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Honeycomb arrays of carbon nanotubes in alumina templates for field emission based devices and electron sources
ARTICLE IN PRESS Physica E 42 (2010) 1469–1476
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Physica E journal homepage: www.elsevier.com/locate/physe
Honeycomb arrays of carbon nanotubes in alumina templates for field emission based devices and electron sources R. Angelucci a,e, I. Boscolo b, A. Ciorba c, M. Cuffiani d,e, L. Malferrari e, A. Montanari e, F. Odorici e, S. Orlanducci f, R. Rizzoli a,e, M. Rossi c,n, V. Sessa f, M.L. Terranova f, G.P. Veronese a,d,e a
CNR–IMM sezione di Bologna, via P. Gobetti 101, 40127 Bologna, Italy INFN and Dipartimento di Fisica Universita di Milano, via Celoria 16, 20133 Milano, Italy c Dipartimento di Energetica, Universita di Roma ‘‘Sapienza’’ via A. Scarpa 16, 00161 Roma, Italy d Dipartimento di Fisica Universita di Bologna, v.le B. Pichat 6/2, 40127 Bologna, Italy e INFN, v.le B. Pichat 6/2, 40127 Bologna, Italy f Dipartimento di Scienze e Tecnologie Chimiche, MINASlab, Universita di Roma ‘‘Tor Vergata’’-INFN, via della Ricerca Scientifica, 00133 Roma, Italy b
a r t i c l e in fo
abstract
Article history: Received 27 August 2009 Received in revised form 27 November 2009 Accepted 27 November 2009 Available online 11 December 2009
Field Emission (FE) properties of vertically aligned Carbon Nanotubes (CNTs) grown in a nanoporous anodic aluminium oxide (AAO) template have been investigated. A 50-mm-thick AAO template was fabricated by electrochemical techniques. The nanotubes were synthesized in a CVD quartz hot wall furnace using C2H2/N2 mixtures as feeding gas. I–V measurements have been performed on samples after the nanotubes growth (type I samples) and after a partial Al2O3 removal (type II samples) in order to obtain segments of nanotubes protruding from the nanopores. The effects of the conditioning process and adsorbates release have been investigated. The emission curves have been analysed in the framework of the Fowler–Nordheim model. For the b factor enhancement, a different dependence on time has been evidenced for two types of investigated samples and has been tentatively correlated with materials modifications occurring under the HV polarisation (in case of type I samples) and with the damage induced by chemical etching (in case of type II samples). The values of emitted current density (up to 40 mA/cm2) and the emission properties indicate that the proposed preparation methodology is suitable for the realization of robust and efficient CNT-based field emission devices and electron sources. & 2009 Elsevier B.V. All rights reserved.
Keywords: Field emission Nanoscale materials Nanotube devices
1. Introduction Electron sources operating on the basis of Field Emission (FE) are nowadays a basic component of many technological tools, such as Field Emission Displays (FEDs), cathode ray tubes, X-ray devices, high-power triodes, electron guns for evaporators, welders, powerful radio-frequency/microwave tubes, accelerators, etc. [1]. Among the variety of cold cathode materials having been developed till now, only field emission electron sources based on Carbon Nanotubes (CNTs) have the potential to operate at low voltage, and also under less strict vacuum conditions, emitting high and stable current densities [2]. Proper choices of the CNTs arrangement, in terms of density distribution and orientation, are expected to allow the fabrication of electron sources with extremely high brightness. However, as regards to the material one should consider that the I–V characteristics of a nanotube specific type are strongly influenced by various parameters such as nanotube dimension, shape and chemical state of the emitting
n
Corresponding author. E-mail address: [email protected] (M. Rossi).
1386-9477/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2009.11.119
apex, whereas the overall source emitting behaviour depends on the CNTs density and on the real emitting area. Moreover, to validate operations under poor vacuum conditions one should consider the presence of adsorbates affecting the I–V characteristics as emitted currents and threshold fields. The Fowler–Nordheim (FN) model [3] describes field emission from metallic or semi-conducting surfaces and is currently adopted to describe field emission from populations of CNTs. However, for CNT deposits, when analysing experimental data in the frame of the FN model, disagreements with the model previsions are often evidenced. Such discrepancies are likely related to the presence of collective effects influencing the measured I–V characteristics. The I–V measurements from CNT films involve emission areas with many emission centres and the FE characteristics derived from the measurements are integrated on the active emission centres population. Therefore it is impossible to extract the local FE characteristics, like those of a single emitter, but it is possible to relate the integrated quantities, for example the current, to some sample characteristics like the nanotubes density. Opposed to the common sense, a higher density of emission centres does not correspond necessarily to a higher density current. Indeed,
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increasing the emitter density the nanotubes start to interact each other, decreasing the local electric field on their apex and therefore the emitted current. Under these conditions the nanotubes system loses the electric field enhancing characteristics. This electrostatic screening effect implies the existence of an optimum value for the nanotubes density. CNT-based cathodes fabrication is presently a topic of enormous interest in view of the fact that the combination of electrical properties with toughness and chemical inertness makes possible to use the nanotubes also in harsh environments, where other materials have failed. For instance, the ability to withstand energetic ions, oxygen fluxes and adsorbates would make the cathodes attractive and functional to be used as electron injection devices in plasma reactors and electron–cyclotronresonance ion sources. For that kind of application, the mechanical, thermal and chemical characteristics of cold cathodes based on CNTs grown inside an insulating AAO matrix is very promising. However, the electron injection efficiency, the mechanical strength and chemical inertness of these matrices to be used as electron injection devices in particular plasma environments need to be investigated. As regards the operation in space environment, CNT-based cathodes would be the enabling technology for lowpower propulsion thrusters, electrodynamics tethers for propulsion in low-Earth orbit and charge control of spacecraft solar arrays. The recent research trends show that a series of near term technological challenges should overcome to engineer macroscopic emitting surfaces with characteristics required for the envisaged advanced applications. The most critical development issue to be addressed is a reproducible batch-type manufacturing of large-area cathodes with controlled density, height and size distribution of the emitters on the device surface. An interesting approach is to fabricate the source as an ordered and uniform array of emitters by growing the carbon nanotubes within the nanopores of a template. To this scope we have chosen nanoporous alumina as a template material for ordered nanotubes production. The alumina matrix is well suited for the growth of nanotube architectures [4–6], whereas the dissolution of the underlying alumina template can enable integration of the produced carbon nanostructures into a variety of electronic devices [7]. The samples used in this study consist of multi-walled CNTs ordered arrays embedded in porous alumina membranes with a regular honeycomb pore distribution. The field emission properties of such samples, both as-grown and after a partial removal of the alumina matrix, have been investigated. In view of the fact that the presence of adsorbates and their release kinetics are a real concern for the emission experiments, specific measurements have been also performed, aiming at probing the effects induced on the emission by conditioning treatments. In particular, the adopted measurement methodology and the regular 3D architecture allowed the evaluation of the current emitted by the single emitter (the nanotube inside a single pore). Overall, the characterization methodology presented in the paper is a viable solution to check if the prepared material is functionally suitable for the realization of robust and efficient CNT-based field emission devices and electron sources.
2. Experimental In order to obtain a highly regular nanopore array in anodized aluminium oxide (AAO), a two-step anodization process of Al foils with a technique similar to the one described in [8–9] was performed.
