Synthesis of Aligned Carbon Nanofibers on Electrochemically

Jun 3, 2008 - Aligned carbon nanofibers (CNFs) with a population density of about 3 ... of silicon surface can increase the population density of ener...
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J. Phys. Chem. C 2008, 112, 9247–9252

9247

Synthesis of Aligned Carbon Nanofibers on Electrochemically Preroughened Silicon Q. Wang,* S. T. Ren,* and W. J. Liu Department of Optics and Electronics Science, Harbin Institute of Technology at WeiHai, Weihai 264209, China ReceiVed: March 7, 2008; ReVised Manuscript ReceiVed: April 15, 2008

Aligned carbon nanofibers (CNFs) with a population density of about 3 × 108 cm-2 are synthesized on electrochemically roughened silicon using the bias-assisted chemical vapor deposition system. Isolated CNFs with much lower population density (∼3 × 106 cm-2) are formed on polished silicon wafer at the same growth conditions. The morphologies, microstructures, and components of the CNFs are accordingly characterized using scanning electron microscopy, Raman spectroscopy, transmission electron microscopy, and energy dispersive analysis of X-rays. On the basis of the surface barrier limited diffusion model, it is shown that electrochemical roughening of silicon surface can increase the population density of energetic carbon species that act as self-catalysts for the vertical growth of CNFs. The formation of CNFs with coneshaped tips simultaneously involves the vertical growth and a continuous thinning of fiber tips by the bombarding of CH4+ and H+ ions in CH4/H2 plasma. 1. Introduction nanotubes,1

many other new Since the discovery of carbon low-dimensional carbon materials have been synthesized using the chemical vapor deposition (CVD) system, including carbon nanocones,2 carbon nanofibers (CNFs),3,4 carbon nanorods,5 tubular graphite, or carbon cones.6,7 Because of their high aspect ratio and hollow interior structure, they have wide applications in high-sensitivity chemical sensors,8,9 surface-enhancement Raman spectroscopy devices and photonic functionalities,10,11 viable alternatives to silicon-based microelectronic devices and circuitry,12 electron field microemitters,13 nanoindenter,14 advanced probing biosensors in protein and cell detection,15 controllable drug delivery for gene therapy,16,17 etc. Uniform formation of nanostructure arrays with controllable morphologies (e.g., population density, orientation, size, etc.) using conventional CVD still proves to be a major challenge for the application of carbon nanostructures into nanoelectronics, due to the technical difficulty of sufficiently controlling the flux distribution of reactive species. The existing techniques, such as plasma-enhanced CVD, show promising prospects in achieving this requirement by intentional tuning of plasma parameters. To date, plasma-aided growth of carbon nanostructures has been repeatedly shown in experiments to be superior in controlling of vertical alignment, location site ordering, and shapes of asformed carbon nanomaterials,18–23 especially the pioneering work of K. Ostrikov et. al.24–26 Some important, but relatively unknown, topics associated with the complicated ions conducts—from ionization, ions-surface interactions, surface diffusion of energetic adsorbed species, to growth and etching—also need further investigation. It is easy to predict that prepatterning of the cathode surface with mesoscopic roughening will introduce local electric field modifications, which will present another alternative control of the location site of as-grown nanostructures, but little experimental evidence and physical interpretation of this case are currently available. * Corresponding author. Phone: +86-631-5687349; fax: +86-6315687036; e-mail: [email protected] or [email protected].

