characterization of varying sp2/sp3 · pdf filecharacterization of varying sp2/sp3...

IJRRAS 4 (1) ● July 2010 Niranjana& al. ● Characterization of Varying SP2/SP3 Nanocluster’s Grown

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CHARACTERIZATION OF VARYING SP2/SP3 NANOCLUSTER’S GROWN UNDER VARIOUS PROCESS CONDITIONS 1Niranjana S, 2B.S. Satyanaryana, 3U.C. Niranjan & 4Shounak De

1Selection Grade Lecturer, Manipal Institute of Technology, Manipal University, Manipal, India. 2 Principal, R .V. College of Engineering, Bangalore, India.

3Adjunct Professor, Department of E&C, MIT, Manipal, India. 4Lecturer, Manipal Institute of Technology, Manipal University, Manipal, India.

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

ABSTRACT

Continuous or pulsed vacuum based cathodic arc systems are proven as novel, and important for the thin film growth from insulator, semiconductor to conductor. There are several key process parameters such as Gas composition, conditions at substrate (Temperature, Pressure), Deposition Rate, Deposition time, Position of substrate, and type of arc. The process parameters has a significant influence on the characteristics and growth of nanocluster. The variation in the process parameter affects the nanocluster morphology, structure, composition, electronic and optoelectronic properties, which make it useful for different application. The carbon based nanocluster samples grown at fixed conditions of nitrogen partial pressure (10–4 Torr and 10–3 Torr) under varying conditions of Helium partial pressure from 5x10 – 4 Torr to 50 Torr resulted samples with different morphology and properties. Presented here morphological details of various samples grown under various process parameters, some using continuous and other using pulsed cathodic arc process. Discussed carbon nanocluster characteristics using SEM, Field emission response, and Raman response. Proposed an approach of grouping sp2/sp3 nanocarbons based statistical parameters. Keywords: sp2/sp3 thin film, Carbon based Nanocluster, Raman response, cathodic arc system 1. INTRODUCTION

Nanofabrication techniques such as Hot Filament Chemical Vapour Deposition (HFCVD), Thermal Chemical Vapor Deposition, and Cathodic Arc process, have unfolded different self_aligned nanomaterials which function as the building blocks of future nanoelectronic devices. Many of the naocarbon materials, including nanodiamond, fullerene, buckey ball, C-60, nanotubes, nanohorns, nanofibers, and carbon nanocluster , are finding their importance in nanotechnology. Some of the growing area of technology were these nanocarbon find applications is sensors, devices, vacuum nanoelectronics, biomedical applications and MEMS/ NEMS. Vacuum Nanoelectronics is a revolutionary technology and nanocarbon based vacuum nano applications includes field emission displays, electron beam lithography, electron and ion guns , multitude of sensors, electron microscopes and microprobes, low & medium power microwave sources, micro/ pico satellite propulsion systems, high power devices and Tera Hz communication devices [1-5]. The carbon nanoclusters are grown at relatively low temperature using Cathodic arc process [6]. The Cathodic arc either as continuous or pulsed arc has already been demonstrated as suitable for applications in the areas like tribology, low K dielectrics and conformal metal coating for VLSI/ULSI. This process is of great interest because it offers great opportunity for tailoring nanocarbon based materials over a wide range of properties. The growth of low temperature grown nanocarbons could be of interest for flexible electronics or electronics on glass or plastic. The nanocarbon film’s structural, compositional and morphological properties are highly dependent on the process parameter such as Temperature, Pressure, Compositional Gas ratios, ion energy, type of arc (continuous arc or pulsed arc), arc current and arc voltage. Studied the morphology and properties of carbon nanoclustres grown at different growth conditions with different growth parameters. Among many techniques Raman Spectroscopy offers the option of nondestructive, instantaneous characterization [7-16] and is an indirect method, used for nanocarbon analysis. Raman measurements are used to study the bonding and compositional nature of the various nanocarbon films. The simple conductivity measurements of nanocarbon thin films under varying conditions could lead to substantial understanding of the electronic properties. The Quality analysis and correlation of SEM and Raman responses helps in automating the production of nanostructures [7, 15, 16]. The combined study could able to define, the possibility of determining approach for characterization and feasibility of nanocarbon thin films for future applications.

