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Fabrication of carbon nanostructures (nanodots, nanowires) by energetic ion irradiation

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JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 40 (2007) 2083–2088

doi:10.1088/0022-3727/40/7/033

Fabrication of carbon nanostructures (nanodots, nanowires) by energetic ion irradiation Amit Kumar1,3 , Fouran Singh1 , J C Pivin2 and D K Avasthi1 1 Materials Science Group, Inter-University Accelerator Centre, PO Box 10502, Aruna Asaf Ali Marg, New Delhi 110067, India 2 CSNSM, IN2 P3-CNRS, Batiment 108,91405 Orsay Campus, France

E-mail: [email protected] and [email protected]

Received 28 December 2006 Published 16 March 2007 Online at stacks.iop.org/JPhysD/40/2083 Abstract Carbon nanostructures were synthesized by energetic ion irradiation of Si-based gel films. These polymer-like films with different side groups and C concentrations were prepared by sol–gel chemistry and irradiated with Si or Au ions of different energies in the range 3 MeV–2 GeV. The shape and size of the formed carbon nanostructures was studied by energy filtered transmission electron microscopy. They exhibited a visible photoluminescence emission, due to their semiconducting nature and the confinement of excitons. The changes in the optical properties were correlated with the structural transformation of films, investigated by means of Fourier transformed infrared (FTIR) and Raman spectroscopies. The role of carbon concentration, structure and energy transferred by ions on the luminescence properties are discussed. (Some figures in this article are in colour only in the electronic version)

1. Introduction Carbon nanostructured materials are attracting considerable interest because of its vast variety of possible application in nanotechnology. Generation of carbon nanostructures has been attempted by many techniques e.g. by arc discharge, chemical vapour deposition, pulsed laser deposition chemical routes, etc [1–3]. Nowadays, the ion beams are an emerging tool for synthesizing or altering materials properties on nano/atomic levels [4–7]. It is known that when a swift heavy ion (SHI) (heavy ion with energy >1 MeV/nucleon) passes through material, it induces a continuous trail of damage with a few nanometres diameter and typically several tens of micrometres in length. Such an ion track consists of material with properties that are drastically changed compared with the surrounding virgin material. Recent reports show that ion beams are a versatile tool to synthesize nanostructured materials with controlled size distribution [7, 8]. In the last few years, it has been shown that ion irradiation of Si-based polymers leads to generation of carbon nanoclusters [9]. These 3

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nanoclusters formed by irradiation of silicon-based polymers exhibit a strong visible photoluminescence spectrum, when the amount of energy transferred by ions to target atoms exceeds a threshold value [10]. Photoluminescence (PL) in carbon nanostructures films has recently received great attention [11]. The strong visible PL efficiency at room temperature makes such materials suitable for optoelectronics devices (interconnects, light emitting diodes etc). It is worth noting that the yield of nanostructure formation depends essentially on the amount of electronic excitation. The displacements due to elastic collision scarcely contribute to the conversion [9–12]. The PL spectrum depends strongly on ion fluence or film transformation into ceramics or free C cluster formation in matrices with stoichiometric SiCx or SiCx Oy glass (y + x/2 = 1) [12, 13]. The present work gives a comprehensive picture on the creation of carbon nanostructures by irradiation of Si-based polymers. In the present work, different types of carbon nanostructures were synthesized (randomly distributed carbon clusters and aligned carbon cluster forming almost nanowires) by irradiation of Si-based gels. The energy and mass of ions

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Table 1. The electronic and nuclear energy loss of used ions for MTES, PTES and MP films. Energy and ions

Polymers

Electronic energy loss Se (eV Å−1 )

Nuclear energy loss Sn (eV Å−1 )

3 MeV Si 3 MeV Si 3 MeV Au 100 MeV Au 120 MeV Au 120 MeV Au 120 MeV Au 2 GeV Au

PTES MP MTES MP MTES PTES MP MP

125.6 118.5 103.8 787.5 755 921.3 186.5 1024

1.9 1.6 98.8 12.36 9.8 11.2 10.7 1

were varied to span range of electronic excitation. The energy filtered transmission electron microscopy (EFTEM), Fourier transform infrared (FTIR), Raman and photoluminescence spectroscopies were used for characterization of the irradiated films. The role of carbon concentration and density of energy deposited by ions on the formed nanostructures and optical properties are discussed.

