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Influence of electric field and emission current on the configuration of nanotubes in carbon nanotube layers
Carbon 43 (2005) 3112–3123 www.elsevier.com/locate/carbon
Influence of electric field and emission current on the configuration of nanotubes in carbon nanotube layers N.A. Kiselev a, A.L. Musatov b, E.F. Kukovitskii c, J.L. Hutchison O.M. Zhigalina a, V.V. Artemov a, Yu.V. Grigoriev a, K.R. IzraelÕyants b, S.G. LÕvov c b
d,*
,
a Institute of Crystallography, RAS, Leninskii prosp. 59, Moscow 119333, Russia Institute of Radioengineering and Electronics, RAS, Mokhovaya Street 11, Moscow 103907, Russia c Physical–Technological Institute, RAS, Kazan 420029, Russia d Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK
Received 22 March 2005; accepted 21 June 2005 Available online 2 August 2005
Abstract The influence of applied electric field (Eav) and emission current (IFE) on the configuration of conical layers carbon nanotubes (CLNTs) grown by CVD on the edge of Ni foil has been investigated. TEM profile imaging revealed a high concentration of nanotubes near the foil edge surface, whereas on the nanotube layersÕ outer surfaces single, non-oriented nanotubes with open ends free of catalytic particles, were observed. After sufficient electric field application many nanotubes became oriented towards the anode, but one or two of them were found to be always a few microns more extended. In situ SEM investigation showed that below Eav = 3.2–3.9 V/lm, emission was achieved at the expense of originally existing free nanotube ends. Configuration changes began at larger electric fields. On the observed foil edge length (14.6–17.8 lm, with an edge thickness of 200 lm) one or two nanotubes extended towards the anode and probably became the main emitters. Upon further increasing the field to Eav = 5.7–8 V/lm and at an emission current IFE = 2 · 10 5 A these tubes disappeared (or essentially shortened). At Eav = 8 V/lm and higher and at an exposure time up to 40 min, several tens of extended nanotubes appeared, with one or two extended well beyond the others. This nanotube configuration pattern is connected with electrostatic screening between the nanotubes. Our interpretation of the data suggests that in the investigated range of Eav and IFE, a limited number of nanotubes are emitting and these nanotubes are constantly changing as Eav, IFE and exposure time increase. 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Chemical vapor deposition; Electron microscopy; Field electron emission; Microstructure
1. Introduction Carbon nanotubes (CNTs) are characterized by a small diameter and a very large aspect ratio; therefore * Corresponding author. Tel.: +44 1865 273705; fax: +44 1865 283333. E-mail address: [email protected] (J.L. Hutchison).
0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.06.029
they are promising materials for use as electron emitters [1–11]. In this respect the important feature of CNTs is the existence near their apex of great electrostatic forces which stretch them towards the anode. Estimates [5] show that, at an electric field strength corresponding to the appearance of field emission current, electrostatic forces acting on the end of a 20 nm diameter CNT reach 10 8 N. Stretching of CNTs towards the anode under the influence of electrostatic force has been reported in
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a number of publications [10,15] in the case of single, narrow CNTs. In this paper the influence of electric field, emission current and exposure time on CNTs configuration was investigated for large assemblages of non-oriented, curved conical layers nanotubes (CLNTs). Carbon nanotube layers were grown by CVD on Ni foil [12]. The layers demonstrated low voltage, field electron emission characteristics: the value of the field amplification coefficient b was in the range 1000–4000 [5]. High-resolution transmission electron microscopy (HRTEM) investigations showed that CNTs in the layers consisted of conical graphene layers stacked together. There were some open CNTs ends formed by one or several graphene layer edges. These were tentatively considered as a source for electron emission [5]. At the same time the specimen preparation technique for HRTEM could generate open ends as a result of CNTs breaking during their removal from the substrate or during the subsequent ultrasonic treatment. Also it was not entirely obvious that an end-on process of CLNTs growth dominated. The aim of the work reported here was to investigate the structure of CLNT layers as a function of Eav (electrostatic forces), emission current (IFE) and exposure time (t) using TEM profile imaging and in situ SEM investigations. It is also shown that an end-on process of CLNT growth does take place and that CNTs with open ends free of catalytic particles are present at the outer layer surface.
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eters are incompatible with equilibrium conditions for a liquid droplet, it could easily be ejected from the CNT tip [11]. 2.2. Electron microscopy For TEM profile imaging pieces of the 0.2 mm thick Ni foil were glued to a copper supporting ring, so that the foil edge was located at the centre of the ring. The specimens were investigated at 200 kV. The specimens for HRTEM were prepared by scraping CNTs from the Ni foil surface and dispersing them in acetone using an ultrasonic bath. The specimens were examined at 300 and 400 kV. SEM investigations were performed in a secondary electron mode at 30 kV. The influence of electric field on CLNTs configuration was investigated using a small device consisting of a brass cathode and anode with an adjustable spacing. The cathode was 4 mm in diameter with a flat end on which a piece of Ni foil with carbon nanotubes was glued with its edge pointing upwards. The anode was also 4 mm in diameter, this time with a conical end having a flat termination 0.5 mm in diameter. Alternatively an anode with a hemispherical end of diameter about 200 lm was also used. In order to obtain SEM images, a discontinuous mode of applied voltage was needed. Normally, at any Eav the exposure time was 30 s. After that the voltage was switched off and the SEM image of CNTs configuration recorded. 2.3. Investigation of field electron emission from conical layer nanotubes layers
2. Experimental 2.1. Layers preparation Carbon nanotube layers were synthesized on the flat part and on the edge of a 0.2 mm thick Ni foil by CVD using the products of thermal decomposition of polyethylene as a source of carbon. This version of the CVD process is described in detail elsewhere [12]. In the first stage of this process polyethylene pyrolysis took place in the first furnace at 600 C. Gaseous products of pyrolysis were transported by helium flow to the second furnace where a catalytically active Ni substrate was maintained at 800–900 C. It has been suggested [13,14] that at 800 C transformation of the Ni catalyst into the liquid state takes place and that CNTs grow via liquid catalyst droplets. In this context we note the pioneering work of Tammann [17] and its subsequent confirmation in a series of elegant, controlled atmosphere TEM experiments [16]. The liquid droplet growth model provides a possible explanation for the absence of tip particles. Surface tension and capillary forces depend on the temperature and carbon content in catalyst particles. If changes of these param-
Field electron emission measurements were carried out in an ultrahigh vacuum chamber at approximately 1.3 · 10 7 Pa. A hemispherical stainless steel anode probe 2 mm in diameter was placed at an anode–sample distance of 200–400 lm. Using a manipulator it was possible to vary the anode–sample distance and to move the anode along the specimen surface to make measurements at different points. The anode–sample distance was measured by the manipulator micrometer with respect to the position at which electrical contact was registered. As this contact could damage the layer it was done only after the emission investigation had been completed. Measurements of field emission were carried out with a high-voltage source (Keithley 248) and picoammeters (Keithley 485 and Keithley 6485). For measurements of current–voltage (I/V) characteristics in the SEM, a stabilized high-voltage rectifier with regulated voltage output up to 3 kV in 1 V steps was used. For measurements of emission current, a load resistance was included in the circuit of the sample and the voltage drop on the resistance was measured by a digital voltmeter.