Experiments were performed on 99.999% pure Al foils (by Goodfellow) with 2.5 2.5 cm2 area and 100 mm thickness, previously degreased and electro-polished in order to remove defects and improve their planarity. Anodization was obtained in an electrolytic cell with 0.3 M (COOH)2 solutions at an applied voltage of 40 V. The resulting pores have average diameter of 40 nm and inter-pore distance of about 100 nm. At the process temperature of about 5 1C, the AAO excavation rate was about 3 mm/h, and a depth of about 45 mm was obtained in 15 h. To the purpose of electrically contacting the CNTs embedded in AAO, the oxide barrier layer at the bottom of the AAO pores has been removed by the following steps: a pore-widening step (in H3PO4, at 30 1C, for about 40 min) which enlarges the pore diameter up to 50 nm and reduces the barrier thickness; the residual Al layer removal (in CuCl2) on the sample backside; the pores bottom opening by chemical etching (in H3PO4 at 30 1C, for about 50 min) to obtain the AAO membrane. An electrically conductive uniform pad on the backside of the AAO membrane was obtained by thermal evaporation of 100-nmthick Cr layer followed by 150-nm-thick Au layer. This electrical contact also allows to deposit at the bottom of the pores a small quantity of cobalt, which acts as a catalyst for the subsequent synthesis of carbon nanotubes. Cobalt seeds have been DC electrodeposited from CoSO4 solution. The process duration enables producing the ordered arrays of Co nanoparticles with controlled height. The CVD synthesis of carbon nanotubes was obtained by the following multi-step process: samples loading in a quartz furnace with hot walls at atmospheric pressure, a 100 min controlled warming in H2 flow up to the synthesis temperature of 700 1C, a 60 min annealing in H2 flow (150 sccm) at constant temperature and a 30 min deposition step with a mixture of 15% C2H2 in N2 carrier gas (170 sccm total mass flow) and a final cooling performed in Ar flow. After the CNTs deposition process, an amorphous carbon layer covering the Al2O3 top surface was found. In order to eliminate possible shorts between CNTs, the carbon layer has been removed by O2 annealing at 420 1C in 2 h. Then, for each sample, a welldefined portion of the AAO template was partly dissolved by 4 h etching in 0.3 M H3PO4 at 23 1C. The CNT array inside the AAO template has been observed by field emission scanning electron microscopy (FE-SEM, LEO 1530) and characterized by micro-Raman spectrometry (Renishaw 1000 spectrometer, He–Ne laser at 632.8 nm, maximum output power of 15 mW) both after annealing in O2 and after the complete etching of alumina template. FE measurements have been carried out in a high-vacuum chamber provided with a rotative-turbomolecular pumping system. The sample has been moved along three axes with a micrometric translator. In order to measure the FE properties in different small and localised areas of the sample surface and to avoid the negative effects induced by an imperfect probe orientation in the plane-toplane geometry, the employed anode is a molybdenum sphere with a diameter of 1.5270.01 mm, mounted on a 30-mm-long rod of stainless steel, with a diameter of about 1.1 mm. The distance between the anode and the flat cathode (sample) and the position in the (x,y) plane are PC-controlled by a linear precision actuator (DC-Mike Actuator M-222.23, PI) with a minimum incremental step of 0.1 mm and two linear standard translators with a minimum incremental step of 1 mm. The diode is fed by 2 kV–3 mA generator via a protective ballast resistor of 100 M. The effective anode–cathode distance is determined via the measurement of the anode–cathode capacitance and the current is measured by a Keithley 6485 picoammeter with rms noise of about 1 pA in the lowest current range under measurement
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conditions [10]. FE measurements at room temperature and in a pressure range 2.6–3.5 10 7 mbar have been performed.
3. Results and discussion Fig. 1 shows typical FE-SEM images obtained from the anodized alumina template, revealing a regular honeycomb-like structure with pore diameters and pitches of about 50 and 100 nm, respectively. The FE-SEM analysis confirmed that the adopted preparation methodology is a viable route to reliably achieve a uniform 3D architecture of vertically aligned parallel nanopores in alumina templates. The typical morphology obtained after the CNT deposition and the O2 annealing step is reported in Fig. 2a. The nanotube top ends are aligned with alumina surface, while the nanotube density is uniform depending only on the template pitch. Here the nanotube diameter is in the range 40–50 nm and the interdistance is about 100 nm. Fig. 2b and c show the effects of H3PO4 treatment on such architecture to partially remove the AAO template. The nanotube diameter appears again in the range 40–50 nm but the tips of nanotubes exceed the ceramics level by some nanometers. Their apexes are open, so the tops of nanotubes are the rims of carbon cylinders. The first-order Raman spectrum of the as-grown sample (before the AAO etching) shows D (at about 1340 cm 1) and G (at about 1580 cm 1) peaks partially masked by a broad shoulder, associated to the presence of the embedding alumina template, which makes difficult to evaluate the quality of the CNTs inside the AAO pores. On the contrary, the complete removal of the alumina template gives the possibility to detect better defined Raman signals from the cylindrical carbon deposits, released from the original ceramic skeleton. The features of the Raman spectrum (Fig. 3) taken after the AAO removal, i.e. the relatively high intensity of the D peak, the intensity ratio of the D to G peaks (ID/IG = 1.36) and the presence of a low intensity second-order D peak (G0 ) suggest that the carbon structures inside the AAO pores are defective nanotubes. A poor graphitization also explains the fact that the nanotube top ends, after the O2 annealing, become approximately aligned with the alumina surface. Indeed, the O2 atmosphere has etched the extruding tubules while the embedded ones were partially protected by the alumina template.
Fig. 2. FE-SEM image of CNT arrays in AAO templates: As obtained at the end of CVD growth; (b–c) top (b) and inclined view (c) after partial removal of the AAO matrix by chemical treatments.
Fig. 1. FE-SEM side view and top view (inset) of a typical AAO template used for the CNT synthesis.
The electron emission properties of CNTs on alumina template samples, before and after the partial AAO etching, have been measured, being their morphologies reported in Fig. 2a–c, respectively, and thereafter referred to as sample of type I and sample of type II. FE measurements have been carried out on different areas of each sample. Each area of the investigated samples has been analysed by the protocol described below, according to experimental methodology fully described in [10]. No significant differences have been revealed analysing different areas of the same type samples. As a consequence, in the following, we report and discuss the FE results corresponding to a typical area for each type of sample.
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and Eq. (1) can be written as dA2 2 A2 JðEÞ ¼ A1 E exp 2pR dE
ð3Þ
where d is the anode–cathode distance and R is the sphere radius. Parameters A1 and A2 are extracted by fitting the experimental I–V curves with Eq. (2). From equations reported above the following expression of the enhancement factor is derived pffiffiffiffiffiffiffiffiffi 3=2 8pd 2me f b¼ ð4Þ 3eh A2
Fig. 3. Raman spectrum taken after complete removal of the AAO template, with indication of the intensity ratios of D to G, D to G0 and G to G0 peaks, respectively.