In this paper, aligned synthesis of CNFs with controllable population density is presented. The formation process of these carbon nanostructures is intensively studied therein to show the key role of prepatterning for controlling the density of CNFs. This knowledge will shed new light on the manageable formation of functional nanostructures. Prominent advantages of this method include as-grown CNFs that are ready for use without the need for further nanoscaled assembly, and no conventional preseeding of catalyst particles for density controlling of as-grown carbon nanostructures is needed. 2. Experimental Section N-type silicon (100) wafers with resistivity of 6-9 Ω cm were ultrasonically cleaned with acetone and deionized water successively. Some of them were chemically etched using a typical Teflon cell containing hydrofluoric acid (48% aqueous solution) and ethanol with a volume ratio of 2:1 to produce roughened silicon (or etched silicon) substrate. The etching current density was 60 mA/cm2, and the etching duration was 30 s. As-etched silicon substrates were mounted on the holder and then put into a conventional hot filament CVD (HFCVD) system. Before the deposition of CNFs, the chamber was first pumped to a base pressure of 10-2 Torr, and then H2/CH4 with a flow rate of 90/10 sccm was introduced into the chamber. The total gas pressure was kept at 30 Torr throughout the growth of CNFs. The growth was kept for 10-120 min at the substrate temperature of 750 °C. During the growth, the negative bias of about -300 V was applied, with a discharge current density of 50 mA/cm2. High-resolution scanning electron microscopy (SEM) images of the samples were obtained using a FEI DB235 focused ion beam (FIB) milling system. The ions-milling of one as-formed fiber for the cross-section SEM characterization was also conducted using this FIB system. The roughness of our roughened samples was characterized using the Dektak-8 surface profiler system. The microstructure of a dispersed CNF was characterized using a Hitachi HF-2000 transmission electron microscopy (TEM) system equipped with the energy dispersive analysis of X-rays (EDAX) system. JY-T64000 three grades

10.1021/jp802044e CCC: $40.75  2008 American Chemical Society Published on Web 06/03/2008

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Wang et al.

Figure 2. Raman scattering spectra taken from different CNF arrays on (a) polished and (b) roughened silicon surfaces.

Figure 1. (a) SEM images of an electrochemically roughened silicon surface, (b) CNFs grown on polished silicon (the inset is the magnified image of the conical tip), (c) CNFs grown on roughened silicon (the inset is the top-view image of as-formed CNFs), and (d) cross-section of one CNF cut by FIB milling (the inset is the image of the CNF before FIB milling).

Raman spectroscopy is employed to characterize the phase purity for the as-deposited CNFs. 3. Results and Discussion 3.1. The general Characterizations of As-formed CNFs. Figure 1 shows the SEM images of as-deposited CNFs on both polished and chemically roughened silicon. All SEM images shown in Figure 1 are taken when the sample holders are tilted forward 52°. Figure 1a is the SEM image of a chemically roughened silicon wafer, where we can see the average horizontal diameter of as-etched pits is about 400 nm, which indicates the pit distribution density is about 2 × 108cm-2. The root-mean-square roughness (R), is defined as[∑(Zi - Zave)2/ N ]1/2, where Zave is the average Z height values of points within the selected measurement region, Zi is the specific value of Z height at certain point, and N is the number of measured spots. The measured R of our roughened samples is about 50 nm. As shown in Figure 1b, the CNFs grown on polished silicon have a very low population density of about 3 × 106 cm-2. The CNFs have an average base diameter of about 300 nm; a head curvature radius (r) of less than 20 nm, as shown in the inset image; and show an average height (h) of 4 µm, which indicates a high aspect ratio (h/r) of about 200. Around the grown-up CNFs, there are many undeveloped tips with much lower height (similar phenomenon has been studied by Tsakadze et al.,27 and it has been termed as secondary growth). Figure 1c shows the CNFs with a much higher density of about 3 × 108 /cm2 on chemically roughened silicon substrate. They show similar height and larger tip radius of about 50 nm, in comparison with those shown in Figure 1b. We should note the population density of CNFs is rather similar to that of an as-etched pit on a silicon substrate. Figure 1d shows the SEM image of one nanofiber on polished silicon, which had intentionally been cut top-down by FIB milling, and we can see that nanofiber has a solid interior. In Figure 2, panels a and b, the Raman spectra of the samples are presented to study the chemical bonding states of asdeposited CNFs on polished and roughened silicon. When the spectrum shown in Figure 2a is Lorentzian fitted, we can obtain