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3. Field electron emission from conical layer nanotube layers Reproducible I/V characteristics of field emission measured in ultrahigh vacuum on the flat part of the Ni foil with carbon CNTs layer, for several best samples, are shown in Fig. 1a. This figure shows that an emission current 10 11 A appeared at the average electric field Eav = 0.7–1.2 V/lm, and emission current of 10 l A (current density j 10 mA/cm2) was obtained at Eav = 1.5–2.2 V/lm. I/V characteristics plotted in Fowler–Nordheim coordinates are shown in Fig. 1b. Here, the I/V characteristics are linear practically across the whole range of the emission currents. Only at the highest current values (10 5 A) is there a deviation from a straight line, towards high current. A similar effect has been reported elsewhere [18] where it was explained in terms of Joule heating of a nanotube at high-emission currents. At high current
Fig. 1. Field emission current (IFE) as a function of the average electric field (Eav) for CLNTs layer measured on flat part of Ni foil is ultrahigh vacuum (a) and Fowler–Nordheim plot of these curves (b).
the field emission mode turns into the thermo-field emission mode. From the slope of the I/V characteristics we have calculated the value of the field amplification coefficient b and obtained for these samples b = 2500–4000. The value of b for most samples manufactured by this method lies in the range b = 1000–4000. The I/V characteristics of field emission measured on the edge of a Ni foil with the carbon nanotube layer are shown in Fig. 2a and b. Such a characteristic measured in ultrahigh vacuum (curve 1) does not differ from that measured on the flat part of the Ni foil with the carbon CNT layer (Fig. 1a and b). The Fowler–Nordheim plot (Fig. 2b, curve 1), is a straight line practically across the whole range of emission currents. However, the I/V field emission characteristics measured on such samples placed in the SEM chamber showed a marked discontinuity in the Fowler–Nordheim plot (Fig. 2b, curve 2)
Fig. 2. Field emission current (IFE) as a function of the average electric field (Eav) for CLNTs layers measured on the edge of Ni foil in ultrahigh vacuum, 1 and in SEM chamber, 2 (a) and Fowler–Nordheim plot of these curves (b). For ‘‘2’’ reversed plots are shown.
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and a loss of stability compared with ultrahigh vacuum conditions. We suggest that these differences are a result of the contamination of nanotube tips under the relatively poor vacuum conditions in the SEM chamber.
4. Structural investigations 4.1. HRTEM of CNTs To prepare specimens for HRTEM, dispersed CNTs were mounted on holey carbon films and examined at 400 kV. Both pre- and post-emission CNTs layers were investigated. A general view is shown in Fig. 3a. There appears to be one type of nanotube, with diameters ranging between 6 and 50 nm. It should be noted that catalyst material is virtually absent, and open ends of CNTs appear to be free of catalyst particles. Open CNT ends usually have a conical depression. Occasionally opposite (both) ends of a nanotube are visible. They are sharp and have opposite conicity. These outlines are typical for ‘‘bamboo’’-shaped CLNTs [19]. Since these CNTs are rather short to be complete original tubes, we tentatively consider these sharp ends to be broken ends. An HRTEM image (Fig. 3b) shows a typical CNT to consist of conical graphene layers stacked together, i.e., it is a conical layer CNT [12]. Its conicity varies between 19 and 41, correspondingly the outer diameter is 36– 40 nm and an inner channel diameter 15–21 nm. Open
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graphene layers edges are observed both on the outer surface profile and on the inner surfaces of the central channel. This particular CLNT is from the specimen following a field emission experiment. However, we could not detect any significant structural differences between CLNTs before and after emission experiments. On the other hand, we cannot be certain that any particular CLNTs imaged by HRTEM after emission measurements have actually emitted electrons. HTREM of open CLNTs ends shows that these ends are formed by one, two or three conical graphene layer edges (Fig. 4); so the radius of curvature (in radial direction) is 0.34–1.02 nm and the aspect ratio is high. Such ends can be considered as effective field electron emission source [5]. However, taking into account the specimen preparation techniques there is the possibility that these ends are actually broken CLNT ends. The situation is further aggravated by the fact that these ends have no Ni catalyst particle. In this context we note that it has been previously shown [20] that there is a tipgrowth process, when the catalyst particle is located at the growing end of a CNT, and a base-growth process where it remains in the supporting substrate and the end of the tube is closed. In order to clarify this problem it would be necessary for us to demonstrate conclusively that the tip-growth process is realized in the present work and that the open ends are not simply experimentally produced artefacts. To do so would require the elimination of the process of scraping the CLNTs from the Ni foil.
Fig. 3. CLNTs obtained by CVD on Ni foil (after emission). To prepare specimen CNTs were scraped of the substrate. (a) TEM general view of CLNTs. Open and broken ends are arrowed. (b) HRTEM image of CLNT. Open graphene layers edges are marked by arrows.
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Fig. 4. HRTEM image of CLNT open end. Arrows mark edge-on views of conical graphene layers which terminate the tube.
Fig. 5. Prior to emission TEM profile imaging of CNTs layers grown on the edge of 0.2 mm thick Ni foil. In (a) and (b) horizontal arrows tentatively marked boundaries of foil edge, dense region with coiled CLNTs, less dense area and area of separate loops and single CLNTs area. Small arrows indicate open ends.
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4.2. Profile imaging of CNTs layers prior and after to emission As was described above in Sections 2.1 and 2.2, for profile imaging CLNTs were grown on the edge of a 0.2 mm thick Ni foil. The small foil thickness facilitates TEM investigations of the foil edge profile and the CLNTs grown on it (Fig. 5). CNTs in the layers are distributed unevenly. In the immediate vicinity of the Ni support dense tangles of CLNTs are observed. The thickness of this area is 800–2000 nm. Next is a 800– 1650 nm thick, less dense area in which separate CLNTs 6–40 nm in diameter are easily traced. The CNTsÕ ends are usually free of catalytic particles and their outlines appear identical to images of ends revealed by HRTEM (Fig. 4). The direction of CLNT growth can be deduced from the conicity orientation, in most cases visible at the tubesÕ ends and sometimes also by inner cup orientation. Beyond this area separate loops and 400–2000 nm long CLNTs are observed, usually terminated with open ends. The total number of visible open ends ranges up
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to six per 1 lm of edge length; so the conclusion can be drawn that in our case CLNTs grow according to a tip-growth process, but the catalytic particles for some reason have disappeared. Open ends with atomically sharp endings can be very efficient electron sources. After SEM investigation of electric field influence on the CLNTs configuration some specimens were transferred into the TEM and examined by profile imaging. In one experiment the specimen was exposed to Eav 4.5 V/lm (see Section 4.3). A profile image of this specimen is shown in Fig. 6. Compared to profile images recorded prior to emission, more open-ended CNTs are revealed (12–16 per 1 lm of edge profile). The number of CNTs extended towards the anode is up to 6 per micron of length. Fig. 6 also indicates that some CNT end shapes are identical to those observed by profile imaging and by HREM, prior to emission. It is significant that although the number of CNTs stretched towards the anode increased, the height of CNTs is not uniform; we invariably found single CNTs (Fig. 6a) or a group of CNTs (Fig. 6b) which were a few microns higher that the others.
Fig. 6. TEM profile image of nanotubes layer grown on the edge of Ni foil after emission (Eav 4.5 V/lm).
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4.3. In situ SEM investigations of electric field, field emission current and exposure time influence on the CLNTs configuration More than 10 experiments were performed involving in situ SEM investigations of electric field influence on CLNTs configuration; these agreed well with one another except for a spread of Eav for the onset of configuration change (Eav = 3.4–3.9 V/lm). This spread may
arise as a result of incorrect estimation of d or, more probably, from different initial configurations of CNTs in the layer and differences in apex radius of individual CNTs. A detailed account of two of these experiments is given below. The initial configuration of CLNTs in one of these experiments is shown in Fig. 7a. The anode–cathode distance is 102 lm. The length of the investigated part of the foil edge is 14.6 lm. Arrow ‘‘1’’ indicates the end
Fig. 7. In situ SEM investigations of electric field influence on the CLNTs configuration. Anode–cathode distance d = 102 lm. The length of investigated part of Ni foil edge is 14.6 lm. Exposure time (t) is 30 s except (f). (a) CLNTs prior to electric field application. Arrows 1 and 2 marked tubes with free ends, which later on undergo serious configuration changes. (b) Eav = 4.90 V/lm, IFE = 1.6 · 10 5 A. Tube 1 and rope no. 2 straighten out (this process started at Eav = 3.92 V/lm). (c) Eav = 5.10 V/lm, I = IFE = 1.8 · 10 5 A. Tube 1 becomes straight. (d) Eav = 5.71 V/lm, IFE = 2.1 · 10 5 A. Tube 1 disappeared, rope no. 2 continued to straightening out. Tubes 3 and 4 protruded. (e) Eav = 6.12 V/lm, IFE = 2.7 · 10 5 A, t = 30 s. (f) Eav = 6.12 V/lm, IFE = 2.7 · 10 5 A, t = 120 s, rope no. 2 disappeared. Tube 3 straightened out, tubes 4 and 5 protruded.