2. Experimental The gels were prepared from ethoxides with the following formulae: Si(CH3 )(OC2 H5 )3 and Si(C6 H5 )(OC2 H5 )3 labelled respectively MTES (methyltriethoxysilane) and PTES (phenyltriethoxysilane). Hydrolyzed solutions of MTES and PTES were mixed in 50 molar percentages of each to vary the carbon concentration in gel films, which we will give the name MP for simplicity. These films were spun at 1500–3000 rpm on clean 1 0 0 Si- wafers. The typical thickness of the films was ∼900 nm. These films were irradiated by 3 MeV Si, 3 MeV Au, 100 MeV, 120 MeV and 2 GeV Au ions at different fluences using the accelerator at CSNSM Orsay France, GSI Germany and 15 UD Pelletron at IUAC, New Delhi. The range and energy loss of the ions were estimated using the transport and range of ions in matter (TRIM) code [14]. The estimated electronic energy loss (Se ) and nuclear energy loss (Sn ) values for used ions are given in table 1. The energy of all used ions was such that the ions penetrate the thin film and be implanted in the substrate. The irradiated samples were investigated by cross-sectional transmission electron microscopy (TEM) imaging from a Philips CM20FEG microscope operated at 200 kV. The TEM samples were prepared by the conventional ion milling technique, thinned down to about 50 nm. Inelastically scattered electrons (having excited C K transition at 284 eV and Si L23 transition at 99 eV) were filtered in energy to yield energy filtered TEM (EFTEM) images of regions enriched in C or Si. The energy filtering was performed with a post-column electron energy filter, namely, Gatan Imaging Filter GIF-200, model 667. The Fourier transformed infrared (FTIR) spectra have been recorded before and after irradiation using a FTIR, Nexus 670 model in the region of 3100–900 cm−1 with a resolution of 4 cm−1 wavenumber. Raman spectra of pristine and irradiated films were recorded using the LABRAM100 micro Raman system with 488 nm argon ion laser. The PL spectra from polysiloxane films were measured using excitation with the 325 nm line with a power of 27 mW of a Kimmon He-Cd laser. 2084

Figure 1. Cross-sectional EFTEM images of the MTES films irradiated with (a) 120 MeV Au ions at 3 × 1011 ions cm−2 fluence and (b) 3 MeV Au ions at 2.5 × 1015 ions cm−2 fluence. In the picture the regions in red colour correspond to C atoms and those in indigo colour correspond to Si.

For the PL measurement, a Michelle 900 monochromator system and a Si charge coupled detector were used. The accumulation time was 2 min for all the films.

3. Results and discussion 3.1. Transmission electron microscopy Figure 1 shows the electron EFTEM pictures of the MTES films, irradiated with (a) 120 MeV and (b) 3 MeV Au ions at the fluences of 3 × 1011 and 2.5 × 1015 ions cm−2 , respectively. In the image of the sample irradiated at 120 MeV (figure 1(a)), the carbon cluster are aligned in strings, forming almost continuous nanowires with an average diameter of 4 nm, whereas in images of the sample irradiated at 3 MeV (figure 1(b)), randomly distributed carbon clusters like nanodots of sizes 3–7 nm are observed. These images have a clear evidence of carbon nanostructure formation under energetic ion irradiation. In case of higher energy (120 MeV Au ions) the clusters are aligned along the ion paths due to dense electronic excitation resulting in the formation of an ion track, whereas in the case of 3 MeV Au ion irradiation carbon nanodots-like structures are formed, due to the compound effect of elastic and inelastic collisions. In brief, for 3 MeV Au irradiated films, the electronic energy loss is responsible for the creation of nanodots and the nuclear energy loss is responsible for redistributing or spreading the nanodots randomly. In case

Fabrication of carbon nanostructures

Figure 2. The FTIR spectra PTES and MP films irradiated with 120 MeV Au ions at indicated fluences.

of high energy (120 MeV Au ions) the nuclear energy loss is insignificant (as compared with 3 MeV Au ions) and therefore the C clusters are confined in a track. When the energetic ion penetrates the film, a spike of energy or temperature occurs along the ion path. This high temperature (>600 K) melts the film and irreversible structural transformation occurs along the ion path in cylindrical region. The radius of ion tracks is generally around 3–10 nm, which depends on the ion energy, species and target material. Due to transient spikes of temperature, evolution of H gas takes place [15] from the ion track region. The kinetics of gas evolution depends more on the nature of chains and organic side groups than on the mass of irradiating ion. Carbon atoms do not tend to evolve as COx , CHx during irradiation ions but more clusters of free C are formed due to high C–C bond enthalpy. The obtained carbon nanostructures (carbon dots and aligned carbon clusters) find useful applications as field emitters, light emitters and nanocontacts. 3.2. Infrared spectra The Infrared spectra of the films were recorded in the 900– 3100 cm−1 range. Figures 2(a) and (b) show the IR spectra of pristine and irradiated PTES and MP films, respectively, at fluences of 3 × 1011 and 1 × 1013 ions cm−2 with 120 MeV Au ions. The different types of bonds (vibrations) in pristine and irradiated PTES and MP films are given in table 2. In the pristine film, there are some strong bands at around 893, 1107, 1132, 1430 and at 1600 cm−1 , which are assigned as given in table 2. A weak intensity peak at 3000–3100 cm−1 is due to =C–H stretching. As we increase the fluence, the intensity of peaks at 1132 and 1431 cm−1 decreases sharply due to breaking of different types of bonds and release of H. The kinetics of hydrogen release is reported