three peaks that can be well-merged into the original spectrum. The peak centered at 1335-1340 cm-1 is characteristic for the A1g D breathing mode of sp3 diamond-like carbon or disordered sp3 carbon.25 The peak centered at 1450 cm-1 is assigned to amorphous carbon.28 The E2g G mode at about 1596-1601 cm-1 shows the existence of an sp2 carbon cluster.29 Figure 2b is the fitted Raman spectrum of nanofibers on a roughened silicon substrate. There are two peaks at about 1347 (disordered carbon) and 1600-1605 cm-1 (sp2 carbon cluster), respectively. We can conclude that there are three kinds of carbon bonding states in the as-formed CNFs, namely, sp2 graphitic carbon, sp3 diamondlike carbon, and amorphous carbon. At negative substrate bias, it was previously shown that the carbon film could be changed from a polymer-like structure, through a diamond-like one, to a graphite-like structure due to the enhanced ion bombardment.30 So the high sp2 content in our cases can be assigned to the effect of ions bombardment. The microstructures of as-formed CNFs are studied using TEM and EDAX characterizations. As shown in Figure 3a, an as-formed CNF with a diameter of about 15 nm at the tip region shows typical amorphous structure. The EDAX spectrum of the CNF is shown in Figure 3b, where we can see C, O, and Si atom components are 87.6, 3.1, and 9.3%, respectively. It further verifies that the nanofiber is mainly composed of carbon, as shown in the Raman characterization. In the following, we will theoretically reproduce the surface roughing induced density difference of as-formed CNF arrays. It is organized as follows: In Section 3.2, the kinetics process of ions generation in a direct current (DC) bias-assisted CVD system will be discussed. In Section 3.3, local electric field modification introduced by surface roughing is analyzed to establish the surface barrier limited diffusion model, which can well explain the experimental results. In Section 3.4, the etching effect of striking ions for the conical tip structure of assynthesized CNFs is studied. 3.2. Kinetic Modeling of Ions Generation in a Plasma Sheath. In a DC bias-assisted CVD system, hydrogen and hydrocarbon ions are generated from electron impact with CH4 and H2 molecules in the plasma sheath, due to inelastic collisions that cause ionization and dissociation of CH4 and H2 molecules into CH4+, CH3+, CH2+, CH+, C+, H2+, H+, and other uncharged species (neutrals).31 The temperature of electrons Te in H2/CH4 plasma similar to our case is about (1.0-1.1) × 105 K, that is, the energy of electrons is 13-14.2 eV, as suggested by Wang et al.32 The first ionization energy of CH4 and H2 are

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J. Phys. Chem. C, Vol. 112, No. 25, 2008 9249 variant expression (eq 3),

F(E, r) ) f(E)g(r)

(3)

where f(E) is the energy distribution section, and g(r) is the spatial distribution function. In the full energy distribution j give the dominant spectrum, ions with average energy E contribution to the following surface diffusion and etching, j , r)is considered. Taking the average ion therefore only F(E energy as that acquired during the last free path after a charge transfer collision, it yields the following equation (eq 4),

∫0V f(E)E dE 5λ E) V ≈ eVc ∫0 f(E) dE 3L c

c

(L . λ)

(4)

where the ions mean free path (λ) is given by eq 5,36

λ ) 1 ⁄ nnσ and the neutral density (nn) follows eq

(5) 6,37

nn ) p ⁄ kBT

Figure 3. (a) High resolution TEM image and (b) EDAX spectrum of one CNF.

12.7 and 13.6 eV, respectively.33 CH3+ ions can only be formed by electron impact with energy above 25.7 eV,34 which is higher than the electron kinetic energy of 13.6 eV. So in the following discussion we neglect the other ionic species except CH4+ and H2+. After the generation of ions, electrons, and neutrals by ionization, as-produced ions are accelerated in the near cathode sheath region to strike the cathode surface. It is easy to deduce that the surface electric field distribution is intensively related with the surface roughness that can induce ununiform local field distribution, and therefore this surface roughing can certainly affect the spatial and energy distribution of striking ions. The ions are accelerated as they traverse the plasma sheath and can be deflected by microscopic electric fields of the protrusions in their immediate proximity. To make a physical interpretation of the formation mechanism of as-formed CNFs, we use here a modified expression of an ions-neutral collision model (first proposed by Davis and Vanderslice35) to obtain an ideal ions energy and spatial distribution function F(E, r) at the cathode surface with protuberances and concavities. The energy distribution of as-generated ions at a flat cathode surface follows eq 1,35

f(E) )