The field emission behaviour of CNT-based materials is strongly influenced by the presence of adsorbates, which are bonded by different surface energies depending on their chemical state. It is well known that the adsorbates modify both the measured emitted current and the threshold fields. Therefore they hamper the real field emission properties evaluation. As a consequence, a key factor in the FE measurements is represented by setting up the proper conditioning procedures able to reduce the amount of surface-adsorbed chemical species substantially. To achieve the aim, the samples are firstly conditioned by applying different constant voltages between anode and cathode. When the measured I–V curves become fairly repeatable and the emission current noise is reduced, a higher constant voltage is applied up to the emission of a current in the range of few microamperes. Under such conditions the local heating also contributes to eliminate the more energetically bonded chemical species. Fig. 4 shows the typical behaviour of the measured current under applied voltage of 2000 V during the conditioning process. The emitted current is noisy (especially at the beginning of the conditioning process) and shows steps and ramps. The conditioning process is considered as terminated when in a temporal window of 180 s the emitted current is rather constant with a relative variation less than 1%, like in the last segment of the curve in Fig. 4. The field emission behaviour is described by the well-known Fowler–Nordheim law [3] " pffiffiffiffiffiffiffiffiffi # 2 3=2 e3 b E2 4 2me f ð1Þ exp JðEÞ ¼ 3e_ bE 8ph f where e and me are the charge and mass of the electron, respectively, h is Planck’s constant, E the applied electric field, f the work function and b the electric field enhancement factor, defined as the ratio of the local field to the applied field. Considering that in our setup the anode–cathode system can be modelled as a sphere-to-plane geometry, the following relationship between the total emitted current I and the anode– cathode voltage V has been found [10]: 2V A2 exp ð2Þ IðVÞ ¼ A1 V 3 1 V A2
d and f are unknown values, but d can be indirectly determined using the procedure described below. The distance d has been determined by a non-contact method in order to avoid any damage of the material surface. This method is described in details elsewhere [10], and it is based on the assumption that the anode and a sample surface can be modelled as a sphere and a plane, respectively. The relationship between the capacitance C of a sphere/plane capacitor and the distance d between the two conductors [11] is d ð5Þ CðdÞ ffi 2peR kln R where R is the sphere radius, e is the medium permittivity and k is a constant which takes into account the roughness of a conductor surface. A linear actuator with a 0.056 mm accuracy yields the quote z of the probe (the sphere), but the quote o of the material surface (the plane) and hence the anode–cathode distance d= z o are unknown. According to the procedure described in [10], the distance d= z o can be evaluated by fitting the experimental C(z) curve reported in Fig. 5. In this way the relative error of b is given by @b Db 1 @b Dd þ @b DA2 ¼ 3 Df þ Dd þ DA2 ¼ Df þ ð6Þ @d @A b @f A2 2 f d b 2 From literature data, the CNTs work function ranges [12–14] from 4.51 to 5.05 eV, corresponding to a mean value of 4.7870.27 eV with a relative error of 5.6% on f, while the typical error on d is about 1–2%. The maximum propagated error on the b factor value is about 17%. The FE measurements performed in a typical area for the two samples types are reported in Fig. 6: the FE properties of the asgrown sample (type I), shown in Fig. 6a–d, are compared to those of a partially etched sample (type II), shown in Fig. 6e–h. The measurements of type I and type II samples have been performed with an anode–cathode distance, determined according to the above described procedure, d =39 and 51 mm, respectively. An apparent peculiarity of the reported field emission measurements is the presence of two slopes in the Fowler– Nordheim (FN) plots. The standard Fowler–Nordheim model has been developed for metals and in this case a single potential barrier has been considered for the electron tunnelling towards the vacuum. It establishes a linear dependence of ln (J/E2) on inverse electric field (1/E). In spite of it, the field emission measurements on CNTs showing two slopes in the corresponding FN plots are discussed by many scientific articles. This behaviour cannot be attributed to the space charge effect due to the low current densities. But it has been ascribed to the presence of two potential barriers, instead of one as in metals [15–16]. The additional barrier can be produced by an adsorbates layer on the surface [17], but it should disappear after a total desorbing process (which is only partially achievable at UHV conditions)
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Fig. 4. Current vs. time, I(t), taken at V = 2000 V during the final step of the conditioning process (curve on the left) and steadily emitted from the conditioned sample (curve on the right)
b match very well the values obtained by fitting the FN plots at
Fig. 5. Capacitance C versus quote z. The cathode surface quote o extracted from the fit is reported. The anode–cathode distance is d =z o. r is the correlation coefficient between the z and C data arrays and it gives a measure of goodness of the data fit with the given formula C(z).
[18]. In this way, it is possible to obtain FN plots with a single slope from metallic surfaces. Another barrier could be present at the interface between the nanotube and the substrate, where an oxide or carbide (depending on the substrate material) interlayer is formed during the nanotubes growing process [15–16]. A ‘‘real’’ field emission is achieved after a conditioning process, in order to reduce the adsorbates on the emitting surface, and only at high electric fields [15–16]. Therefore, the enhancement b factor has to be extracted only on this part of the FN plot. At low electric fields the electron emission is influenced by the presence of the second potential barrier and it is no more described by the Fowler–Nordheim model. The b extracted in this range is no more the electric field enhancement factor on the material surface and it loses any physical meaning. As described above, b is extracted by fitting the whole I(V) experimental curve. These values match the b calculated at high electric fields very well. In this way, an arbitrary choice of the data used in FN plot fitting for b evaluation will be avoided. By fitting the whole I(V) curves, at low fields and much lower currents the measurements have little weight, because of the used algorithm (w2 minimization with uniform weights) and the fitted
high fields. Fig. 6a and e show the experimental I–V (the total emitted current I vs. the anode–cathode voltage V) curves. The reported curves (A–D) correspond to a temporal sequence of measurements carried out at the end of the conditioning procedure, performed once before beginning the measurements sequence itself. Fig. 6b and f show the related I–E curves of the total emitted current I vs. applied macroscopic electric field E= V/d with d anode–cathode distance. The insets highlight, for each type of sample, a value range of electric field threshold Eth corresponding to that required to have an emission current of 1 nA. The J–E curves represent the emitted current density J vs. the applied macroscopic electric field E (Fig. 6c and g), and the related insets show the FN plots derived from two sets of the measured I–V curves. Finally, Fig. 6d and h display the values of the field enhancement b factor, i.e. a purely geometrical factor related to the emitter surface geometry, which is derived from I–V curves of Fig. 6a and e by the method described above. The plots reported in Fig. 6c and g insets show a good agreement with the Fowler–Nordheim model in both samples types. The acid treated samples (type II) show current densities one order of magnitude smaller and threshold electric fields larger than the as-grown ones (type I), highlighting a worse emission efficiency, probably due to the nanotubes damage caused by the acid. Fig. 6d and h show the behaviour of b values corresponding to the different sequential measurements; the dependence of b on time deserves some comments. In the case of as-grown samples, b values show an increasing trend, as also summarised in Table 1, which can be explained on the base of two possible physical reasons, reported as follows. The first one is that the surface morphology undergoes modifications during the measurements. In this case a viable hypothesis is the nanotubes movement induced by applied electric field [19–20]. In our case the field is expected to induce a stretching of nanotubes out of the alumina nanopores. The second one is intrinsically connected to the necessity of conditioning procedures before the experimental measurements for b evaluation. As described above, the b values depend on the work function f. In our experimental data analysis, we used the typical values of f reported in the nanotubes literature, but it is well known [18,21–24] that adsorbates presence can influence
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Fig. 6. Comparison between the main field emission characteristics of the as-grown (on the left, a–d) and the partially etched (on the right, e–h) samples. The reported curves (A–D) correspond to a sequence of measurement runs, carried out after the conditioning procedure. (a, e) experimental I–V (the total emitted current I vs. the anode– cathode voltage V) curves; (b, f) I–E curves (the total emitted current I vs. the applied macroscopic electric field E =V/d, where d is the anode–cathode distance); the insets highlight the electric field thresholds for a total emitted current of 1 nA; (c, g) J–E (the emitted current density J vs. the applied macroscopic electric field E) curves. Insets: related Fowler–Nordheim plots; (d, h) values of the enhancement b factor.
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Table 1 Values derived from measurements on the as-grown sample.
b Eth (V/mm) Ec (V/mm)
A
B
C
D
402 15.8 42.3
273 18.5 62
355 19.0 47.9
668 17.2 25.5
such values. As a consequence, the determined b values could be underestimated in comparison to the real b value that is, merely, a geometrical factor. The adsorbates desorption during measurements could change the work function and determine an apparent increase in the b value. The conditioning procedure we followed let us reasonably exclude a predominant effect of adsorbates determining increase in the b value. The possibility of ballistic emission has been also considered. The critical field Ec required for ballistic emission has been calculated for the different measurements (A–D), according to the model proposed by Forbes [25]. The calculated Ec values are reported in Table 1 and compared with the Eth values, remaining practically unchanged in our experimental conditions. On the basis of the reported data, the presence of a significant and/or predominant ballistic emission can be reasonably excluded. Even if a strict conclusion about the physical origin of the b increase cannot be drawn at this stage, from a phenomenological point of view there are no doubts that the measurements indicate a real increase in the b parameter for the as-grown sample. Up to now the FE model used to analyse our data, established in [10], has been applied in the case of continuous CNT deposits and has demonstrated its feasibility to determine the emission current density of the cathodes with a good accuracy. The ordered structure of cathodes assembled in the present experiments enables a further advance, consisting in application of the ‘‘continuous’’ model to the emission phenomenon from discrete CNT emitters. The samples morphology makes reasonable the hypothesis that the emission centres are placed in the pores at the centre of the template honeycomb cells (Fig. 7). The total emitted current is the sum of the contributes of each single emitter, I= SkIk. The kth cell contribute Ik to I depends on the position rk of the emission centre on the sample surface with respect to the projection of the spherical probe centre on it, and on its emission characteristics (the work function fk and the enhancement bk factor). Due to the small dimensions of a single emitter, its contribution can be expressed as Ik = AkJk, where Jk is the emission current density J(rk, fk, bk) calculated at the centre of the cell and Ak is its effective emitting area. Assuming fk = f = 4.7870.27 eV (the median value of carbon nanotubes work function), bk = b as derived from measurements and Ak =A, the mean effective emitting area, the expression of the total emitted current is given by X X I¼ AJ½bEðrk ; V; dÞ; f ¼ A J½bEðrk ; V; dÞ; f ð7Þ k
k
where E(r,V,d) is the electric field on the material surface at the position r when an anode–cathode voltage V is applied and the anode–cathode distance is d. P The term k J½bEðrk ; V; dÞ; f is calculated by truncating the sum when the contribute of the cells bounding the considered area (white cells in Fig. 7) represents a given percentage (0.01%) of the maximum contribution (darkest cell in Fig. 7, considered beneath as probe centre) and the sum is about saturated. Taking into account the ratio between the pore and the cell areas, the area Atot of the surface considered in this sum is compatible with the areas Acont calculated in the framework of the ‘‘continuous’’ model. It supposes electron emission from the whole surface.