Table 2. The characteristic peaks of PTES and MP film before and after irradiation. Peak position (cm−1 ) As-deposited

Irradiated

Assignment

1450–1600 1750 2860 2980 3000–3100

Si–H2 Bending doublet Si–O–Si Asymmetric stretching Si–C6 H5 Bending Si–CH3 Stretching C6 H5 Stretching doublet C=C Stretching (aromatic rings) C=O Stretching C : H Symmetric stretching C : H Asymmetric stretching =C–H Stretching

893 1107 1132 1270 1430

elsewhere [15]. After irradiation, some new peaks in the region 1450–1600, 2860 and 2980 are noticed, which are attributed to C=C stretching (aromatic rings), symmetric stretching and antisymmetric stretching of C–H, respectively. With a further increase in the fluence, peaks at 1132 and 1430 vanish and C–H peak dominates. The IR spectra show that different types of bonds break and increase in the C : H (hydrogenated carbon clusters) takes place with ion fluence. The latter is due to the formation of small carbon clusters such as C5 H12 , C17 H36 , C35 H36 , C41 H60 [16]. From the figures, we see that in MP films, C : H vibration strength is larger than that in PTES films and these oscillations start at lower fluence. The analyses show that the ion irradiation of these films leads to precipitations of carbon clusters. The x-ray emission spectroscopic studies also show that the ion irradiation on a similar type of Si-based polymer (PTES) leads to free C cluster formation, whereas Si-O bonding remains the same as silica matrix [13]. 2085

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Figure 3. (a) Raman spectra of PTES film irradiated with 3 MeV Si ions and (b) variation of I (D)/I (G) peak with ion fluence.

3.3. Raman spectra Raman studies were carried out to understand the properties of carbon nanostructures formed under ion irradiation. It is known that carbon materials (amorphous) exhibit two Raman modes labelled, D (diamond) peak at ∼1330 cm−1 and G (graphitic) peak at ∼1580 cm−1 [17, 18]. The peak positions and intensities vary due to the bond disordering, clustering of the sp2 phase, presence of sp2 rings or chains and sp2 /sp3 ratio [19]. Raman spectra of PTES films irradiated with 3 MeV Si at different fluences are shown in figure 3(a). Raman spectra of irradiated film show the broad characteristic peaks of amorphous carbon phases in the 900–1800 cm−1 range. Raman scattering analysis shows that particles formed in irradiated PTES exhibit a noticeable degree of tetragonal hybridization. The spectra shown in figure 3(a) are normalized to understand the evolution of the D peak with fluence. The figure shows that the intensity of the D peak increases with fluence and the G peak shows a shift towards a lower wavenumber. Figure 3(b) shows that the ratio of I (D), the integral intensity under the D peak, to I (G), the integral intensity under the G peak, increases with the ion fluence. The inset of figure 3(b) shows the Gaussian fitting of the D and the G peaks for a fluence of 1 × 1014 ions cm−2 . The arrows show the shifting of the G peak towards lower wavenumber and an increase in the intensity of the D peak with a further increase in ion fluence. The shifting of the G peak is attributed to sp3 bonded carbon formation, increase in the bond disordering and increase in the I (D) attribute to clustering as reported by Ferrari et al [18]. In the present experiments, we observed a broad peak in the region 1100–1600 cm−1 . The G mode at about 1550–1580 cm−1 in amorphous graphite (a-G) as well as in ‘amorphous diamond’ (a-D, with a high degree of 2086