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of an individual CNT and ‘‘2’’ the end of the rope of CNTs initially protruding from the layer. As mentioned
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in Section 2.2, in order to obtain SEM images a discontinuous mode of Vapp was used, with a typical exposure
Fig. 8. In situ SEM investigations of large electric field influence on CLNTs configuration at long exposure (t). Anode–cathode distance d = 80 lm the length of investigated part of Ni foil edge is 17.8 lm, t is exposure time. (a) Initial configuration. The bundle of CNTs is observed with one or two protruding single CNTs. (b) Eav = 7.06 V/lm, IFE = 5.5 · 10 5 A, t = 30 min. Part of CNTs is oriented towards anode, in a greater extend CNTs 1, 3 and ropes no. 2 and no. 4. (c) Eav = 7.61 V/lm, IFE = 5.5 · 10 5 A, t = 45 min CNT 3 disappeared. CNT 5 is protruding. (d) Eav = 8.14 V/lm, IFE = 6.0 · 10 5 A, t = 25 min. CNT 4 is protruding. Several tens of shorter CNTs are also oriented towards anode. (e) Eav = 8.75 V/lm, IFE = 7.0 · 10 5 A, t = 20 min. CNTs 1, 2 and 5 essentially shortened. Main emitters probably are CNTs 4, 6 and 7. (f) Eav = 9.2 V/lm, IFE = 9.0 · 10 5 A, t = 20 min (the overall time 140 min). Main emitters are CNTs 4 and 6. CNT 7 disappeared.
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time of 30 s. In the range Eav = 0.98–1.96 V/lm no configuration changes are revealed. From Eav = 2.16– 3.73 V/lm, the emission current rises, i.e., field emission commences. In this range of Eav configuration changes are again not observed. It is possible that in this case emission is achieved owing to already existing free open ends of CNTs protruding from the layer. At Vapp = 400 V (Eav = 3.92 V/lm) (Fig. 7b) tube 1 is beginning to straighten out. At Vapp = 500 V (Eav = 4.9 V/lm) this tube continue to straighten and rope 2 also starts to change its configuration. The configuration of other tubes does not appear to change. After Eav = 5.10 V/lm tube 1 becomes virtually straight (Fig. 7c). This pattern is preserved after application of Eav = 5.29 V/lm. After application of Eav = 5.49 V/lm tube 1 disappears completely but tubes 3, 4 and 5 are now evident (Fig. 7d). It may be concluded that on the investigated part of the foil edge only tube 1 is emitting. As a result of Joule heating by the emission current IFE = 1.8–2.1 · 10 5 A along the length of the CLNT in combination with electrostatic forces, the tube first straightened and then failed. At a later stage, with Eav = 5.88 V/lm and IFE = 4 · 10 5 A tube 3 is noticeably straightened, whereas rope 2 is elongated (Fig. 7e). It is possible that in this case rope 2 is emitting more intensively and tube 3 has started to emit. To estimate the influence of longer time emission on CNTsÕ configuration at constant Eav (5.88 V/lm), the specimen was exposed for an additional 120 s (Fig. 7f). As a result rope 2 disappeared and tubes 3, 4 and 5 straightened. Presumably at the range Eav = 5.49–5.88 V/lm the main emission from this part of the edge was from rope 2 and then the main emitter became tube 3. Straightening of this tube continued at Eav = 6.86 V/lm. It is also of interest to find out the CLNTsÕ configuration in the layer after application of relatively large fields (Eav = 7.06–9.2 V/lm) and longer exposure time. The next experiment describes such investigations. The length of foil edge investigated was 17.8 lm, and d was set at 80 lm. In Fig. 8a the initial configuration of CNTs in the layer is shown. A large bundle of CNTs is observed with one or two single ones protruding. After 30 min exposure at Eav = 7.06 V/lm and IFE = 5.5 · 10 5 A the CNTs configuration significantly changed (Fig. 8b). A few CNTs became oriented towards the anode, in particular CNT 1, rope 2 and straight CNT 3. An additional 45 min exposure at Eav = 7.61 V/lm and IFE = 5.5 · 105 A gave rise to the protruding CNTs 4 and 5 (Fig. 8c); straight CNT 3 disappeared. After an additional 25mins at Eav = 8.14 V/ lm and IFE = 6 · 10 5 A, rope 6 joined the above number (Fig. 8d). Several tens of shorter CNTs and CNTs loops were also now oriented towards the anode. At Eav = 8.75 V/lm, IFE = 7 · 10 5 A and t = 20 min. CNTs 1, 2 and 5 essentially shortened
(Fig. 8e), CNTs 4 and 6 became most protruding, and nearest to them is no. 7. The main emitters in this case were probably precisely these tubes. After Eav = 9.2 V/lm, IFE = 9 · 10 5 A and t = 20 min (overall time 140 min) tubes 4 and 6 survived; tube 7 had disappeared and a few ‘‘new’’ CNTs potentially able to replace them as emitters appeared. Summing up the results of more than 10 sets of experiments, it is reasonably safe to suggest that in the initial stage of increasing electric field (up to Eav = 3.2–3.9 V/ lm) the CNTs configuration does not change—or such changes are small—and after the electric field release necessary to obtain an SEM image the original configuration is restored. In only one of the experiments a significant, permanent configuration change was observed, at Eav = 1.67 V/lm, but newly formed, free single CNTs ends were not oriented towards the anode. After application of an electric field greater than Eav = 3.2–3.9 V/ lm and IFE 10 5 A two or three CNTs appeared to have protruded and straightened (at the foil edge length 10–18.5 lm). Sometimes they become absolutely straight. It is most likely that precisely these tubes were the main emitters in each sequence of stages. At the electric field Eav = 5–8 V/lm, and IFE = 2– 3 · 10 5 A, these tubes disappeared or essentially shortened. At the same time one or two CNTs appeared to which apparently emission function transferred. At this range of Eav exposure time become important. For example, increasing it from the usual 30 to 120 s activates significant configuration changes. At large Eav, current IFE and t = 20–45 min this situation is essentially the same. One or two CNTs emit. Then these CNTs as a rule disappear or shorten and two or three fresh CNTs appear, to which the main emission function transfers.