(

3L E 15λ eVc

)

-2 ⁄ 5

[ ( (

exp -

L E 1- 1λ eVc

) )] 3⁄5

(1)

where E is the ion energy, e is the elementary charge, L is the sheath thickness, λ is the ion mean free path, and Vc is the applied voltage (when the glow discharge occurs, it is followed by a small collapse). In all of the following equations, the relation L . λ is satisfied, so eq 1 can be reduced to eq 2.

f(E) ≈

[

3L 3L E exp 5λ 5λ eVc

]

(E , eVc)

(2)

If the spatial distribution of as-generated ions is considered, distribution function F(E,r) will reasonably follow the separated

(6)

where p is the gas pressure, kB is Boltzmans constant, σ is the inelastic collision cross-section of ions and neutrals [in CH4 (10%)-H2 (90%) plasma, the neutrals are mainly H2 molecules], and T is the absolute temperature. In our case, nn can be calculated to be about 9 × 1022 m-1.3 The ionization ratio is fairly small with the value of ∼10-7,38 that is, the plasma density np is about 9 × 1015 m-3. All ions will be accelerated in the sheath and then strike the cathode surface, whereas all neutrals will gain much lower energy when they come to the cathode surface.39 The generation of CH4+ is the most prolific process compared to other ionic species, because it possesses the highest electron-impact ionization cross-sectionσ(CH+ x ,e ). The inelastic collision cross-section σ(H2+,H2)of H2+ ions with H2 neutrals is -19 m2.40 6 × 10-20 m2,36 and the value of σ(CH+ 4 ,H2) is 4.5 × 10 By combining eqs 5 and 6, the mean free path (λ) for H+ and CH4+ ions can be calculated to be 1.8 × 10-4 and 2.5 × 10-5 m, based on our present experimental results. The sheath thickness (L), depending on the bias voltage and plasma density and being the main factor in determining the microscopic topology of the ion flux, can be expressed as the following equation:41

L)

V30

⁄ 4

[(9j ⁄ 4ε0)(M ⁄ 2e)1 ⁄ 2]1 ⁄ 2

(7)

Here, j is the discharge current density (500 A/m2) that can be calculated from the experiment parameters, ε0 is the dielectric constant, e is the elementary charge, and M is the ion mass. Thus, L can be estimated as 6.37 × 10-4 and 4.3 × 10-4 m for CH4+ and H+ ions, respectively. By comparing eqs 5 and 7, the values of L/λ for H2+ and CH4+ are 3.54 and 17.2 respectively. It indicates that the preassumed relation L . λ is j of H2+ well-satisfied. Then, using eq 4, the average energies E + and CH4 ions striking the cathode are 141 and 29 eV, respectively. It should be pointed out that the mean ions energy j is negatively proportional to the gas pressure p, as can be E deduced from eqs 4–6. It indicates that increasing p will j , on the condition that E j is much lower significantly decrease E than Vc. The minimum energy (Emin) of CH4+ to sputter silicon atoms is about 38 eV, and the threshold energy (Eth) of H+ should reach about 271 eV for the sputtering of silicon to be realized, according to the following formula (eq 8).42

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(Mi + Ms)2 Emin ) 7.7 Wf 4MiMs

Wang et al.

(8)

Here, Mi and Ms are the mass of the active ion and silicon atom, respectively, and Emin is the minimum ion energy for substrate sputtering. Wf is the surface bond energy of substrate atoms (for silicon ∼4.7 eV). As a result, when the low-energy ions strike the silicon substrate, few silicon atoms are sputtered out. In summary of Section 3.2, the ion generation and acceleration in the plasma region are discussed, and little sputtering of the silicon substrate will occur due to the rather low-energy ions. In Section 3.3 we will establish the microscopic interaction models of the plasma-generated carbon species with substrate surface and simulate the growth kinetics of the carbon nanostructures on a roughened surface. 3.3. Surface Barrier Limited Nucleation on Prepatterned Substrate. Just as discussed above, little silicon sputtering will occur in the cathode region. The energetic CH4+ and H+ ions mainly contribute to the high immigration energy Ei of adsorbed C species on the silicon surface and to the increase of the local site temperature. It should be noted that this temperature increase will also increase the surface diffusion energy Ei (or diffusion length Dl) for as-adsorbed carbon species, as shown in the following eq 10. In this part, for describing the surface kinetic process of an ion hitting a solid and spreading through the surface, a Gaussian distribution of deposited energy that acts as the immigration driving force for certain as-adsorbed energetic carbon species (ions and neutrals) is assumed as shown in eq 9.