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2. EXPERIMENTAL DETAILS

Nanocluster carbon films were grown using Pulsed Laser Assisted cluster beam assembly source process at room temperature and Nanocluster carbon films grown at room temperature using the Cathodic Arc process. The nanocluster samples were grown for two fixed conditions of nitrogen partial pressure (10 –4 Torr and 10–3 Torr) under varying conditions of Helium partial pressure from 5x10 – 4 Torr to 50 Torr. Cathodic arc is a Plasma based technology. The arc process can be carried out either at high vacuum or in a low pressure gaseous environment. The triggering is done mechanically or electronically and at cathode the current is concentrated at a small number of sites called Cathode spot. Generally the Spot size range 1-10 microns and temperature at the spot site can go upto 40000C. The Life time of individual spot is around 10nsec-microsec. “New daughter spots are formed near previous parent spot, resulting in spot motion” . Arc current is composed of both an electron component and an ion component. Max Arc current has 10% of ion component. Macro particles generally 0.1-10 micron are filtered using magnetic filter & S bend duct. The carbon plasma plumes away from cathode with velocity in the range of 1-3cm/µs. The various process parameters influencing on the nanocluster are shown in figure 1.

Figure1, Various Process parameters Influence sp2/sp3 the carbon based nanocluster..

The Morphological characteristics of the carbon based nanocluster samples are studied with SEM images. The field assisted electron emission measurements were carried out in a vacuum chamber of vacuum 10-7 torr, in a parallel plate configuration, with an anode cathode spacing of 100 µm. Measured coplanar and sandwich configuration based electronic and optoelectronic measurements. The Raman measurements were carried out using a Reinshaw Raman spectroscopy equipment with a 514.5nm excitation source. Conductivity measurements were made with coplanar configuration for the nanocluster carbon thin films grown on glass. The electrical measurements were carried out using Keithley source meter and electrometers. Whereas the other nanocarbons considered for statistical analysis includes nanodiamond, nanowall and carbon nanotube. The Nanodiamond films grown using Hot Filament Chemical Vapor Deposition (HFCVD) at around 8000

C. The Carbon Nanowalls films were grown using DC plasma CVD at 9000C. The Carbon Nanotubes were grown using Thermal CVD at 7500C.

§FILMSp2/Sp3

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3. RESULT AND DISCUSSION

The morphological and dimensional features of carbon based nanocluster grown at various process parameter has varying sp2/sp3 are studied using Scanning Electron Microscopic images (SEM). Shown in Figure2(a-f), morphological details of various nanocluster films with varying cluster sizes from near atomic smooth shown in figure 2(a), nanoclusters to nanofibers in figure(2.b,2.c,2.d and 2.f), including nanowalls in figure2(e). It may be seen from the image we have clusters of varying sizes, shapes and varying levels of surface roughness and protrusions. These protrusions vary from a few nanometers to over a few microns depending on the material and the process of growth. These surface morphological features influence the electron emission. The sharp protrusion regions act as preferentially field enhanced active emission sites. At the same time, these very narrow (few nanometers) regions may also burn out, when high current flows through them, leading to instability in emission. The cluster dimension analysis with histogram plot is shown in figure 3(a). The field assisted electron emission measured in the case of the various nano materials mentioned above are shown in Figure 3(b). The current density is plotted against the applied field, for different nano carbon based materials. It may be seen from the figure that all the nano materials irrespective of the growth process typically show turn on fields as low as 1V/µm for a current density of 1µA/cm2. However the trend beyond turn on is not exactly the same. It may be seen from the I-V plot that nanocluster, initially rise exponentially, but saturate around 4V/µm, while the nanostructured carbon or Nanowall, though a bit slow to turn on then rises exponentially and is also observed to deliver higher current densities. Shown also Fowler Nordhems plots (figure3.c). Further an effort was made to look at possibility of correlation between the nanocarbon dimension estimated from SEM images to field assisted electron emission properties of these films.