5. Discussion HRTEM revealed that CLNTs grown on Ni foil by CVD have many open ends free of catalytic particles. These ends were characterized by a high-aspect ratio. TEM profile imaging of foil edge revealed a high concentration of CLNTs near the foil surface while from the outer part of the layer single CNTs with open ends having conical depressions, protruded. This indicates that there is most likely a tip-growth process and that the open CLNTs ends can be effective field emitters. The influence of electric field, emission current and exposure time on the configuration of CNTs was investigated for nanotube layers grown on the edge of a Ni foil by the same process as on the flat foil surface. In situ SEM investigations were performed in the as-grown condition in the device with 10 4–10 5 Torr vacuum. Under these conditions constant contamination of CNTs tips was likely and an in situ cleaning condition-
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ing process [7,18] was not effective. At the same time field emission experiments performed in ultrahigh vacuum with CNTs grown on the edge of the foil as a specimen allowed us to obtain a standard Fowler–Nordheim plot. The results described in Sections 4.2 and 4.3 can be interpreted in the following way: any essential nanotubes configuration changes are observed before Eav = 3.2–3.9 V/lm. As was mentioned in Section 4.3 it is possible that after release of the applied field the structure was restored. Possibly at this stage emission is achieved owing to already existing, free, open ends of CNTs, which where revealed by TEM profile imaging (Fig. 6) and by SEM (Figs. 7 and 8). At higher Eav and IFE 10 5 A, a small number of CNTs oriented towards the anode are always observed. One can suppose that the straightening of CNTs is due to electrostatic forces and Joule heating by the emission current. Field emission current is an exponential function of the field, so only those CNTs whose ends are closer to the anode and have sharper apex radius are emitting. Sometimes single CNTs become absolutely straight. As a rule these CNTs usually disappear first. Upon further increasing the field and current (Eav = 5–8 V/lm and IFE 2–3 · 10 5 A) previously emitting CNTs are replaced by another one. In this range of Eav exposure time is critical: even a small increase in exposure time leads to a rapid increase in nanotubes replacement. With field Eav = 7.5–9.2 V/lm and IFE 5.5– 9.0 · 10 5 A and large exposure (20–140 min) basically the same processes are realized. In this case, under the influence of large fields re-orientation of tens of nanotubes occurs, but all of them are shorter than those CNTs which are emitting. One can suppose that their re-orientation occurred only as a result of electrostatic forces. Even in the field Eav 9.2 V/lm (IFE 9 · 10 5 A) in the inner part of the CNT layer there appear to be many CNTs whose configurations remain unchanged, and therefore structural realignment can be considered as completed, only for a given limiting value of Eav. The overall picture of CNTs configuration distribution appears to be closely connected with electrostatic screening between the CNTs. The electric field at the apices of the emitters decreases with decreasing spaces between the apices. The field amplification decreases rapidly for a spacing less than two times the height of a CNT. The field enhancement is largest for a single CNT and decreases for closely positioned CNTs [3,11,21,22]. TEM profile imaging (Fig. 6) and in situ SEM investigations (Figs. 7 and 8) reveal that some CNTs protruded towards the anode and most probably emitting CNTs appear to be arranged much closer together than the ‘‘twice-the-height’’ distance. One possible explana-
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tion for this phenomenon is the following: normally we have observed the foil edge at the length of 14.6– 17.8 lm (i.e., an average length 16.5 lm). However, the thickness of the observed region is 200 lm, which is much larger. We thus observed superimposed images of CNTs, arranged on different levels along the electron beam direction. This means that CNTs can be separated
Fig. 9. In situ SEM investigation of electric field influence on CLNTs configuration using preliminary exposure at high Eav. Anode–cathode distance d = 75 lm: (a) Eav = 6 V/lm, 30 s; (b) Eav = 6 V/lm, exposure time 30 min; (c) Eav = 5.3 V/lm, exposure time 60 min.
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by a much larger distance that what is observed in the electron microscopes. A limited number of emitting CNTs is available, as was mentioned above, by the fact that even in a set of approximately equal-height CNTs one of two are invariably slightly higher and/or their ends are sharper. Such CNTs become the main emitters and simultaneously even more straighten up. Following an increase of the field or exposure time these are usually destroyed. Over the whole range of Eav and IFE investigated, a limited number of CNTs emit and constant replacement of these nanotubes occurs. To increase the stability of CNTs configuration, a ‘‘structural conditioning’’ process is required which consists of short-term exposure at high Eav followed by Eav decreasing to the required operation condition. An example of this ‘‘structural conditioning’’ is shown in Fig. 9. At Eav = 6 V/lm the initial tangled coil of CNTs is visible along with a number of protruding CNTs (Fig. 9a). After 30 min exposure these CNTs are seen to be extended towards the anode (Fig. 9b). Subsequently the electric field was decreased to Eav = 5.3 V/ lm. After 60 min exposure the configuration of these CNTs was basically unchanged (Fig. 9c) although one of them became 25% shorter. Most likely this is due to Joule heating by the emission current. The precise measurement of IFE in the SEM vacuum after the ‘‘conditioning’’ process could not be performed because of the simultaneous contamination of the nanotube tips (see Section 3 above). Precise investigation of emission current stability in ultrahigh vacuum are being carried out for CNTs synthesized on the flat part of a Ni foil (manuscript in preparation). Finally, we note that ‘‘structural conditioning’’ at larger Eav could further increase the stability of CNTs configuration, although this process could also result in more damage of small diameter single CNTs.
6. Conclusion Conical layers carbon nanotubes were grown by CVD on the edge and on the flat part of Ni foil. Field electron emission measurements carried out in UHV on the flat part of Ni foil revealed reproducible I/V characteristics. For best samples the value of the field amplification coefficient b was 2500–4000. TEM profile imaging of CNT layers grown on the foil edge showed that in spite of an apparent tip-growth process, nanotube ends were free of catalyst particles. Profile imaging and in situ SEM investigations revealed that the configuration of the CNTs is influenced by electric field (electrostatic forces) and screening effects. Joule heating by emission current in combination with electrostatic forces leads to tube straightening and then to their
disappearance. Due to the specific layer structure (random CNTs orientation and close positioning) and to screening effects a limited number of emitting CNTs for each Eav exist. Such CNTs become the main emitters. Following an increase of the field these are usually destroyed. Over the whole range of Eav and IFE investigated, a limited number of CNTs emit and constant replacement of these nanotubes by ‘‘new’’ ones occurs. We have demonstrated that the CNTs configuration can to some extent be stabilized by preliminary, brief ‘‘conditioning’’ at higher Eav.
Acknowledgements This work was supported by ISTC Project No. 1024-2 and by Scientific School Grant 14.04.2003.2.
References [1] Bonard JM, Salvetat JP, Sto¨ckli T, Forro L, Chaˆtelain A. Field emission from carbon nanotubes: representatives for applications and clues to emission mechanism. Appl Phys A 1999;69:245–54. [2] Zhu W, Bower C, Zhou O, Kochanski G, Jin S. Large current density from carbon nanotubes field emitters. Appl Phys Lett 1999;75:873–5. [3] Gro¨ning O, Ku¨ttel OM, Emmenegger Ch, Gro¨ning P, Schlapbach L. Field emission properties of carbon nanotubes. J Vac Sci Technol B 2000;18:665–78. [4] Rao AM, Jacques D, Haddon RC, Zhu W, Bower C, Jin S. In situ-grown carbon nanotube array with excellent field emission characteristics. Appl Phys Lett 2000;76:3813–5. [5] Musatov AL, Kiselev NA, Zakharov DN, Kukovitskii EF, Zhbanov AI, Izrael`yants KR, et al. Field electron emission from nanotube carbon layers grown by CVD process. Appl Sur Sci 2001;183:111–9. [6] de Jonge N, Lamy Y, Schoots K, Oostcrcamp TH. High brightness electron beam from a multi-walled carbon nanotube. Nature 2002;420:393–5. [7] Semet V, Binh VT, Vincent P, Guillot D, Teo KBK, Chhowalla M, et al. Field electron emission from individual carbon nanotubes of a vertically alignment array. Appl Phys Lett 2002;81:343–5. [8] Milne W, Teo KBK, Chhowalla M, Amaratunga GAJ, Lee SB, et al. Electrical and field emission investigation of individual carbon nanotubes from plasma enhanced vapour deposition. Diamond Rel Mater 2003;12:422–8. [9] Teo KBK, Lee SB, Chhowalla M, Semet V, Binh VT, Groening O, et al. Plasma enhanced chemical vapor deposition carbon nanotubes/nanofibres—how uniform do they grow? Nanotechnology 2003;14:204–11. [10] Bonard JM, Klinke C, Dean KA, Coll BF. Degradation and failure of carbon nanotube field emitters. Phys Rev B 2003;67. article no. 115406. [11] de Jonge N, Bonard JM. Carbon nanotube electron sources and applications. Phil Trans Roy Soc Lond A 2004;362:2239–66. [12] Kiselev NA, Sloan J, Zakharov DN, Kukovitskii EF, Hutchison JL, Hammer J, et al. Carbon nanotubes from polyethylene precursors: structure and structural changes caused by thermal and chemical treatment revealed by HREM. Carbon 1998;36:1149–57.