( )

Ei(r) ) NsE exp -

r2 R2

(9)

Here, r is a position variant around the exact hitting spot of the incoming ion, Ns ) 4/(π1/2R3) is the energy normalizing factor, j is the average kinetic energy carried by the ions, and the values E of material constant (R) describes the horizontal spreading lengths of the energy. Similar formulation had also been suggested elsewhere.43 In our case, the vertical energy spreading into the bulk is neglected, due to the rather low ions energy and, therefore, the low vertical penetration ability of as-adsorbed energetic carbon ions. Equation 10 works, only if the ions mean j is larger than the surface activation energy striking energy E Ed. The deposited energy enables carbon species to surpass the surface potential barrier Ed and to diffuse along the surface, as long as E(r,t) > Ed. So it is easy to deduce that the higher the mean ions energy the longer the diffusion length Dl. In the calculation of Dl, we have assumed that the surface diffusion activation energy is Ed ) 0.6 eV, which is the case for carbon species migration on Si (100) surface heated to a typical substrate temperature Ts ) 700 °C.41 The diffusion coefficient of active ions on the cathode surface in relation to substrate temperature is expressed as eq 10,44

( )

D(Ts) ) a2Vi exp -

Ed kTs

(10)

where Ed is the active energy of ions diffusion, Vi is the vibration frequency of ions, and a is the distance between two position of nucleation. It is clear to deduce that the higher the local substrate temperature, the higher the diffusion coefficient D(Ts). As a result, substrate heating by ions bombardment can also enhance the surface diffusion. The diffusion length Dl of energetic carbon species will be much enhanced considering the striking ions energy of several

Figure 4. Schematic of the ions distribution on rough surface driven by a local electric field.

tens of electronvolts. In the following, it will be illustrated that nonuniform surface electric field distribution can also affect the diffusion length (Dl) of adsorbed ions. Now we should first discuss the case of a planar silicon surface. Ions entering the sheath with a low velocity relative to the presheath potential drop (∼Te/2)45 begin to accelerate toward the planar substrate. Because of the random sticking of incoming ions on the planar surface, carbon nuclei will be randomly formed, hence the random growth of carbon nanostructures. As previously discussed by Jiang et al.,46 the mean isolating (or depletion) distance between neighboring nuclei can be significantly increased by energetic ions bombardment, due to the larger ions diffusion coefficient. As a result, isolated CNFs with a rather low population density of about 3 × 106 cm-2 can be synthesized on a polished silicon surface. For the case of ions striking the roughened silicon surface, it will be rather different. The negative biasing of the cathode will result in an as-established ununiform electric field due to the converged negative charging at the convex position. This dynamical equilibrium of negative charging on the convex positions will produce a different ion population density for both convex (with higher ion density) and concave (with lower ion density) positions, which is decisive in determining the population density of as-formed CNFs. It is quite different from the homogeneous ion distribution at a flat surface. To obtain a more explicit understanding of this effect, the microscopic field created at individually etched pit nanostructures with convex and concave is theoretically simulated as follows.47

b Ei )

σi dSib ri i 4πε r 0 i

∫S

(11)

Here, σi is the surface charge density, Si denotes the surface of the ith nanostructure, and b ri is the position vector. The total field at the ith nanostructure surface is b Ei. A schematic field distribution is illustrated in Figure 4. As can be seen therein, the vertical vector section of b Ei will act as the accelerating mechanism to incoming positively charged ions along the z-direction. The ions deflection is only introduced by the horizontal vector section of b Ei, whose vector direction is perpendicular to the z-direction. Consequently, the ions will be deflected toward the convex position, as also illustrated in Figure 4. From this point of view, more carbon-containing ions will swarm to the convex position driven by the surface local field. If the energy of incoming ions is high enough, selective etching will occur, when considering “down-stream” collision.48 It should also be pointed out this nonuniform distribution of electric field will produce no effect on the distribution of neutrals. As previously discussed in this paper, more ions bombarding at the convex will bring about local heating that increases the diffusion ability of carbon species. Ion bombard-