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Figure 2, SEM Micrographs of nanocluster carbon films grown with a under varying Nitrogen and Helium partial pressures (a) Near atomic smooth,(b,c&d) Clusters,(e) nanowalls and (f) Nanofibers.

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Figure 3. a) cluster Histogram plot , b) Current density Vs Applied field plots of Nanocarbon based low field electron emitters (cold cathodes) , c) Fowler Nordhems plot, and d) Conductivity plot for various nanocluster samples.

The conductivity measurements shown in figure 3(d), indicates the semiconducting nature of these nanothin films. The measurement were carried out at a fixed voltage under varying temperature from room temperature to 180oC. Emission uniformity in nanocluster films (sp2 rich) are studied. However most of the above said techniques take long time in terms of sample preparation for the characterization. Raman spectroscopy offers the option of nondestructive, instantaneous characterization of these carbon films. As it is increasingly seen as an effective tool to characterise nanomaterials varying from conductors to semiconductors to insulators and also polymeric clusters. It can be used for both qualitative and quantitative analysis of the materials. In the case of carbon, Raman can be used to identify if the material is amorphous, nanocrystalline or crystalline, the size of the cluster, the nature of bonding: if it is graphite like (sp2 bonding or π bonding )or diamond like (sp3 bonding or σ bonds )and the ratio between the two (sp2/sp3)etc. Raman response studies of carbon based nanocluster, shows one or two prominent features and some minor modulations. The prominent features are G (Graphite ) Peak, and D (Disorder) Peak. Shown in figure 4(a), Raman response of various nanocarbons including nanocluster, carbon nanotube, nanowall, nanodimond, nanoplillars. Shown in figure 4(b) typical sp2 rich and sp3 rich Raman responses.

The nanodiamond film exhibits a narrow diamond peak around 1332cm-1 and a broad peak around 1580cm-1(G peak) showing the presence of graphite like and amorphous phase materials around the grain boundaries. In the case of nanostructured carbon (nanowalls) we see relatively the narrowest G peak around 1580cm-1, showing the well formed nanowalls. The very low and narrow peak at 1350cm-1(D peak), showing that this higher temperature grown nanowall, has relatively the least amorphous phase or disordered cluster phase material. The material is

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Figure 4(a). Raman Response of the low field emiting nanocarbon films Nanodiamond films,Cluster assembled carbon films, Nanostructured Graphitic sheets(nanowalls), Nanotubes and Nano cluster carbon films and (b) Raman response of sp2 and sp3 rich films.

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closest to graphite. The carbon nanotubes show both the G and D peaks like the nanowalls, but area broader. This shows that the materials have more amorphous phase materials and also clusters with varying dimensions besides the carbon nanotubes. In the case of room temperature grown nanocluster carbon we see an even broader G and D peak than nanotubes. Clearly showing that we have clusters with vastly varying dimensions and also amorphous phase materials. With further numerical analysis aided calculations, it would be possible to calculate the cluster size, the bonding ratio etc. So this clearly shows that the Raman could be a powerful tool to characterize these nanomaterials instantaneously even before field assisted electron emission measurements. The data clearly shows the possibility of establishing a correlation between the electrical, vacuum nanoelectronic and Raman measurement. Further the point of technological importance is that the room temperature grown material also shows nearly similar, low field electron emission to that of high temperature grown nanodiamond or nanowalls. With further characterization especially with the assistance of a tool like Raman, it should be possible to exactly tailor a new nanocluster materials with the desired cluster size and bonding ratio, exhibiting better vacuum nanoelectronics or even other nanoelectronic or opto-electronic properties. Staistical analysis of nanocluster carbon grown under various Helium and Nitrogen concentration are studied. The Lorenzian curve fitting technique have been used for extracting Raman feature Id/Ig[15]. Analysed responses of samples grown for two fixed conditions of nitrogen partial pressure(10 –4 Torr and 10–3 Torr) under varying conditions of Helium partial pressure from 5x10 – 4 Torr to 50 Torr along with the sp2 and sp3 rich samples. Shown in figure5(a), the Raman Response for constant Nitrogen with variation in Helium and Raman Response for Nitrogen Variation without Helium is in figure 5(b). The variation of Helium partial pressure and corresponding Raman response mean values are related in the plot shown in figure 5(c). The scatter plot shown in figure 5(d) clearly indicates possibility of using Id/Ig and mean values as features for grouping nanocarbons with various sp2/ sp3. The sp2 rich and sp3 rich centroid’s are surrounded by other samples includes carbon based nanocluster with sp2/sp3 variation and few nanodiamond.