Fig. 7. Schematic drawing of the area considered in the calculation of the total emitted current. Different grey-level areas correspond to different steps in the summation. The white cells contribution is too small and the sum is stopped at the corresponding step.
Imposing I equal to the measured current Im, it is possible to calculate the mean effective emission area A¼
Im N P
ð8Þ
J½bEðrk ; V; dÞ; f
k¼0
Of course this value is underestimated, because not all the cells contain the emitter (some pores are empty). However, the values found for A are compatible with the section of the tubes inside the pores. Using this value it is possible to estimate the current I emitted, at a given voltage, from a single emitter, i.e. a single nanotube inside a pore. The evaluated current values are also underestimated, for the same reasons reported above for the value A. For the as-grown sample (type I) we determined a value I43 pA for the single emitter. It is fully compatible with the current density values found with the continuous model. In case of the etched sample (type II) the emission process shows remarkable differences in comparison to the case of asgrown sample. Assuming a threshold current of 1 nA, the threshold field is in the range 19.5–22.4 V/mm and shows a trend of field increasing with time. These field values are higher in comparison to those found for the as-grown samples. The values of both emission current (Fig. 6e) and calculated current density (Fig. 6g) are considerably lower in comparison to those measured for the as-grown samples (Figs. 6a and 6c, respectively). The evaluation of b values (Fig. 6h) reveals a decreasing trend vs. time. From the point of view of field emission behaviour the H3PO4 treatment effect seems to be negative. This could be ascribed to the physical properties degradation of the nanotubes segments protruding from nanopores representing the emission current source. In this context, the b decrease can be explained taking into account a possible emitting centres degradation. However, considering the applied fields values the currents emitted from the exposed CNT arrays are still advantageous over cold cathodes assembled using other materials.
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4. Conclusions In this paper some open questions about the fabrication of efficient CNT-based cold cathodes have been addressed. The electron sources have been realized using porous alumina templates for the growth of arrays of vertically ordered nanotube deposits. The employed anodization methodology enables to produce nanopatterned alumina templates and to control diameter, density and height of the emitters (where pore dimensions, that is diameter and deepness, and pitches can be varied). On the other hand, the coupling of alumina with CNTs assures robustness, chemical inertness and feasibility of operation even in harsh environments for the emitting devices. Field emission testing has been performed, using the same sphere-to-plane diode configuration, both on the as-produced cathodes and on nanotube systems protruding from the pores after partial removal of the underlying Al2O3 templates by acid etching. The experimental data, obtained with the capacitance technique for measuring the anode–cathode distance with the accuracy of few microns, have been analysed in the frame of the Fowler–Nordheim model. The general trends exhibited by the main parameters of the field emission process, i.e. the threshold field, the enhancement factor and the current density at different fields, have been found to agree with the emission features expected on the base of the physics involved. In particular, quite high values of current density (up to some tens of mA/cm2) have been obtained at relatively low applied fields for the as-grown samples. The obtained current density is not far from the value considered by some authors as a realistic challenge [26–27]. It has been also possible to give an evaluation of the current emitted from a single nanoemitter (i.e. a CNT inside the pore). The reported value of I =3 pA represents an underestimation of the real value considering that the model is based on the assumption that all the pores have been filled with an emitting nanotube. On the contrary, only part of the pores is effectively filled with nanotubes and, therefore, the experimentally measured current is due to a number of nanotubes lower than those considered in the model. The reported value is the minimum physically possible assuming that all the nanopores are filled. In our case, the fraction of filled holes has been estimated to be typically 490%. On the basis of the present results, the cold cathodes assembled by growing CNTs into the alumina scaffolds meet the requirements to be used as efficient electron sources. With the aim of fabricating electron injection devices for application in plasma reactors and electron–cyclotron-resonance ion sources, the robustness and chemical inertness of those cold cathodes have been also investigated. Preliminary experiments have demonstrated both a good electron injection efficiency and robustness of the CNTs arrays inside AAO templates when inserted in plasma environments. On the other end, for the partially etched samples a worsening of the emission has been detected. With an applied electric field of about 30 V/mm, the current density is about 10 mA/cm2. These
samples seem therefore less suitable for the use as cold cathodes. Nevertheless, the results of the experiments of alumina dissolution are considered very important for two different reasons. First of all, it has been demonstrated that the ordered symmetry of the whole deposit is preserved and that the structural properties of the nanotubes are only partially affected by the treatment, as seen from the Raman analysis and from the emission features. Secondly, the reduction of the emission from ordered nanotube samples may be of interest when dealing with micro-nano systems requiring an arrangement of highly ordered CNTs along the Z direction for efficient transport of both heat and electrical charges, but at the same time low electron emissions. In this context, self-standing arrays of low-emitting CNT protruding from a thin alumina support contacted at the bottom with a metallic layer (in our case Cr/Au) can be viewed as multifunctional systems and potentially useful for interconnections, nanocontacts or flip-chip architectures.