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J. Phys. Chem. C, Vol. 112, No. 25, 2008 9251

ment with energy of several tens of electronvolts can create hot spots with local temperatures of ∼105 K,49 which can greatly enhance the surface diffusion coefficient as formulized in eq 10. Therefore, this inhomogeneous ion impinging at a roughened surface, that is, higher ion density at the convex and lower ion density at the concave, can introduce another effect: higher ion diffusion length Dl at the convex position. On the basis of above analysis, it indicates a diffusion barrier is established between the convex and concave that prevents the ions diffusion outward of the concave positions. Asestablished barrier distribution density should be accorded with the prepatterned pit density. This inhomogeneous surface diffusion effect can be modeled as surface barrier limited diffusion, which is physically introduced by the inhomogeneous ion heating effect. Thus, the selective growth of carbon nanostructures on the concave positions could occur, which means geometrical confinement should also modify the impingement of incoming carbon species and therefore their sticking probability. As discussed later in this section, the population density of as-formed CNFs is in good agreement with the prepatterned pit density, which shows deterministic supporting for the surface barrier limited diffusion model. To avoid unnecessary discussion, it is reasonably assumed in this paper that the ions-electrons recombination time is always long enough to sustain a dynamic equilibrium of negative charging on the convexes, which provides the everlasting driving force for the above-mentioned ions converging on the convexes. A numerically analytic approach for the determination of varying sticking density of carbon islands on silicon surface with different growth duration was suggested to provide a definite support for the surface barrier limited diffusion model. From this point of view, the evolution of CNFs population density can be analytically modeled according to eq 12.46

dN(r, t) ) 2πrF(t) exp[-F(t)πr2] dr

(12)

Here dN(r,t0) is the sticking number of incoming C species in the differential area ds ) 2πr dr at time t0, F(t) is the sticking density at the center of the circle model [F(t) can be modulated by changing the R value of the surface roughness], and t is the growth duration. Thus, we define population density n(r, t) as eq 13.

n(r, t) )

dN(r, t) ) F(t) exp[F(t)πr2] ds

(13)

For an experimental case, it is easy to assume that a growth process can become a dynamical equilibrium state, on condition that long enough growth duration is provided. Thus, for a specific growth position (r ) 0 is chosen) the population density at t can be expressed as eq 14

(

( ))

n(0, t) ) F(t) ) n∞ 1 - exp

-t T0

(14)

where n∞ denotes the ultimate equilibrium sticking density of CNFs. T0 is the population saturation time, and it can be increased by the increase of diffusion length (Dl) if we reasonably assume the same diffusion velocity. The experimental relation of sticking density n(t) versus growth duration t is plotted in Figure 5a, and it can be wellfitted into an exponential equation with standard expression of n(t) ) Y0 + A exp (-t/T0). Y0, A, and T0 are three fitted parameters. As can be obtained from Figure 5a, the sticking density versus growth duration for the CNF arrays on polished and roughened silicon are fitted as n1(t) ) 0.01(1 - exp(-t/ 40)) and n2(t) ) 0.632(1 - exp(-t/15)), respective; the

Figure 5. The dependence of CNFs population density on growth duration (a) and electrochemical etching duration (b).