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Figure 5, a) Raman Response with helium partial pressure variation, b) Raman response with Nitrogen partial pressure change (no helium), c) Helium variation with the mean of the response , d) Scatter plot, e)Peak value and Mean of response correlation plot and f) field emission based display using diode configuration .

The samples with more graphitic nanoclusters are near the sp2 centroid and more sp3 nanocarbons are clustered around sp3 centroid. Shown in figure 5(e), the partial pressure variation of Helium ( in torr) with the peak position and Mean value of data. The parametric based nanocarbon analysis is a promising approach can play a major role during insitu analysis of a nanocarbon. Thus proposed a novel approach for grouping or for evaluating nanocarbons. Figure 5(f), is the image of glowing phosphor due to the excitation by electrons due to field assisted emission. Used the vacuum diode configuration for this experiment with anode consisting of patterned phosphorous screen and cathode being a nanostructured carbon film. The experiment was carried out under a vacuum of about 10-6 Torr. MIT 50 characters are displayed at an applied electric field of 2.6 V/µm. This proof of concept promises the material for vacuum nanoelectronics applications.

4. CONCLUSION The study of low field assisted electron emitting nanocarbons includes: nano diamond, nanowalls, nanotubes, and carbon based nanoclusters grown by various techniques are analyzed. It is shown that, in spite of the diverse morphology and process technology, these materials have threshold field of around 1 V/µm for an emission current density of 1 µA/cm2 . The sp2 /sp3 varying nanocarbons are clustered with sp3 rich material and sp2 rich material as reference. Thus proposed a novel approach for grouping or evaluating nanocarbons based on their statistical details. The vacuum diode display, and the conductivity details, clearly shows that these materials are useful for nanoelectronics or vacuum nanoelectronics based applications. Thus with the more detailed study of sp2/sp3 nanocarbons, and an efficient database, Raman study at in situ could possibly directly predict some of the vacuum electronic or nanoelectronic properties or it can evaluate and classify or quantify nanocarbons.

5. ACKNOWLEDGEMENT The authors would like to thank Manipal Institute of Technology, Manipal University, Manipal for providing support during this research study.

6. REFERENCES [1]. M.Terrones,A.Jorio,M.Endo,A.M.Rao,Y.A. Kim, T.Hayashi, H.Terrones, J.C.Charlier, G. Dresselhaus and

M.S.Dresselhaus, “New Directions in Nanotube Science”, MaterialsToday,Oct,2004. [2]. Walt A De Heer, “Nanotubes and Pursuit of Applications”,MRS Bulletin,April,2004. [3]. W.Zhu Editor, Vacuum Microelectronics, Wiley, 2003. [4]. N.S.Xu & S.Ejaz Huq, Novel cold cathode materials and applications, Material Science. & Engg. R 48 ,47, 2005. [5]. J.Robertson, “Diamond Like amorphous carbon,” Materials Science and Engineering, R 37, pp 129-281, 2002. [6]. B.S.Satyanarayana,J.Robertson,W.I.Milne, Low Threshold Field Emission From Nanoclustered Carbon Grown by Cathodic