Acknowledgments This work has been partially supported by the INFN through the CANTES project. References [1] S.C. Lim, K. Lee, I.H. Lee, Y.H. Lee, Nano 2 (2007) 69. [2] J.M. Bonard, H. Kind, T. Stoeckli, L.-O. Nilsson, Solid State Electron. 45 (2001) 893. [3] R.H. Fowler, L.W. Nordheim, Proc. R. Soc. London Ser. A 119 (1928) 173. [4] J.S. Suh, J.S. Lee, Appl. Phys. Lett. 75 (1999) 2047. [5] T. Iwasaki, T. Motol, T. Den, Appl. Phys. Lett. 75 (1999) 2044. [6] H. Gao, C. Mu, F. Wang, D. Xu, K. Wu, Y. Xie, S. Liu, E. Wang, J. Xu, D. Yu, J. Appl. Phys. 93 (2003) 5602. [7] C.-C. Lin, K.-C. Chang, F.-M. Pan, C.-T. Kuo, M. Liu, C.-N. Mo, Diam. Relat. Mater. 16 (2007) 1388. [8] O. Jessensky, et al., Appl. Phys. Lett. 72 (1998) 1173. [9] H. Masuda, K. Fukuda, Science 268 (1995) 146. [10] I. Boscolo, S. Cialdi, A. Fiori, S. Orlanducci, V. Sessa, M.L. Terranova, A. Ciorba, M. Rossi, J. Vac. Sci. Technol. B 25 (2007) 1253. [11] L. Boyer, F. Houze´, A. Tonck, J.-L. Loubet, J.–M. George, J. Phys. D: Appl. Phys. 27 (1994) 1504. [12] M. Shiraishi, M. Ata, Carbon 39 (2001) 1913. [13] Z. Xu, X.D. Bai, E.G. Wang, S.L. Wang, Appl. Phys. Lett. 87 (2005) 163106. [14] S. Suzuki, Y. Watanabe, Y. Homma, S. Fukuba, S. Heun, A. Locatelli, Appl. Phys. Lett. 85 (2004) 127. [15] S. Altman, P.V. Pikhitsa, M. Choi, Appl. Phys. Lett. 84 (2004) 1126. [16] J. Zhang, C. Yang, W. Yang, T. Feng, X. Wang, X. Liu, Solid State Commun. 138 (2006) 13. [17] W.P. Dyke, W.W. Dolan, Adv. Electron. Electron Phys. 8 (1956) 89. [18] K.A. Dean, B. Chalamala, Appl. Phys. Lett. 76 (2000) 375. [19] P. Poncharal, Z.L. Wang, D. Ugarte, W.A. de Heer, Science 283 (1999) 1513. [20] Y. Guo, W. Guo, J. Phys. D: Appl. Phys. 36 (2003) 805. [21] R. Gomer, in: Field Emission and Field Ionization, Harvard University Press, Cambridge, Massachussetts, 1961. [22] J.W. Gadzuk, E.W. Plummer, Rev. Mod. Phys. 45 (1973) 487. [23] L.W. Swanson, A.E. Bell, Adv. Electron. Electron Phys. 32 (1973) 193. [24] K.S. Yeong, J.T.L. T.hong, Appl. Surf. Sci. 233 (2004) 20. [25] R.G. Forbes, Ultramicroscopy 79 (1999) 11. [26] W. Zhu, C Bower, O. Zhou, G. Kochansky, S. Jin, Appl. Phys. Lett. 75 (1999) 873. [27] N. de Jonge, J.-M. Bonard, Phil. Trans. R. Soc. London A 362 (2004) 2239.
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Physica E journal homepage: www.elsevier.com/locate/physe
Honeycomb arrays of carbon nanotubes in alumina templates for field emission based devices and electron sources R. Angelucci a,e, I. Boscolo b, A. Ciorba c, M. Cuffiani d,e, L. Malferrari e, A. Montanari e, F. Odorici e, S. Orlanducci f, R. Rizzoli a,e, M. Rossi c,n, V. Sessa f, M.L. Terranova f, G.P. Veronese a,d,e a
CNR–IMM sezione di Bologna, via P. Gobetti 101, 40127 Bologna, Italy INFN and Dipartimento di Fisica Universita di Milano, via Celoria 16, 20133 Milano, Italy c Dipartimento di Energetica, Universita di Roma ‘‘Sapienza’’ via A. Scarpa 16, 00161 Roma, Italy d Dipartimento di Fisica Universita di Bologna, v.le B. Pichat 6/2, 40127 Bologna, Italy e INFN, v.le B. Pichat 6/2, 40127 Bologna, Italy f Dipartimento di Scienze e Tecnologie Chimiche, MINASlab, Universita di Roma ‘‘Tor Vergata’’-INFN, via della Ricerca Scientifica, 00133 Roma, Italy b
a r t i c l e in fo
abstract
Article history: Received 27 August 2009 Received in revised form 27 November 2009 Accepted 27 November 2009 Available online 11 December 2009
Field Emission (FE) properties of vertically aligned Carbon Nanotubes (CNTs) grown in a nanoporous anodic aluminium oxide (AAO) template have been investigated. A 50-mm-thick AAO template was fabricated by electrochemical techniques. The nanotubes were synthesized in a CVD quartz hot wall furnace using C2H2/N2 mixtures as feeding gas. I–V measurements have been performed on samples after the nanotubes growth (type I samples) and after a partial Al2O3 removal (type II samples) in order to obtain segments of nanotubes protruding from the nanopores. The effects of the conditioning process and adsorbates release have been investigated. The emission curves have been analysed in the framework of the Fowler–Nordheim model. For the b factor enhancement, a different dependence on time has been evidenced for two types of investigated samples and has been tentatively correlated with materials modifications occurring under the HV polarisation (in case of type I samples) and with the damage induced by chemical etching (in case of type II samples). The values of emitted current density (up to 40 mA/cm2) and the emission properties indicate that the proposed preparation methodology is suitable for the realization of robust and efficient CNT-based field emission devices and electron sources. & 2009 Elsevier B.V. All rights reserved.
Keywords: Field emission Nanoscale materials Nanotube devices
1. Introduction Electron sources operating on the basis of Field Emission (FE) are nowadays a basic component of many technological tools, such as Field Emission Displays (FEDs), cathode ray tubes, X-ray devices, high-power triodes, electron guns for evaporators, welders, powerful radio-frequency/microwave tubes, accelerators, etc. [1]. Among the variety of cold cathode materials having been developed till now, only field emission electron sources based on Carbon Nanotubes (CNTs) have the potential to operate at low voltage, and also under less strict vacuum conditions, emitting high and stable current densities [2]. Proper choices of the CNTs arrangement, in terms of density distribution and orientation, are expected to allow the fabrication of electron sources with extremely high brightness. However, as regards to the material one should consider that the I–V characteristics of a nanotube specific type are strongly influenced by various parameters such as nanotube dimension, shape and chemical state of the emitting
n
Corresponding author. E-mail address: [email protected] (M. Rossi).
1386-9477/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2009.11.119
apex, whereas the overall source emitting behaviour depends on the CNTs density and on the real emitting area. Moreover, to validate operations under poor vacuum conditions one should consider the presence of adsorbates affecting the I–V characteristics as emitted currents and threshold fields. The Fowler–Nordheim (FN) model [3] describes field emission from metallic or semi-conducting surfaces and is currently adopted to describe field emission from populations of CNTs. However, for CNT deposits, when analysing experimental data in the frame of the FN model, disagreements with the model previsions are often evidenced. Such discrepancies are likely related to the presence of collective effects influencing the measured I–V characteristics. The I–V measurements from CNT films involve emission areas with many emission centres and the FE characteristics derived from the measurements are integrated on the active emission centres population. Therefore it is impossible to extract the local FE characteristics, like those of a single emitter, but it is possible to relate the integrated quantities, for example the current, to some sample characteristics like the nanotubes density. Opposed to the common sense, a higher density of emission centres does not correspond necessarily to a higher density current. Indeed,
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increasing the emitter density the nanotubes start to interact each other, decreasing the local electric field on their apex and therefore the emitted current. Under these conditions the nanotubes system loses the electric field enhancing characteristics. This electrostatic screening effect implies the existence of an optimum value for the nanotubes density. CNT-based cathodes fabrication is presently a topic of enormous interest in view of the fact that the combination of electrical properties with toughness and chemical inertness makes possible to use the nanotubes also in harsh environments, where other materials have failed. For instance, the ability to withstand energetic ions, oxygen fluxes and adsorbates would make the cathodes attractive and functional to be used as electron injection devices in plasma reactors and electron–cyclotronresonance ion sources. For that kind of application, the mechanical, thermal and chemical characteristics of cold cathodes based on CNTs grown inside an insulating AAO matrix is very promising. However, the electron injection efficiency, the mechanical strength and chemical inertness of these matrices to be used as electron injection devices in particular plasma environments need to be investigated. As regards the operation in space environment, CNT-based cathodes would be the enabling technology for lowpower propulsion thrusters, electrodynamics tethers for propulsion in low-Earth orbit and charge control of spacecraft solar arrays. The recent research trends show that a series of near term technological challenges should overcome to engineer macroscopic emitting surfaces with characteristics required for the envisaged advanced applications. The most critical development issue to be addressed is a reproducible batch-type manufacturing of large-area cathodes with controlled density, height and size distribution of the emitters on the device surface. An interesting approach is to fabricate the source as an ordered and uniform array of emitters by growing the carbon nanotubes within the nanopores of a template. To this scope we have chosen nanoporous alumina as a template material for ordered nanotubes production. The alumina matrix is well suited for the growth of nanotube architectures [4–6], whereas the dissolution of the underlying alumina template can enable integration of the produced carbon nanostructures into a variety of electronic devices [7]. The samples used in this study consist of multi-walled CNTs ordered arrays embedded in porous alumina membranes with a regular honeycomb pore distribution. The field emission properties of such samples, both as-grown and after a partial removal of the alumina matrix, have been investigated. In view of the fact that the presence of adsorbates and their release kinetics are a real concern for the emission experiments, specific measurements have been also performed, aiming at probing the effects induced on the emission by conditioning treatments. In particular, the adopted measurement methodology and the regular 3D architecture allowed the evaluation of the current emitted by the single emitter (the nanotube inside a single pore). Overall, the characterization methodology presented in the paper is a viable solution to check if the prepared material is functionally suitable for the realization of robust and efficient CNT-based field emission devices and electron sources.