sp3 hybridization) or at 1530–1550 cm−1 in sp3 hybridized aC : H, as reported by other groups [17, 18]. The frequency of the breathing mode is 1350–1450 cm−1 in a-G and 1200– 1300 cm−1 in a-D or 1250–1300 cm−1 in a-C : H [18]. The above discussion shows the amorphous carbon formation in irradiated PTES films, which is a-C : H like (sp3 hybridized) in nature. 3.4. PL spectra Our earlier results show that these polymers exhibit PL in the visible region after ion irradiation [10]. Here, we will discuss the role of carbon concentration, structures and energy transferred by ions, on the PL spectra of these polymers. 3.4.1. Role of carbon concentration. To study the role of carbon concentration with respect to hydrogen, we chose two polymers (PTES and MP). According to calculations MP films have higher H/C and SiO/C ratios compared with PTES films. Figures 4(a) and (b) show the visible light emission spectra of PTES and MP films. The PL intensity increases with increasing fluence due to the segregation of C into clusters. The segregation of C into clusters depends upon the energy deposited by the energetic ion to the target atoms. These carbon clusters also contain hydrogen, as evidenced by the FTIR analysis, and have the properties of a-C : H (amorphous hydrogenated carbon) films. The Raman analysis shows that carbon clusters have the properties of amorphous carbon with sp3 hybridization. It is known that a-C : H (or tetragonalC as known by Raman analysis) clusters exhibit visible PL emission due to sp3 hybrid C atoms organized in clusters of different sizes and shapes [19, 20]. Raman and FTIR results show that irradiated gel films have hydrogenated sp3

Fabrication of carbon nanostructures

Figure 4. PL spectra of PTES and MP films irradiated with the indicated fluences of 120 MeV Au ions.

hybrid carbon nanoclusters, which emit visible light at room temperature. It has been shown earlier that the PL emission from carbon nanoclusters depends on their shape, size and the number of atoms in the cluster [20]. PL spectra show some quenching in PL intensity at higher fluence. There are several possible reasons for quenching of PL intensity. (i) The growth/clustering of carbon nanoparticles at higher fluence, as evidenced by FTIR/Raman measurements, leads to a reduction in the excitation and recombination of charge carriers. (ii) The depletion of H atoms in the carbon cluster or matrix act as a non-radiative recombination centre. (iii) It may be due to defect generation or increasing the bond disordering at higher fluence in the carbon cluster or matrix as evidenced by Raman analysis. This leads to the trapping of the charge carriers and nano radiative transitions occur. From the figure, we observed that MP films exhibit better light emission compared with PTES films. These results suggest that higher H/C ratio and carbon concentration with respect to SiO (matrix) play an important role in light emission. Therefore the polymers, which have higher H/C and SiO/C ratios, exhibit better light emission. These results are in agreement with earlier results reported by Sendova et al [21], the PL emission in SiO2 - C co-sputtered films is optimal when the carbon concentration is 5–7% and PL intensity decreases by a factor of 1000 when the films contain 25% carbon. These reports showed that the films containing less carbon with respect to the SiO2 matrix exhibit higher light emission. 3.4.2. Role of the type of carbon nanostructure formed. According to the electron filtered transmission electron microscopy analysis, we observed the carbon nanodots and aligned carbon nanoclusters with 3 MeV and 120 MeV Au ion irradiated MTES films, respectively. To study the role of carbon structured formed, we studied the luminescence

Figure 5. PL spectra of 3 MeV Au and 120 MeV Au ion irradiated films at fluence of 5 × 1013 ions cm−2 and 3 × 1012 ions cm−2 , respectively.

properties of the same energy ions irradiated MTES films. Figure 5 shows the normalized PL spectra of 3 and 120 MeV Au ion irradiated MTES films for optimal fluence (the fluence at which irradiated films exhibit higher light emission). It is seen that 3 MeV Au ion irradiated films show broader PL spectra with red shift as compared with 120 MeV Au ion irradiated films. The main reason for broad luminescence in 3 MeV Au ion irradiated films is the random size distribution of carbon nanoparticles whereas in the case of 120 MeV Au ions, the narrow PL spectra is due to the sharp distribution or aligned clusters formation. 2087

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Raman and photoluminescence spectroscopies. The formed carbon nanostructures (< 10 nm) exhibit a strong visible photoluminescence, with a-C : H (sp3 hybrid) properties, as evidenced by Raman and FTIR analysis. It is observed that photo luminescence intensity goes through the maximum as the amount of electronic excitation increases and the maximum for the same electronic energy transfer (1–3 eV/atom) for all used ions. The PL intensity is higher for MP films, indicating that the films containing more hydrogen and less carbon with respect to SiO (matrix) exhibit better light emission after irradiation.

Acknowledgments The authors (DKA and JCP) are grateful to the Indo-French Centre of Advanced Research (IFCAR) for providing financial support for carrying out research on Generation of Nano Phase by Energetic Ion Beams. AK is grateful to the Council of Scientific and Industrial Research, India, for providing a fellowship for pursuing the research.