N.A. Kiselev et al. / Carbon 43 (2005) 3112–3123 ` vov SG, Saitov NA, Chernozatonskii VA. [13] Kukovitsky EF, L Correlation between metal catalyst particle size and carbon nanotube growth. Chem Phys Lett 2002;355:497–503. [14] Kukovitsky EF, L`vov SG, Saitov NA, Chernozatonskii VA. CVD growth of carbon nanotube films on nickel substrates. Appl Surface Sci 2003;215:201–8. [15] Wei Yi, Xie C, Dean KA, Coll B. Stability of carbon nanotubes under electric field studied by scanning electron microscopy. Appl Phys Lett 2001;79:4527–9. [16] Tammann G. Lehrbuch der Metallkunde. 4th ed. Leipzig: Leopold Voss; 1932. [17] Baker RTK. The relationship between particle motion on a graphite surface and Tammann temperature. J Catal 1982;78:473–6.
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[18] Purcell ST, Vincent P, Journet C, Binh VT. Hot nanotubes: stable heating of individual multiwall carbon nanotubes to 2000 K induced by the field-emission current. Phys Rev Lett 2002;88:105502–4. [19] Saito Y, Yoshikawa TJ. Bamboo-shaped carbon tube filled partially with nickel. J Cryst Growth 1993;134:154–6. [20] Baker RTR. Catalytic growth of carbon filaments. Carbon 1989;27:315–23. [21] Nilsson L, Groening O, Emmenegger C, Kuettel O, Schaller E, Schlapbach L, et al. Scanning filed emission from patterned carbon nanotube films. Appl Phys Lett 2000;76:2071–3. [22] Bonard JM, Weiss N, Kind H, Sto¨ckli T, Forro L, Kern K, et al. Tuning the field emission properties of patterned carbon nanotube films. Adv Mater 2001;13:184–8.
Influence of electric field and emission current on the configuration of nanotubes in carbon nanotube layers N.A. Kiselev a, A.L. Musatov b, E.F. Kukovitskii c, J.L. Hutchison O.M. Zhigalina a, V.V. Artemov a, Yu.V. Grigoriev a, K.R. IzraelÕyants b, S.G. LÕvov c b
d,*
,
a Institute of Crystallography, RAS, Leninskii prosp. 59, Moscow 119333, Russia Institute of Radioengineering and Electronics, RAS, Mokhovaya Street 11, Moscow 103907, Russia c Physical–Technological Institute, RAS, Kazan 420029, Russia d Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK
Received 22 March 2005; accepted 21 June 2005 Available online 2 August 2005
Abstract The influence of applied electric field (Eav) and emission current (IFE) on the configuration of conical layers carbon nanotubes (CLNTs) grown by CVD on the edge of Ni foil has been investigated. TEM profile imaging revealed a high concentration of nanotubes near the foil edge surface, whereas on the nanotube layersÕ outer surfaces single, non-oriented nanotubes with open ends free of catalytic particles, were observed. After sufficient electric field application many nanotubes became oriented towards the anode, but one or two of them were found to be always a few microns more extended. In situ SEM investigation showed that below Eav = 3.2–3.9 V/lm, emission was achieved at the expense of originally existing free nanotube ends. Configuration changes began at larger electric fields. On the observed foil edge length (14.6–17.8 lm, with an edge thickness of 200 lm) one or two nanotubes extended towards the anode and probably became the main emitters. Upon further increasing the field to Eav = 5.7–8 V/lm and at an emission current IFE = 2 · 10 5 A these tubes disappeared (or essentially shortened). At Eav = 8 V/lm and higher and at an exposure time up to 40 min, several tens of extended nanotubes appeared, with one or two extended well beyond the others. This nanotube configuration pattern is connected with electrostatic screening between the nanotubes. Our interpretation of the data suggests that in the investigated range of Eav and IFE, a limited number of nanotubes are emitting and these nanotubes are constantly changing as Eav, IFE and exposure time increase. 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Chemical vapor deposition; Electron microscopy; Field electron emission; Microstructure
1. Introduction Carbon nanotubes (CNTs) are characterized by a small diameter and a very large aspect ratio; therefore * Corresponding author. Tel.: +44 1865 273705; fax: +44 1865 283333. E-mail address: [email protected] (J.L. Hutchison).
0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.06.029
they are promising materials for use as electron emitters [1–11]. In this respect the important feature of CNTs is the existence near their apex of great electrostatic forces which stretch them towards the anode. Estimates [5] show that, at an electric field strength corresponding to the appearance of field emission current, electrostatic forces acting on the end of a 20 nm diameter CNT reach 10 8 N. Stretching of CNTs towards the anode under the influence of electrostatic force has been reported in
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a number of publications [10,15] in the case of single, narrow CNTs. In this paper the influence of electric field, emission current and exposure time on CNTs configuration was investigated for large assemblages of non-oriented, curved conical layers nanotubes (CLNTs). Carbon nanotube layers were grown by CVD on Ni foil [12]. The layers demonstrated low voltage, field electron emission characteristics: the value of the field amplification coefficient b was in the range 1000–4000 [5]. High-resolution transmission electron microscopy (HRTEM) investigations showed that CNTs in the layers consisted of conical graphene layers stacked together. There were some open CNTs ends formed by one or several graphene layer edges. These were tentatively considered as a source for electron emission [5]. At the same time the specimen preparation technique for HRTEM could generate open ends as a result of CNTs breaking during their removal from the substrate or during the subsequent ultrasonic treatment. Also it was not entirely obvious that an end-on process of CLNTs growth dominated. The aim of the work reported here was to investigate the structure of CLNT layers as a function of Eav (electrostatic forces), emission current (IFE) and exposure time (t) using TEM profile imaging and in situ SEM investigations. It is also shown that an end-on process of CLNT growth does take place and that CNTs with open ends free of catalytic particles are present at the outer layer surface.
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eters are incompatible with equilibrium conditions for a liquid droplet, it could easily be ejected from the CNT tip [11]. 2.2. Electron microscopy For TEM profile imaging pieces of the 0.2 mm thick Ni foil were glued to a copper supporting ring, so that the foil edge was located at the centre of the ring. The specimens were investigated at 200 kV. The specimens for HRTEM were prepared by scraping CNTs from the Ni foil surface and dispersing them in acetone using an ultrasonic bath. The specimens were examined at 300 and 400 kV. SEM investigations were performed in a secondary electron mode at 30 kV. The influence of electric field on CLNTs configuration was investigated using a small device consisting of a brass cathode and anode with an adjustable spacing. The cathode was 4 mm in diameter with a flat end on which a piece of Ni foil with carbon nanotubes was glued with its edge pointing upwards. The anode was also 4 mm in diameter, this time with a conical end having a flat termination 0.5 mm in diameter. Alternatively an anode with a hemispherical end of diameter about 200 lm was also used. In order to obtain SEM images, a discontinuous mode of applied voltage was needed. Normally, at any Eav the exposure time was 30 s. After that the voltage was switched off and the SEM image of CNTs configuration recorded. 2.3. Investigation of field electron emission from conical layer nanotubes layers
2. Experimental 2.1. Layers preparation Carbon nanotube layers were synthesized on the flat part and on the edge of a 0.2 mm thick Ni foil by CVD using the products of thermal decomposition of polyethylene as a source of carbon. This version of the CVD process is described in detail elsewhere [12]. In the first stage of this process polyethylene pyrolysis took place in the first furnace at 600 C. Gaseous products of pyrolysis were transported by helium flow to the second furnace where a catalytically active Ni substrate was maintained at 800–900 C. It has been suggested [13,14] that at 800 C transformation of the Ni catalyst into the liquid state takes place and that CNTs grow via liquid catalyst droplets. In this context we note the pioneering work of Tammann [17] and its subsequent confirmation in a series of elegant, controlled atmosphere TEM experiments [16]. The liquid droplet growth model provides a possible explanation for the absence of tip particles. Surface tension and capillary forces depend on the temperature and carbon content in catalyst particles. If changes of these param-
Field electron emission measurements were carried out in an ultrahigh vacuum chamber at approximately 1.3 · 10 7 Pa. A hemispherical stainless steel anode probe 2 mm in diameter was placed at an anode–sample distance of 200–400 lm. Using a manipulator it was possible to vary the anode–sample distance and to move the anode along the specimen surface to make measurements at different points. The anode–sample distance was measured by the manipulator micrometer with respect to the position at which electrical contact was registered. As this contact could damage the layer it was done only after the emission investigation had been completed. Measurements of field emission were carried out with a high-voltage source (Keithley 248) and picoammeters (Keithley 485 and Keithley 6485). For measurements of current–voltage (I/V) characteristics in the SEM, a stabilized high-voltage rectifier with regulated voltage output up to 3 kV in 1 V steps was used. For measurements of emission current, a load resistance was included in the circuit of the sample and the voltage drop on the resistance was measured by a digital voltmeter.