2. Experimental In order to obtain a highly regular nanopore array in anodized aluminium oxide (AAO), a two-step anodization process of Al foils with a technique similar to the one described in [8–9] was performed.
Experiments were performed on 99.999% pure Al foils (by Goodfellow) with 2.5 2.5 cm2 area and 100 mm thickness, previously degreased and electro-polished in order to remove defects and improve their planarity. Anodization was obtained in an electrolytic cell with 0.3 M (COOH)2 solutions at an applied voltage of 40 V. The resulting pores have average diameter of 40 nm and inter-pore distance of about 100 nm. At the process temperature of about 5 1C, the AAO excavation rate was about 3 mm/h, and a depth of about 45 mm was obtained in 15 h. To the purpose of electrically contacting the CNTs embedded in AAO, the oxide barrier layer at the bottom of the AAO pores has been removed by the following steps: a pore-widening step (in H3PO4, at 30 1C, for about 40 min) which enlarges the pore diameter up to 50 nm and reduces the barrier thickness; the residual Al layer removal (in CuCl2) on the sample backside; the pores bottom opening by chemical etching (in H3PO4 at 30 1C, for about 50 min) to obtain the AAO membrane. An electrically conductive uniform pad on the backside of the AAO membrane was obtained by thermal evaporation of 100-nmthick Cr layer followed by 150-nm-thick Au layer. This electrical contact also allows to deposit at the bottom of the pores a small quantity of cobalt, which acts as a catalyst for the subsequent synthesis of carbon nanotubes. Cobalt seeds have been DC electrodeposited from CoSO4 solution. The process duration enables producing the ordered arrays of Co nanoparticles with controlled height. The CVD synthesis of carbon nanotubes was obtained by the following multi-step process: samples loading in a quartz furnace with hot walls at atmospheric pressure, a 100 min controlled warming in H2 flow up to the synthesis temperature of 700 1C, a 60 min annealing in H2 flow (150 sccm) at constant temperature and a 30 min deposition step with a mixture of 15% C2H2 in N2 carrier gas (170 sccm total mass flow) and a final cooling performed in Ar flow. After the CNTs deposition process, an amorphous carbon layer covering the Al2O3 top surface was found. In order to eliminate possible shorts between CNTs, the carbon layer has been removed by O2 annealing at 420 1C in 2 h. Then, for each sample, a welldefined portion of the AAO template was partly dissolved by 4 h etching in 0.3 M H3PO4 at 23 1C. The CNT array inside the AAO template has been observed by field emission scanning electron microscopy (FE-SEM, LEO 1530) and characterized by micro-Raman spectrometry (Renishaw 1000 spectrometer, He–Ne laser at 632.8 nm, maximum output power of 15 mW) both after annealing in O2 and after the complete etching of alumina template. FE measurements have been carried out in a high-vacuum chamber provided with a rotative-turbomolecular pumping system. The sample has been moved along three axes with a micrometric translator. In order to measure the FE properties in different small and localised areas of the sample surface and to avoid the negative effects induced by an imperfect probe orientation in the plane-toplane geometry, the employed anode is a molybdenum sphere with a diameter of 1.5270.01 mm, mounted on a 30-mm-long rod of stainless steel, with a diameter of about 1.1 mm. The distance between the anode and the flat cathode (sample) and the position in the (x,y) plane are PC-controlled by a linear precision actuator (DC-Mike Actuator M-222.23, PI) with a minimum incremental step of 0.1 mm and two linear standard translators with a minimum incremental step of 1 mm. The diode is fed by 2 kV–3 mA generator via a protective ballast resistor of 100 M. The effective anode–cathode distance is determined via the measurement of the anode–cathode capacitance and the current is measured by a Keithley 6485 picoammeter with rms noise of about 1 pA in the lowest current range under measurement
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conditions [10]. FE measurements at room temperature and in a pressure range 2.6–3.5 10 7 mbar have been performed.
3. Results and discussion Fig. 1 shows typical FE-SEM images obtained from the anodized alumina template, revealing a regular honeycomb-like structure with pore diameters and pitches of about 50 and 100 nm, respectively. The FE-SEM analysis confirmed that the adopted preparation methodology is a viable route to reliably achieve a uniform 3D architecture of vertically aligned parallel nanopores in alumina templates. The typical morphology obtained after the CNT deposition and the O2 annealing step is reported in Fig. 2a. The nanotube top ends are aligned with alumina surface, while the nanotube density is uniform depending only on the template pitch. Here the nanotube diameter is in the range 40–50 nm and the interdistance is about 100 nm. Fig. 2b and c show the effects of H3PO4 treatment on such architecture to partially remove the AAO template. The nanotube diameter appears again in the range 40–50 nm but the tips of nanotubes exceed the ceramics level by some nanometers. Their apexes are open, so the tops of nanotubes are the rims of carbon cylinders. The first-order Raman spectrum of the as-grown sample (before the AAO etching) shows D (at about 1340 cm 1) and G (at about 1580 cm 1) peaks partially masked by a broad shoulder, associated to the presence of the embedding alumina template, which makes difficult to evaluate the quality of the CNTs inside the AAO pores. On the contrary, the complete removal of the alumina template gives the possibility to detect better defined Raman signals from the cylindrical carbon deposits, released from the original ceramic skeleton. The features of the Raman spectrum (Fig. 3) taken after the AAO removal, i.e. the relatively high intensity of the D peak, the intensity ratio of the D to G peaks (ID/IG = 1.36) and the presence of a low intensity second-order D peak (G0 ) suggest that the carbon structures inside the AAO pores are defective nanotubes. A poor graphitization also explains the fact that the nanotube top ends, after the O2 annealing, become approximately aligned with the alumina surface. Indeed, the O2 atmosphere has etched the extruding tubules while the embedded ones were partially protected by the alumina template.
Fig. 2. FE-SEM image of CNT arrays in AAO templates: As obtained at the end of CVD growth; (b–c) top (b) and inclined view (c) after partial removal of the AAO matrix by chemical treatments.
Fig. 1. FE-SEM side view and top view (inset) of a typical AAO template used for the CNT synthesis.
The electron emission properties of CNTs on alumina template samples, before and after the partial AAO etching, have been measured, being their morphologies reported in Fig. 2a–c, respectively, and thereafter referred to as sample of type I and sample of type II. FE measurements have been carried out on different areas of each sample. Each area of the investigated samples has been analysed by the protocol described below, according to experimental methodology fully described in [10]. No significant differences have been revealed analysing different areas of the same type samples. As a consequence, in the following, we report and discuss the FE results corresponding to a typical area for each type of sample.
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and Eq. (1) can be written as dA2 2 A2 JðEÞ ¼ A1 E exp 2pR dE
ð3Þ
where d is the anode–cathode distance and R is the sphere radius. Parameters A1 and A2 are extracted by fitting the experimental I–V curves with Eq. (2). From equations reported above the following expression of the enhancement factor is derived pffiffiffiffiffiffiffiffiffi 3=2 8pd 2me f b¼ ð4Þ 3eh A2
Fig. 3. Raman spectrum taken after complete removal of the AAO template, with indication of the intensity ratios of D to G, D to G0 and G to G0 peaks, respectively.