References Figure 6. PL spectra of MP films irradiated with Au and Si ions.

3.4.3. Electronic energy deposition. We know that when an energetic ion passes the films it deposits energy to the system and generates a transient high temperature. This high temperature modifies the films’ properties drastically. So energy deposition plays an important role in modification of the material. Here we study the role of electronic energy deposition on PL spectra. Figure 6 shows the integral intensity of PL versus electronic energy deposition (Se × φ, where Se is electronic energy loss and φ is ion fluence) on the films. From the figure, we observe that peak integral intensity increases with energy deposition up to certain fluence, beyond which it decreases. The reason for decrease in the PL intensity is the same as we had discussed in section 3.4.1. It is pointed out that integral intensity of PL spectra is maximum at around 1– 3 eV/atoms for all mass ions. This shows that total electronic energy deposition (Se × φ) plays a significant role in creating photoluminescent films. It is also pointed out that linear energy deposition to the target atom plays an important role rather than different mass and energy ions. Our earlier studies [10] for MTES films show that the PL is maximum at 4.9–8.4 eV/atom energy transfer. These results show that MP films have lower threshold of energy transfer for maximum PL emission and precipitation of carbon clusters is faster, which is in agreement with the conclusion from infrared results in section 3.2.

4. Conclusions In the present work, we report synthesis of carbon dots and aligned carbon nanoclusters in Si-based polymers by energetic ion irradiation. The properties of these synthesized carbon nanostructures are studied by EFTEM, FTIR,

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[1] Ebbsen T W and Ajayan P M 1992 Nature 358 220 [2] Ren Z F, Huang Z P, Wang Z D, Wen J G, Xu J W, Wang J H, Calvel L E, Chen J, Klemic J F and Reed M A 1999 Appl. Phys. Lett. 75 1086 [3] Chen G X, Hong M H, Liu X H, Wang W J, Lu Y F and Chong T C 2003 Proc. SPIE 4830 196 [4] Kumar A, Avasthi D K, Pivin J C, Tripathi A and Singh F 2006 Phys. Rev. B 74 153409 [5] Srivastava S K, Avasthi D K and Pipple E 2006 Nanotechnology 17 2518 [6] Seki S, Maeda K, Tagawa S, Kudoh H, Sugimoio M, Morita Y and Shibata H 2001 Adv. Mater. 3 1663 [7] Kumar A, Avasthi D K, Tripathi A, Kabiraj D, Singh F and Pivin J C 2007 J. Appl. Phys. 101 014308 [8] Kumar A, Carey J D, Pivin J C, Tripathi A and Avasthi D K 2007 Appl. Phys. Lett. submitted [9] Pivin J C, Sendra-Vassileva M, Colombo P and Martucci A 2000 Mater. Sci. Eng. B 69 574–7 [10] Kumar A, Singh F, Pivin J C and Avasthi D K 2005 Radiat. Meas. 40 785 [11] Henley S J, Carey J D and Silva S P R 2004 Appl. Phys. Lett. 85 6236 [12] Pivin J C, Colombo P and Soraru J D 2000 J. Am. Ceram. Soc. 83 713 [13] Kurmaev E Z, Moewes A, Krietemeyer M, Endo K, Ida T, Shimada S, Winarski R P, Neumann M, Shamin S N and Ederer D L 1999 Phys. Rev. B 60 15100 [14] Ziegler J F, Biersack J P and Littmark U 1985 The Stopping and Range of Ions in Solids (Oxford: Pergamon) [15] Srivastava S K, Avasthi D K and Pivin J C 2002 Nucl. Instrum. Methods Phys. Res. B 191 718 [16] Zhang R O, Bertran E, Lee S-T 1998 Diamond Relat. Mater. 7 1663 [17] Hong S-H and Winter J 2005 J. Appl. Phys. 98 124304 [18] Ferrari A C and Robertson J 2000 Phys. Rev. B 61 14095 [19] Robertson J and O’Reilly E P 1987 Phys. Rev. B 51 2143 [20] Giorgis F, Giuliani F, Pirri C F, Tagliaferro A and Tresso E 1998 J. Appl. Phys. 72 2520 [21] Sendova-Vassileva M, Tzenov N, Dimova-Mali-Novska D and Josepovits K V 1996 Mater. Res. Soc. Symp. Proc. 417 395

Fabrication of carbon nanostructures (nanodots ...

Mar 16, 2007 - 1 Materials Science Group, Inter-University Accelerator Centre, PO Box 10502,. Aruna Asaf Ali Marg, ... The strong visible PL efficiency at room temperature makes ... Hydrolyzed solutions of MTES and PTES were mixed in 50 ...

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