saturation duration (T0) for the sticking density for these two cases are 40 and 15 min, respectively. It deterministically verifies that the diffusion barrier introduced by surface roughening effectively lowers the saturation time and therefore the diffusion length. The proposed model predicts the time evolution of active sites on the substrate surface and includes the capture of C species and the etching effect of the striking ions. If the suggested surface diffusion is truly responsible for the depletion of carbon species in the convex, and furthermore for the selective growth of CNFs on the concave position, one should expect that the population density of as-formed CNFs will depend on the density of as-etched pits. It is because the diffusion length (Dl) of adatoms is expected to decrease due to the increase of diffusion barrier density. To test this prediction, several samples were prepared under various durations of electrochemical roughening (as has been seen experimentally, the larger the roughing duration, the larger the density of as-etched pits). The distribution density n(r) for as-formed CNFs at a growth duration of 120 min versus etching durations were numerically calculated and are shown in Figure 5b. It is clearly shown the population density of CNFs increases with electrochemical etching duration at the initial stage and reaches a rather stable value of ∼5 × 108 cm-2. 3.4. Formation of Cone-shaped Tip due to Ion Etching. It is well-known that low-energy ion bombardment shows its ability to remove chemisorbed H atoms from H-terminated carbon surface, which creates available lattice sites for the binding of incoming carbon species and thus induces the synthesis of well-faceted diamond film.50,51 Therefore, the aligned growth of CNFs can also be an ion-induced process. The roughened surface structure of as-grown carbon materials in Ar+ plasma has been previously studied and showed the

9252 J. Phys. Chem. C, Vol. 112, No. 25, 2008 key role of ion sputtering for the formation of conical structure.52 The conical tip of as-synthesized CNFs, as shown in Figure 1, can also be explained by taking into account of the converged ions etching effect of as-grown carbon atoms at the fiber tips (for the ions distribution on the tip of asgrown CNFs, it is quiet similar to that shown in Figure 4). To summarize, the selective growth of CNFs on the convex positions involves both carbon species stacking (growth) and the simultaneous ion bombarding (etching). 4. Conclusions Vertically aligned CNF arrays with different population density are formed on both polished and chemically roughened silicon surfaces. The as-formed CNFs are characterized to show amorphous structure with high content of sp2 carbon. On the basis of the surface barrier limited diffusion introduced by local ununiform electric field distribution at specific sites, the different nanofiber population density on both polished and roughened surfaces is well interpreted. The conical tip structuring can be explained by considering the ion etching effect. Our results can serve as typical microscopic models of surface/interface phenomena on the roughened surfaces with a large number of holelike nanostructures, which are directly relevant to the development of deterministic strategies toward precise and cost-efficient plasma-aided nanofabrication. Acknowledgment. We are thankful to the financial support of “The program of excellent team in Harbin Institution of Technology”. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Denysenko, I. B.; Xu, S.; Long, J. D.; Rutkevych, P. P.; Azarenkov, N. A.; Ostrikov, K. J. Appl. Phys. 2004, 95, 2713. (3) Uchida, T.; Anderson, D. P.; Minus, M. L.; Kumar, S. J. Mater. Sci. 2006, 41, 5851. (4) Yoon, S. H.; Lim, S.; Hong, S. H.; Qiao, W. M.; Whitehurst, D. D.; Mochida, I. Carbon 2005, 43, 1828. (5) Huang, S. J.; Shimizu, T.; Sakaue, H.; Shingubara, S.; Takahagi, T. Jpn. J. Appl. Phys. 2005, 44, 5289. (6) Zhang, G. Y.; Jiang, X. Science 2003, 300, 472. (7) Li, J. J.; Gu, C. Z.; Wang, Q.; Xu, P.; Wang, Z. L.; Xu, Z. Appl. Phys. Lett. 2005, 87, 143107. (8) Larciprete, R.; Petaccia, L.; Lizzit, S.; Goldoni, A. J. Phys. Chem. C 2007, 111, 12169. (9) Cattanach, K.; Kulkarni, R. D.; Kozlov, M. Nanotechnology 2006, 17, 4123. (10) Zhang, X.; Zhang, W.; Liu, L.; Shen, Z. X. Chem. Phys. Lett. 2003, 372, 497. (11) Lefrant, S.; Baltog, I.; Baibarac, M.; Schreiber, J.; Chauvet, O. Phys. ReV. B 2002, 65, 235401. (12) Lu, W.; Lieber, C. M. Nat. Mater. 2007, 6, 841. (13) Hofmann, S.; Ducati, C.; Kleinsorge, B.; Robertson, J. Appl. Phys. Lett. 2003, 83, 4661.

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