The field emission behaviour of CNT-based materials is strongly influenced by the presence of adsorbates, which are bonded by different surface energies depending on their chemical state. It is well known that the adsorbates modify both the measured emitted current and the threshold fields. Therefore they hamper the real field emission properties evaluation. As a consequence, a key factor in the FE measurements is represented by setting up the proper conditioning procedures able to reduce the amount of surface-adsorbed chemical species substantially. To achieve the aim, the samples are firstly conditioned by applying different constant voltages between anode and cathode. When the measured I–V curves become fairly repeatable and the emission current noise is reduced, a higher constant voltage is applied up to the emission of a current in the range of few microamperes. Under such conditions the local heating also contributes to eliminate the more energetically bonded chemical species. Fig. 4 shows the typical behaviour of the measured current under applied voltage of 2000 V during the conditioning process. The emitted current is noisy (especially at the beginning of the conditioning process) and shows steps and ramps. The conditioning process is considered as terminated when in a temporal window of 180 s the emitted current is rather constant with a relative variation less than 1%, like in the last segment of the curve in Fig. 4. The field emission behaviour is described by the well-known Fowler–Nordheim law [3] " pffiffiffiffiffiffiffiffiffi # 2 3=2 e3 b E2 4 2me f ð1Þ exp JðEÞ ¼ 3e_ bE 8ph f where e and me are the charge and mass of the electron, respectively, h is Planck’s constant, E the applied electric field, f the work function and b the electric field enhancement factor, defined as the ratio of the local field to the applied field. Considering that in our setup the anode–cathode system can be modelled as a sphere-to-plane geometry, the following relationship between the total emitted current I and the anode– cathode voltage V has been found [10]: 2V A2 exp ð2Þ IðVÞ ¼ A1 V 3 1 V A2
d and f are unknown values, but d can be indirectly determined using the procedure described below. The distance d has been determined by a non-contact method in order to avoid any damage of the material surface. This method is described in details elsewhere [10], and it is based on the assumption that the anode and a sample surface can be modelled as a sphere and a plane, respectively. The relationship between the capacitance C of a sphere/plane capacitor and the distance d between the two conductors [11] is d ð5Þ CðdÞ ffi 2peR kln R where R is the sphere radius, e is the medium permittivity and k is a constant which takes into account the roughness of a conductor surface. A linear actuator with a 0.056 mm accuracy yields the quote z of the probe (the sphere), but the quote o of the material surface (the plane) and hence the anode–cathode distance d= z o are unknown. According to the procedure described in [10], the distance d= z o can be evaluated by fitting the experimental C(z) curve reported in Fig. 5. In this way the relative error of b is given by @b Db 1 @b Dd þ @b DA2 ¼ 3 Df þ Dd þ DA2 ¼ Df þ ð6Þ @d @A b @f A2 2 f d b 2 From literature data, the CNTs work function ranges [12–14] from 4.51 to 5.05 eV, corresponding to a mean value of 4.7870.27 eV with a relative error of 5.6% on f, while the typical error on d is about 1–2%. The maximum propagated error on the b factor value is about 17%. The FE measurements performed in a typical area for the two samples types are reported in Fig. 6: the FE properties of the asgrown sample (type I), shown in Fig. 6a–d, are compared to those of a partially etched sample (type II), shown in Fig. 6e–h. The measurements of type I and type II samples have been performed with an anode–cathode distance, determined according to the above described procedure, d =39 and 51 mm, respectively. An apparent peculiarity of the reported field emission measurements is the presence of two slopes in the Fowler– Nordheim (FN) plots. The standard Fowler–Nordheim model has been developed for metals and in this case a single potential barrier has been considered for the electron tunnelling towards the vacuum. It establishes a linear dependence of ln (J/E2) on inverse electric field (1/E). In spite of it, the field emission measurements on CNTs showing two slopes in the corresponding FN plots are discussed by many scientific articles. This behaviour cannot be attributed to the space charge effect due to the low current densities. But it has been ascribed to the presence of two potential barriers, instead of one as in metals [15–16]. The additional barrier can be produced by an adsorbates layer on the surface [17], but it should disappear after a total desorbing process (which is only partially achievable at UHV conditions)
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Fig. 4. Current vs. time, I(t), taken at V = 2000 V during the final step of the conditioning process (curve on the left) and steadily emitted from the conditioned sample (curve on the right)
b match very well the values obtained by fitting the FN plots at
Fig. 5. Capacitance C versus quote z. The cathode surface quote o extracted from the fit is reported. The anode–cathode distance is d =z o. r is the correlation coefficient between the z and C data arrays and it gives a measure of goodness of the data fit with the given formula C(z).
[18]. In this way, it is possible to obtain FN plots with a single slope from metallic surfaces. Another barrier could be present at the interface between the nanotube and the substrate, where an oxide or carbide (depending on the substrate material) interlayer is formed during the nanotubes growing process [15–16]. A ‘‘real’’ field emission is achieved after a conditioning process, in order to reduce the adsorbates on the emitting surface, and only at high electric fields [15–16]. Therefore, the enhancement b factor has to be extracted only on this part of the FN plot. At low electric fields the electron emission is influenced by the presence of the second potential barrier and it is no more described by the Fowler–Nordheim model. The b extracted in this range is no more the electric field enhancement factor on the material surface and it loses any physical meaning. As described above, b is extracted by fitting the whole I(V) experimental curve. These values match the b calculated at high electric fields very well. In this way, an arbitrary choice of the data used in FN plot fitting for b evaluation will be avoided. By fitting the whole I(V) curves, at low fields and much lower currents the measurements have little weight, because of the used algorithm (w2 minimization with uniform weights) and the fitted
high fields. Fig. 6a and e show the experimental I–V (the total emitted current I vs. the anode–cathode voltage V) curves. The reported curves (A–D) correspond to a temporal sequence of measurements carried out at the end of the conditioning procedure, performed once before beginning the measurements sequence itself. Fig. 6b and f show the related I–E curves of the total emitted current I vs. applied macroscopic electric field E= V/d with d anode–cathode distance. The insets highlight, for each type of sample, a value range of electric field threshold Eth corresponding to that required to have an emission current of 1 nA. The J–E curves represent the emitted current density J vs. the applied macroscopic electric field E (Fig. 6c and g), and the related insets show the FN plots derived from two sets of the measured I–V curves. Finally, Fig. 6d and h display the values of the field enhancement b factor, i.e. a purely geometrical factor related to the emitter surface geometry, which is derived from I–V curves of Fig. 6a and e by the method described above. The plots reported in Fig. 6c and g insets show a good agreement with the Fowler–Nordheim model in both samples types. The acid treated samples (type II) show current densities one order of magnitude smaller and threshold electric fields larger than the as-grown ones (type I), highlighting a worse emission efficiency, probably due to the nanotubes damage caused by the acid. Fig. 6d and h show the behaviour of b values corresponding to the different sequential measurements; the dependence of b on time deserves some comments. In the case of as-grown samples, b values show an increasing trend, as also summarised in Table 1, which can be explained on the base of two possible physical reasons, reported as follows. The first one is that the surface morphology undergoes modifications during the measurements. In this case a viable hypothesis is the nanotubes movement induced by applied electric field [19–20]. In our case the field is expected to induce a stretching of nanotubes out of the alumina nanopores. The second one is intrinsically connected to the necessity of conditioning procedures before the experimental measurements for b evaluation. As described above, the b values depend on the work function f. In our experimental data analysis, we used the typical values of f reported in the nanotubes literature, but it is well known [18,21–24] that adsorbates presence can influence
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Fig. 6. Comparison between the main field emission characteristics of the as-grown (on the left, a–d) and the partially etched (on the right, e–h) samples. The reported curves (A–D) correspond to a sequence of measurement runs, carried out after the conditioning procedure. (a, e) experimental I–V (the total emitted current I vs. the anode– cathode voltage V) curves; (b, f) I–E curves (the total emitted current I vs. the applied macroscopic electric field E =V/d, where d is the anode–cathode distance); the insets highlight the electric field thresholds for a total emitted current of 1 nA; (c, g) J–E (the emitted current density J vs. the applied macroscopic electric field E) curves. Insets: related Fowler–Nordheim plots; (d, h) values of the enhancement b factor.
