as 107 – 108 cm⫺2. This value is of the same order as the density of tip-like structures on the CVD film surface 共see Fig. 2兲. At the same time, the screen unhomogeneity in FE measurements may be resulted from variation in efficiency of emission sites on the CVD film surface. A very low energy of electrons accelerated by voltage less than 800 V at cathode-to-anode distance of 200 ␮m may be considered as another reason of the screen brightness inhomogeneity. IV. DISCUSSION As we have shown previously, nanostructured carbon thin films in the form of plate-like graphite crystallites with basal plane oriented normally to the film surface may have a very low threshold for electron field emissions.13,14 It was proposed that incorporation of similar graphite crystallites determines also the field emissions properties of CVD diamond films. The threshold lowering may be explained by modification of electronic configurations of carbon atoms located at the crystallite edges. In this study, we were able to improve emission properties of our carbon cathodes via optimization of the deposition

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process. An increase of deposition time leads to appearance of oriented multiwall carbon nanotubes on the film surface. The growth mechanism of such form of carbon material is unclear yet. We can only suppose that the predominant orientation of the carbon nanotubes and other graphite-like species in the films grown is stimulated by the constant electric field direction above the substrate during the CVD process. The direction of this field coincides exactly with the direction of the field during FE measurements. The orientation of CNT perpendicularly to the substrate provides an electron emissions from tube ends. This configuration is optimal for an additional local increase of the electric field. An upper limit for the field enhancement factor ␤ can be estimated as 250 from the tip geometrical characteristics obtained with TEM ( ␤ ⬇h/r, where h is the tip height and r is the tip radius兲. Using this value of ␤ and experimental data of the slope of the I–V dependence in FN coordinates 共see Fig. 5兲 it is possible to estimate an effective energy barrier for electrons escaping from the carbon cathode into vacuum, ␸, with the use of the Fowler–Nordheim equation in its simplest form: log共 I/V 2 兲 ⬀ 共 B ␸ 3/2h/ ␤ 兲 V ⫺1 ,

共1兲

where B⫽6.87⫻109 (V eV⫺3/2 m⫺1), ␸ in 共eV兲, h — the interelectrode distance in 共m兲. Using this approach, we have found that ␸ lies in the range of 0.2–1 eV, which is much smaller than the work function for graphite 共5 eV兲. To obtain, formally, ␸ ⫽5 eV, we should put into Eq. 共1兲 a value of ␤ ⫽2500. This is the case only if electrons escape into vacuum from a very small part of the carbon nanotube emitter surface. This uncertainty in determination of the CNT emitter characteristics could be resolved with use of a more complicated approach to solve the FN equation for tip-like structures described in Ref. 15, which provides a direct determination of all parameters by measuring I, V, and dI/dV at any given point on the FE current–voltage characteristics. Our calculations, performed in accordance with this theory, are also in agreement with the point of view, that the effective energy barrier for electrons, emitted from CNT, is lower than the work function of graphite. The decrease of the energy barrier can result from the curvature of graphene sheets forming the nanotube surface. As was mentioned in Ref. 16, where the sheet bends, the bonding must change its s p 2 configuration and approach the sp 3 one. The change from s p 2 - to s p 3 -like hybridization is accompanied by modification of corresponding density of states 共DOS兲 due to deoverlapping of bands near Fermi level as a result of ␲-bonding disruption 共see Fig. 7兲. It must always involve a pair of carbon atoms and will produce sp 3 -like atomic liens along the nanotube side surface. Taking into account that the work function and the effective energy barrier for electrons on the emitter surface are determined by its chemical nature, such modification of atomic hybridization must lead to a change in these energy parameters 共see Fig. 7兲. Namely, they should approach the characteristics of diamond, which consists of s p 3 hybridized carJ. Vac. Sci. Technol. B, Vol. 18, No. 2, MarÕApr 2000

FIG. 7. Schematic diagrams for density of states 共DOS兲 and corresponding energy bands illustrating the field emission mechanism for a CNT film cathode.

bon solely. It is well known that such sp 3 hybridization in diamond may lead to the so-called negative electron affinity 共NEA兲 on its surface which provides a very low energy threshold for electrons escaping from the diamond conduction band into vacuum.17 But, normally, concentration of free electrons in the conduction band of diamond, as well as in conduction bands of other wide-band-gap materials, is very low. This makes NEA practically useless for cold cathodes fabrication from all these dielectric materials. In the case of the CNT graphite-like material discussed, the sp 3 -coordinated carbon atoms provide a local lowering of the energy barrier on the film surface 共see Fig. 7兲. At the same time, these atoms form a very thin layer on the surface which is transparent for electrons due to tunneling effect. As a result, the conductivity of such emitters remains similar to that of graphite which is a semimetal 共free electron concentration is about 1018 cm⫺3). Probably, the same mechanism of field emission takes place in other materials consisting and/or containing graphite crystallites, for which a tendency exits to form the arced 共bended兲 graphene sheets 共see, for example, Ref. 18兲. This consideration can explain the low threshold for field emission of electrons detected in our experiments. Also, the presented model for emitting material 共see Fig. 7兲 predicts a deviation of FE current–voltage dependence from FN law at the higher current densities 共see Fig. 5兲 due to the limited material conductivity and/or the formation of a space charge which should decrease an emission efficiency in the material presurface area. V. CONCLUSIONS For the first time, by using the noncatalytical CVD process, we were able to grow graphite-like thin film material

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consisting of carbon nanotubes oriented perpendicular to the film surface. This film material exhibited excellent field emission properties with the threshold average electric field of 1.5 V/␮m, emission current density higher than 50 mA/cm2, and emission site density higher than 107 cm⫺2 at the average field of 5 V/␮m. The high efficiency of the field emission is explained in terms of rehybridization of interatomic bonds from sp 2 to sp 3 for atom located at the bends of graphene sheets. Simultaneously, the energy barrier decreases due to this effect. The enhancement of the effective field on the ends of tip-like surface structures leads to the lowering of the field threshold, observed experimentally. ACKNOWLEDGMENTS This work was partially supported by Russian Federal Program ‘‘Integratsiya’’ 共Project A0080兲, by INTAS 共Grant No. 1997-1700兲 and by the Russian Foundation for Fundamental Research 共Grant No. N97-02-17282兲. 1

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