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3. Field electron emission from conical layer nanotube layers Reproducible I/V characteristics of field emission measured in ultrahigh vacuum on the flat part of the Ni foil with carbon CNTs layer, for several best samples, are shown in Fig. 1a. This figure shows that an emission current 10 11 A appeared at the average electric field Eav = 0.7–1.2 V/lm, and emission current of 10 l A (current density j 10 mA/cm2) was obtained at Eav = 1.5–2.2 V/lm. I/V characteristics plotted in Fowler–Nordheim coordinates are shown in Fig. 1b. Here, the I/V characteristics are linear practically across the whole range of the emission currents. Only at the highest current values (10 5 A) is there a deviation from a straight line, towards high current. A similar effect has been reported elsewhere [18] where it was explained in terms of Joule heating of a nanotube at high-emission currents. At high current
Fig. 1. Field emission current (IFE) as a function of the average electric field (Eav) for CLNTs layer measured on flat part of Ni foil is ultrahigh vacuum (a) and Fowler–Nordheim plot of these curves (b).
the field emission mode turns into the thermo-field emission mode. From the slope of the I/V characteristics we have calculated the value of the field amplification coefficient b and obtained for these samples b = 2500–4000. The value of b for most samples manufactured by this method lies in the range b = 1000–4000. The I/V characteristics of field emission measured on the edge of a Ni foil with the carbon nanotube layer are shown in Fig. 2a and b. Such a characteristic measured in ultrahigh vacuum (curve 1) does not differ from that measured on the flat part of the Ni foil with the carbon CNT layer (Fig. 1a and b). The Fowler–Nordheim plot (Fig. 2b, curve 1), is a straight line practically across the whole range of emission currents. However, the I/V field emission characteristics measured on such samples placed in the SEM chamber showed a marked discontinuity in the Fowler–Nordheim plot (Fig. 2b, curve 2)
Fig. 2. Field emission current (IFE) as a function of the average electric field (Eav) for CLNTs layers measured on the edge of Ni foil in ultrahigh vacuum, 1 and in SEM chamber, 2 (a) and Fowler–Nordheim plot of these curves (b). For ‘‘2’’ reversed plots are shown.
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and a loss of stability compared with ultrahigh vacuum conditions. We suggest that these differences are a result of the contamination of nanotube tips under the relatively poor vacuum conditions in the SEM chamber.
4. Structural investigations 4.1. HRTEM of CNTs To prepare specimens for HRTEM, dispersed CNTs were mounted on holey carbon films and examined at 400 kV. Both pre- and post-emission CNTs layers were investigated. A general view is shown in Fig. 3a. There appears to be one type of nanotube, with diameters ranging between 6 and 50 nm. It should be noted that catalyst material is virtually absent, and open ends of CNTs appear to be free of catalyst particles. Open CNT ends usually have a conical depression. Occasionally opposite (both) ends of a nanotube are visible. They are sharp and have opposite conicity. These outlines are typical for ‘‘bamboo’’-shaped CLNTs [19]. Since these CNTs are rather short to be complete original tubes, we tentatively consider these sharp ends to be broken ends. An HRTEM image (Fig. 3b) shows a typical CNT to consist of conical graphene layers stacked together, i.e., it is a conical layer CNT [12]. Its conicity varies between 19 and 41, correspondingly the outer diameter is 36– 40 nm and an inner channel diameter 15–21 nm. Open
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graphene layers edges are observed both on the outer surface profile and on the inner surfaces of the central channel. This particular CLNT is from the specimen following a field emission experiment. However, we could not detect any significant structural differences between CLNTs before and after emission experiments. On the other hand, we cannot be certain that any particular CLNTs imaged by HRTEM after emission measurements have actually emitted electrons. HTREM of open CLNTs ends shows that these ends are formed by one, two or three conical graphene layer edges (Fig. 4); so the radius of curvature (in radial direction) is 0.34–1.02 nm and the aspect ratio is high. Such ends can be considered as effective field electron emission source [5]. However, taking into account the specimen preparation techniques there is the possibility that these ends are actually broken CLNT ends. The situation is further aggravated by the fact that these ends have no Ni catalyst particle. In this context we note that it has been previously shown [20] that there is a tipgrowth process, when the catalyst particle is located at the growing end of a CNT, and a base-growth process where it remains in the supporting substrate and the end of the tube is closed. In order to clarify this problem it would be necessary for us to demonstrate conclusively that the tip-growth process is realized in the present work and that the open ends are not simply experimentally produced artefacts. To do so would require the elimination of the process of scraping the CLNTs from the Ni foil.
Fig. 3. CLNTs obtained by CVD on Ni foil (after emission). To prepare specimen CNTs were scraped of the substrate. (a) TEM general view of CLNTs. Open and broken ends are arrowed. (b) HRTEM image of CLNT. Open graphene layers edges are marked by arrows.
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Fig. 4. HRTEM image of CLNT open end. Arrows mark edge-on views of conical graphene layers which terminate the tube.
Fig. 5. Prior to emission TEM profile imaging of CNTs layers grown on the edge of 0.2 mm thick Ni foil. In (a) and (b) horizontal arrows tentatively marked boundaries of foil edge, dense region with coiled CLNTs, less dense area and area of separate loops and single CLNTs area. Small arrows indicate open ends.
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4.2. Profile imaging of CNTs layers prior and after to emission As was described above in Sections 2.1 and 2.2, for profile imaging CLNTs were grown on the edge of a 0.2 mm thick Ni foil. The small foil thickness facilitates TEM investigations of the foil edge profile and the CLNTs grown on it (Fig. 5). CNTs in the layers are distributed unevenly. In the immediate vicinity of the Ni support dense tangles of CLNTs are observed. The thickness of this area is 800–2000 nm. Next is a 800– 1650 nm thick, less dense area in which separate CLNTs 6–40 nm in diameter are easily traced. The CNTsÕ ends are usually free of catalytic particles and their outlines appear identical to images of ends revealed by HRTEM (Fig. 4). The direction of CLNT growth can be deduced from the conicity orientation, in most cases visible at the tubesÕ ends and sometimes also by inner cup orientation. Beyond this area separate loops and 400–2000 nm long CLNTs are observed, usually terminated with open ends. The total number of visible open ends ranges up
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to six per 1 lm of edge length; so the conclusion can be drawn that in our case CLNTs grow according to a tip-growth process, but the catalytic particles for some reason have disappeared. Open ends with atomically sharp endings can be very efficient electron sources. After SEM investigation of electric field influence on the CLNTs configuration some specimens were transferred into the TEM and examined by profile imaging. In one experiment the specimen was exposed to Eav 4.5 V/lm (see Section 4.3). A profile image of this specimen is shown in Fig. 6. Compared to profile images recorded prior to emission, more open-ended CNTs are revealed (12–16 per 1 lm of edge profile). The number of CNTs extended towards the anode is up to 6 per micron of length. Fig. 6 also indicates that some CNT end shapes are identical to those observed by profile imaging and by HREM, prior to emission. It is significant that although the number of CNTs stretched towards the anode increased, the height of CNTs is not uniform; we invariably found single CNTs (Fig. 6a) or a group of CNTs (Fig. 6b) which were a few microns higher that the others.