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Table 1 Values derived from measurements on the as-grown sample.
b Eth (V/mm) Ec (V/mm)
A
B
C
D
402 15.8 42.3
273 18.5 62
355 19.0 47.9
668 17.2 25.5
such values. As a consequence, the determined b values could be underestimated in comparison to the real b value that is, merely, a geometrical factor. The adsorbates desorption during measurements could change the work function and determine an apparent increase in the b value. The conditioning procedure we followed let us reasonably exclude a predominant effect of adsorbates determining increase in the b value. The possibility of ballistic emission has been also considered. The critical field Ec required for ballistic emission has been calculated for the different measurements (A–D), according to the model proposed by Forbes [25]. The calculated Ec values are reported in Table 1 and compared with the Eth values, remaining practically unchanged in our experimental conditions. On the basis of the reported data, the presence of a significant and/or predominant ballistic emission can be reasonably excluded. Even if a strict conclusion about the physical origin of the b increase cannot be drawn at this stage, from a phenomenological point of view there are no doubts that the measurements indicate a real increase in the b parameter for the as-grown sample. Up to now the FE model used to analyse our data, established in [10], has been applied in the case of continuous CNT deposits and has demonstrated its feasibility to determine the emission current density of the cathodes with a good accuracy. The ordered structure of cathodes assembled in the present experiments enables a further advance, consisting in application of the ‘‘continuous’’ model to the emission phenomenon from discrete CNT emitters. The samples morphology makes reasonable the hypothesis that the emission centres are placed in the pores at the centre of the template honeycomb cells (Fig. 7). The total emitted current is the sum of the contributes of each single emitter, I= SkIk. The kth cell contribute Ik to I depends on the position rk of the emission centre on the sample surface with respect to the projection of the spherical probe centre on it, and on its emission characteristics (the work function fk and the enhancement bk factor). Due to the small dimensions of a single emitter, its contribution can be expressed as Ik = AkJk, where Jk is the emission current density J(rk, fk, bk) calculated at the centre of the cell and Ak is its effective emitting area. Assuming fk = f = 4.7870.27 eV (the median value of carbon nanotubes work function), bk = b as derived from measurements and Ak =A, the mean effective emitting area, the expression of the total emitted current is given by X X I¼ AJ½bEðrk ; V; dÞ; f ¼ A J½bEðrk ; V; dÞ; f ð7Þ k
k
where E(r,V,d) is the electric field on the material surface at the position r when an anode–cathode voltage V is applied and the anode–cathode distance is d. P The term k J½bEðrk ; V; dÞ; f is calculated by truncating the sum when the contribute of the cells bounding the considered area (white cells in Fig. 7) represents a given percentage (0.01%) of the maximum contribution (darkest cell in Fig. 7, considered beneath as probe centre) and the sum is about saturated. Taking into account the ratio between the pore and the cell areas, the area Atot of the surface considered in this sum is compatible with the areas Acont calculated in the framework of the ‘‘continuous’’ model. It supposes electron emission from the whole surface.
Fig. 7. Schematic drawing of the area considered in the calculation of the total emitted current. Different grey-level areas correspond to different steps in the summation. The white cells contribution is too small and the sum is stopped at the corresponding step.
Imposing I equal to the measured current Im, it is possible to calculate the mean effective emission area A¼
Im N P
ð8Þ
J½bEðrk ; V; dÞ; f
k¼0
Of course this value is underestimated, because not all the cells contain the emitter (some pores are empty). However, the values found for A are compatible with the section of the tubes inside the pores. Using this value it is possible to estimate the current I emitted, at a given voltage, from a single emitter, i.e. a single nanotube inside a pore. The evaluated current values are also underestimated, for the same reasons reported above for the value A. For the as-grown sample (type I) we determined a value I43 pA for the single emitter. It is fully compatible with the current density values found with the continuous model. In case of the etched sample (type II) the emission process shows remarkable differences in comparison to the case of asgrown sample. Assuming a threshold current of 1 nA, the threshold field is in the range 19.5–22.4 V/mm and shows a trend of field increasing with time. These field values are higher in comparison to those found for the as-grown samples. The values of both emission current (Fig. 6e) and calculated current density (Fig. 6g) are considerably lower in comparison to those measured for the as-grown samples (Figs. 6a and 6c, respectively). The evaluation of b values (Fig. 6h) reveals a decreasing trend vs. time. From the point of view of field emission behaviour the H3PO4 treatment effect seems to be negative. This could be ascribed to the physical properties degradation of the nanotubes segments protruding from nanopores representing the emission current source. In this context, the b decrease can be explained taking into account a possible emitting centres degradation. However, considering the applied fields values the currents emitted from the exposed CNT arrays are still advantageous over cold cathodes assembled using other materials.
ARTICLE IN PRESS 1476
R. Angelucci et al. / Physica E 42 (2010) 1469–1476
4. Conclusions In this paper some open questions about the fabrication of efficient CNT-based cold cathodes have been addressed. The electron sources have been realized using porous alumina templates for the growth of arrays of vertically ordered nanotube deposits. The employed anodization methodology enables to produce nanopatterned alumina templates and to control diameter, density and height of the emitters (where pore dimensions, that is diameter and deepness, and pitches can be varied). On the other hand, the coupling of alumina with CNTs assures robustness, chemical inertness and feasibility of operation even in harsh environments for the emitting devices. Field emission testing has been performed, using the same sphere-to-plane diode configuration, both on the as-produced cathodes and on nanotube systems protruding from the pores after partial removal of the underlying Al2O3 templates by acid etching. The experimental data, obtained with the capacitance technique for measuring the anode–cathode distance with the accuracy of few microns, have been analysed in the frame of the Fowler–Nordheim model. The general trends exhibited by the main parameters of the field emission process, i.e. the threshold field, the enhancement factor and the current density at different fields, have been found to agree with the emission features expected on the base of the physics involved. In particular, quite high values of current density (up to some tens of mA/cm2) have been obtained at relatively low applied fields for the as-grown samples. The obtained current density is not far from the value considered by some authors as a realistic challenge [26–27]. It has been also possible to give an evaluation of the current emitted from a single nanoemitter (i.e. a CNT inside the pore). The reported value of I =3 pA represents an underestimation of the real value considering that the model is based on the assumption that all the pores have been filled with an emitting nanotube. On the contrary, only part of the pores is effectively filled with nanotubes and, therefore, the experimentally measured current is due to a number of nanotubes lower than those considered in the model. The reported value is the minimum physically possible assuming that all the nanopores are filled. In our case, the fraction of filled holes has been estimated to be typically 490%. On the basis of the present results, the cold cathodes assembled by growing CNTs into the alumina scaffolds meet the requirements to be used as efficient electron sources. With the aim of fabricating electron injection devices for application in plasma reactors and electron–cyclotron-resonance ion sources, the robustness and chemical inertness of those cold cathodes have been also investigated. Preliminary experiments have demonstrated both a good electron injection efficiency and robustness of the CNTs arrays inside AAO templates when inserted in plasma environments. On the other end, for the partially etched samples a worsening of the emission has been detected. With an applied electric field of about 30 V/mm, the current density is about 10 mA/cm2. These
samples seem therefore less suitable for the use as cold cathodes. Nevertheless, the results of the experiments of alumina dissolution are considered very important for two different reasons. First of all, it has been demonstrated that the ordered symmetry of the whole deposit is preserved and that the structural properties of the nanotubes are only partially affected by the treatment, as seen from the Raman analysis and from the emission features. Secondly, the reduction of the emission from ordered nanotube samples may be of interest when dealing with micro-nano systems requiring an arrangement of highly ordered CNTs along the Z direction for efficient transport of both heat and electrical charges, but at the same time low electron emissions. In this context, self-standing arrays of low-emitting CNT protruding from a thin alumina support contacted at the bottom with a metallic layer (in our case Cr/Au) can be viewed as multifunctional systems and potentially useful for interconnections, nanocontacts or flip-chip architectures.
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