Fig. 6. TEM profile image of nanotubes layer grown on the edge of Ni foil after emission (Eav 4.5 V/lm).
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4.3. In situ SEM investigations of electric field, field emission current and exposure time influence on the CLNTs configuration More than 10 experiments were performed involving in situ SEM investigations of electric field influence on CLNTs configuration; these agreed well with one another except for a spread of Eav for the onset of configuration change (Eav = 3.4–3.9 V/lm). This spread may
arise as a result of incorrect estimation of d or, more probably, from different initial configurations of CNTs in the layer and differences in apex radius of individual CNTs. A detailed account of two of these experiments is given below. The initial configuration of CLNTs in one of these experiments is shown in Fig. 7a. The anode–cathode distance is 102 lm. The length of the investigated part of the foil edge is 14.6 lm. Arrow ‘‘1’’ indicates the end
Fig. 7. In situ SEM investigations of electric field influence on the CLNTs configuration. Anode–cathode distance d = 102 lm. The length of investigated part of Ni foil edge is 14.6 lm. Exposure time (t) is 30 s except (f). (a) CLNTs prior to electric field application. Arrows 1 and 2 marked tubes with free ends, which later on undergo serious configuration changes. (b) Eav = 4.90 V/lm, IFE = 1.6 · 10 5 A. Tube 1 and rope no. 2 straighten out (this process started at Eav = 3.92 V/lm). (c) Eav = 5.10 V/lm, I = IFE = 1.8 · 10 5 A. Tube 1 becomes straight. (d) Eav = 5.71 V/lm, IFE = 2.1 · 10 5 A. Tube 1 disappeared, rope no. 2 continued to straightening out. Tubes 3 and 4 protruded. (e) Eav = 6.12 V/lm, IFE = 2.7 · 10 5 A, t = 30 s. (f) Eav = 6.12 V/lm, IFE = 2.7 · 10 5 A, t = 120 s, rope no. 2 disappeared. Tube 3 straightened out, tubes 4 and 5 protruded.
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of an individual CNT and ‘‘2’’ the end of the rope of CNTs initially protruding from the layer. As mentioned
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in Section 2.2, in order to obtain SEM images a discontinuous mode of Vapp was used, with a typical exposure
Fig. 8. In situ SEM investigations of large electric field influence on CLNTs configuration at long exposure (t). Anode–cathode distance d = 80 lm the length of investigated part of Ni foil edge is 17.8 lm, t is exposure time. (a) Initial configuration. The bundle of CNTs is observed with one or two protruding single CNTs. (b) Eav = 7.06 V/lm, IFE = 5.5 · 10 5 A, t = 30 min. Part of CNTs is oriented towards anode, in a greater extend CNTs 1, 3 and ropes no. 2 and no. 4. (c) Eav = 7.61 V/lm, IFE = 5.5 · 10 5 A, t = 45 min CNT 3 disappeared. CNT 5 is protruding. (d) Eav = 8.14 V/lm, IFE = 6.0 · 10 5 A, t = 25 min. CNT 4 is protruding. Several tens of shorter CNTs are also oriented towards anode. (e) Eav = 8.75 V/lm, IFE = 7.0 · 10 5 A, t = 20 min. CNTs 1, 2 and 5 essentially shortened. Main emitters probably are CNTs 4, 6 and 7. (f) Eav = 9.2 V/lm, IFE = 9.0 · 10 5 A, t = 20 min (the overall time 140 min). Main emitters are CNTs 4 and 6. CNT 7 disappeared.
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time of 30 s. In the range Eav = 0.98–1.96 V/lm no configuration changes are revealed. From Eav = 2.16– 3.73 V/lm, the emission current rises, i.e., field emission commences. In this range of Eav configuration changes are again not observed. It is possible that in this case emission is achieved owing to already existing free open ends of CNTs protruding from the layer. At Vapp = 400 V (Eav = 3.92 V/lm) (Fig. 7b) tube 1 is beginning to straighten out. At Vapp = 500 V (Eav = 4.9 V/lm) this tube continue to straighten and rope 2 also starts to change its configuration. The configuration of other tubes does not appear to change. After Eav = 5.10 V/lm tube 1 becomes virtually straight (Fig. 7c). This pattern is preserved after application of Eav = 5.29 V/lm. After application of Eav = 5.49 V/lm tube 1 disappears completely but tubes 3, 4 and 5 are now evident (Fig. 7d). It may be concluded that on the investigated part of the foil edge only tube 1 is emitting. As a result of Joule heating by the emission current IFE = 1.8–2.1 · 10 5 A along the length of the CLNT in combination with electrostatic forces, the tube first straightened and then failed. At a later stage, with Eav = 5.88 V/lm and IFE = 4 · 10 5 A tube 3 is noticeably straightened, whereas rope 2 is elongated (Fig. 7e). It is possible that in this case rope 2 is emitting more intensively and tube 3 has started to emit. To estimate the influence of longer time emission on CNTsÕ configuration at constant Eav (5.88 V/lm), the specimen was exposed for an additional 120 s (Fig. 7f). As a result rope 2 disappeared and tubes 3, 4 and 5 straightened. Presumably at the range Eav = 5.49–5.88 V/lm the main emission from this part of the edge was from rope 2 and then the main emitter became tube 3. Straightening of this tube continued at Eav = 6.86 V/lm. It is also of interest to find out the CLNTsÕ configuration in the layer after application of relatively large fields (Eav = 7.06–9.2 V/lm) and longer exposure time. The next experiment describes such investigations. The length of foil edge investigated was 17.8 lm, and d was set at 80 lm. In Fig. 8a the initial configuration of CNTs in the layer is shown. A large bundle of CNTs is observed with one or two single ones protruding. After 30 min exposure at Eav = 7.06 V/lm and IFE = 5.5 · 10 5 A the CNTs configuration significantly changed (Fig. 8b). A few CNTs became oriented towards the anode, in particular CNT 1, rope 2 and straight CNT 3. An additional 45 min exposure at Eav = 7.61 V/lm and IFE = 5.5 · 105 A gave rise to the protruding CNTs 4 and 5 (Fig. 8c); straight CNT 3 disappeared. After an additional 25mins at Eav = 8.14 V/ lm and IFE = 6 · 10 5 A, rope 6 joined the above number (Fig. 8d). Several tens of shorter CNTs and CNTs loops were also now oriented towards the anode. At Eav = 8.75 V/lm, IFE = 7 · 10 5 A and t = 20 min. CNTs 1, 2 and 5 essentially shortened
(Fig. 8e), CNTs 4 and 6 became most protruding, and nearest to them is no. 7. The main emitters in this case were probably precisely these tubes. After Eav = 9.2 V/lm, IFE = 9 · 10 5 A and t = 20 min (overall time 140 min) tubes 4 and 6 survived; tube 7 had disappeared and a few ‘‘new’’ CNTs potentially able to replace them as emitters appeared. Summing up the results of more than 10 sets of experiments, it is reasonably safe to suggest that in the initial stage of increasing electric field (up to Eav = 3.2–3.9 V/ lm) the CNTs configuration does not change—or such changes are small—and after the electric field release necessary to obtain an SEM image the original configuration is restored. In only one of the experiments a significant, permanent configuration change was observed, at Eav = 1.67 V/lm, but newly formed, free single CNTs ends were not oriented towards the anode. After application of an electric field greater than Eav = 3.2–3.9 V/ lm and IFE 10 5 A two or three CNTs appeared to have protruded and straightened (at the foil edge length 10–18.5 lm). Sometimes they become absolutely straight. It is most likely that precisely these tubes were the main emitters in each sequence of stages. At the electric field Eav = 5–8 V/lm, and IFE = 2– 3 · 10 5 A, these tubes disappeared or essentially shortened. At the same time one or two CNTs appeared to which apparently emission function transferred. At this range of Eav exposure time become important. For example, increasing it from the usual 30 to 120 s activates significant configuration changes. At large Eav, current IFE and t = 20–45 min this situation is essentially the same. One or two CNTs emit. Then these CNTs as a rule disappear or shorten and two or three fresh CNTs appear, to which the main emission function transfers.
5. Discussion HRTEM revealed that CLNTs grown on Ni foil by CVD have many open ends free of catalytic particles. These ends were characterized by a high-aspect ratio. TEM profile imaging of foil edge revealed a high concentration of CLNTs near the foil surface while from the outer part of the layer single CNTs with open ends having conical depressions, protruded. This indicates that there is most likely a tip-growth process and that the open CLNTs ends can be effective field emitters. The influence of electric field, emission current and exposure time on the configuration of CNTs was investigated for nanotube layers grown on the edge of a Ni foil by the same process as on the flat foil surface. In situ SEM investigations were performed in the as-grown condition in the device with 10 4–10 5 Torr vacuum. Under these conditions constant contamination of CNTs tips was likely and an in situ cleaning condition-
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ing process [7,18] was not effective. At the same time field emission experiments performed in ultrahigh vacuum with CNTs grown on the edge of the foil as a specimen allowed us to obtain a standard Fowler–Nordheim plot. The results described in Sections 4.2 and 4.3 can be interpreted in the following way: any essential nanotubes configuration changes are observed before Eav = 3.2–3.9 V/lm. As was mentioned in Section 4.3 it is possible that after release of the applied field the structure was restored. Possibly at this stage emission is achieved owing to already existing, free, open ends of CNTs, which where revealed by TEM profile imaging (Fig. 6) and by SEM (Figs. 7 and 8). At higher Eav and IFE 10 5 A, a small number of CNTs oriented towards the anode are always observed. One can suppose that the straightening of CNTs is due to electrostatic forces and Joule heating by the emission current. Field emission current is an exponential function of the field, so only those CNTs whose ends are closer to the anode and have sharper apex radius are emitting. Sometimes single CNTs become absolutely straight. As a rule these CNTs usually disappear first. Upon further increasing the field and current (Eav = 5–8 V/lm and IFE 2–3 · 10 5 A) previously emitting CNTs are replaced by another one. In this range of Eav exposure time is critical: even a small increase in exposure time leads to a rapid increase in nanotubes replacement. With field Eav = 7.5–9.2 V/lm and IFE 5.5– 9.0 · 10 5 A and large exposure (20–140 min) basically the same processes are realized. In this case, under the influence of large fields re-orientation of tens of nanotubes occurs, but all of them are shorter than those CNTs which are emitting. One can suppose that their re-orientation occurred only as a result of electrostatic forces. Even in the field Eav 9.2 V/lm (IFE 9 · 10 5 A) in the inner part of the CNT layer there appear to be many CNTs whose configurations remain unchanged, and therefore structural realignment can be considered as completed, only for a given limiting value of Eav. The overall picture of CNTs configuration distribution appears to be closely connected with electrostatic screening between the CNTs. The electric field at the apices of the emitters decreases with decreasing spaces between the apices. The field amplification decreases rapidly for a spacing less than two times the height of a CNT. The field enhancement is largest for a single CNT and decreases for closely positioned CNTs [3,11,21,22]. TEM profile imaging (Fig. 6) and in situ SEM investigations (Figs. 7 and 8) reveal that some CNTs protruded towards the anode and most probably emitting CNTs appear to be arranged much closer together than the ‘‘twice-the-height’’ distance. One possible explana-
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tion for this phenomenon is the following: normally we have observed the foil edge at the length of 14.6– 17.8 lm (i.e., an average length 16.5 lm). However, the thickness of the observed region is 200 lm, which is much larger. We thus observed superimposed images of CNTs, arranged on different levels along the electron beam direction. This means that CNTs can be separated
Fig. 9. In situ SEM investigation of electric field influence on CLNTs configuration using preliminary exposure at high Eav. Anode–cathode distance d = 75 lm: (a) Eav = 6 V/lm, 30 s; (b) Eav = 6 V/lm, exposure time 30 min; (c) Eav = 5.3 V/lm, exposure time 60 min.
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by a much larger distance that what is observed in the electron microscopes. A limited number of emitting CNTs is available, as was mentioned above, by the fact that even in a set of approximately equal-height CNTs one of two are invariably slightly higher and/or their ends are sharper. Such CNTs become the main emitters and simultaneously even more straighten up. Following an increase of the field or exposure time these are usually destroyed. Over the whole range of Eav and IFE investigated, a limited number of CNTs emit and constant replacement of these nanotubes occurs. To increase the stability of CNTs configuration, a ‘‘structural conditioning’’ process is required which consists of short-term exposure at high Eav followed by Eav decreasing to the required operation condition. An example of this ‘‘structural conditioning’’ is shown in Fig. 9. At Eav = 6 V/lm the initial tangled coil of CNTs is visible along with a number of protruding CNTs (Fig. 9a). After 30 min exposure these CNTs are seen to be extended towards the anode (Fig. 9b). Subsequently the electric field was decreased to Eav = 5.3 V/ lm. After 60 min exposure the configuration of these CNTs was basically unchanged (Fig. 9c) although one of them became 25% shorter. Most likely this is due to Joule heating by the emission current. The precise measurement of IFE in the SEM vacuum after the ‘‘conditioning’’ process could not be performed because of the simultaneous contamination of the nanotube tips (see Section 3 above). Precise investigation of emission current stability in ultrahigh vacuum are being carried out for CNTs synthesized on the flat part of a Ni foil (manuscript in preparation). Finally, we note that ‘‘structural conditioning’’ at larger Eav could further increase the stability of CNTs configuration, although this process could also result in more damage of small diameter single CNTs.
6. Conclusion Conical layers carbon nanotubes were grown by CVD on the edge and on the flat part of Ni foil. Field electron emission measurements carried out in UHV on the flat part of Ni foil revealed reproducible I/V characteristics. For best samples the value of the field amplification coefficient b was 2500–4000. TEM profile imaging of CNT layers grown on the foil edge showed that in spite of an apparent tip-growth process, nanotube ends were free of catalyst particles. Profile imaging and in situ SEM investigations revealed that the configuration of the CNTs is influenced by electric field (electrostatic forces) and screening effects. Joule heating by emission current in combination with electrostatic forces leads to tube straightening and then to their
disappearance. Due to the specific layer structure (random CNTs orientation and close positioning) and to screening effects a limited number of emitting CNTs for each Eav exist. Such CNTs become the main emitters. Following an increase of the field these are usually destroyed. Over the whole range of Eav and IFE investigated, a limited number of CNTs emit and constant replacement of these nanotubes by ‘‘new’’ ones occurs. We have demonstrated that the CNTs configuration can to some extent be stabilized by preliminary, brief ‘‘conditioning’’ at higher Eav.
Acknowledgements This work was supported by ISTC Project No. 1024-2 and by Scientific School Grant 14.04.2003.2.
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