Arc,J. App. Phys. 87, No.6,3126, 2000. [7]. M A Al-Khedher , C Pezeshki, J L McHale and F J Knorr , “Quality classification via Raman identification and SEM

analysis of carbon nanotube bundles using artificial neural networks”, Nanotechnology, 18- 355703 (11pp),2007. [8]. S. Zhang a,, X.T. Zeng a, H. Xie , P. Hing, “A phenomenological approach for the Id/Ig ratio and sp3 fraction of magnetron

sputtered a-C films”, Surface and Coatings Technology 123 ,256–260,2000. [9]. Y.H. Cheng et al., “ Influence of nitrogen ion energy on the Raman spectroscopy of carbon nitride films”, Diamond and

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[10]. A.Hiraki, B.S.Satyanarayana, “Field emmision from multylayered Carbon films consisting of nanoseeded Diamond and nanocluster carbon , Deposited at Room temperature on glass substrate”, IEICE, Vol. E 86-C,No.5,2003.

[11]. B.S.Satyanarayana,X.L.Peng, G.Adamapolous, J.Robertson, W.I. Milne, and T.W. Clyne, Very Low Threshold Field Emission from Microcrystalline Diamond films grown using Hot Filament CVD Process, MRS Symp. Proc., Vol.621, 2000.

[12]. A.C.Ferarrari and J.Robertson, “Interpretation of Raman Spectra of Disordered and Amorphous carbon,” Phy.Rev.B.61, 14 095, 2000.

[13]. B.S.Satyanarayana, Niranjana S, Shounak De, R.Bhattacharya and O.S.Panwar, “ A comparative study of field emission from diverse nanocarbon based electron emitters and a possible correlation with the Raman response”, ASID’06, Asian Symposium on information Display, 448-451,2006.

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[16]. Niranjana S, B S Satyanarayana , U C Niranjan and Shounak De, “Quantitative and indirect Qualitative analysis approach for Nanodiamond using SEM images and Raman Response”, IFBME Proceedings, Volume 23, 782-785, ISSN 1680-0737, ISBN 978-3-540-92841-6 (Online), 782-785,2008.

BIOGRAPHY:

Niranjana S received the BE (E&C) and M.Tech (BME) degree from Mangalore University. He is a research student of Manipal University, Manipal. Currently, he is working as a Selection Grade Lecturer at MIT, Manipal University, Manipal. His interests are in Nanotechnology, Nanocarbons and Pattern Recognition. Dr. B. S. Satyanarayana, holds PhD. degree in Electrical Engg. from Cambridge University, UK. He has worked for 25 years in India and abroad, with collaborators in industry, R&D labs, research institutions and government organizations in US, UK, Japan, Korea and Russia. Successfully implemented many projects. He has worked in India and abroad with collaborators in Industry, R& D labs and research institutions from UK, Japan, US, Korea and Russia. Worked for Rodel-Nitta Japan,

Sistec Co. Ltd. Industry-Academic Collaborative Research Centre, Kochi Technical University, Japan, Cambridge University – Cambridge UK, Environment, Science & Technology Office, US Embassy-New Delhi, N.P.L –New Delhi, HARTRON- Ambala, Kurukshetra University and Manipal University. He has more than 100 research publications in Journals and Conference Proceedings , delivered over 40 invited talks in international conferences and workshops. The area of interests include Nanotechnology, vacuum nanoelectronics, large area microelectronics and flexible electronics, flat panel displays, photovoltaics, energy harvesting and scavenging technologies, instrumentation, MEMS, S&T and higher education policy, networks & sensors, aviation, defense and thin film & vacuum technology.

Dr. Niranjan U. C., holds PhD in Electrical Science from IISc, Bangalore. He is Director of Research and Training at MDN (Manipal Dot Net Pvt Ltd) Manipal and Adjunct Professor, Dept of E&C, MIT, Manipal. He brings more than 20 years of experience and passion in signal-processing. Well known as a teacher and senior researcher, his students have spread far and wide. He is senor member of IEEE and past national secretary of BMESI. He was guest faculty at Mangalore University, NMAMIT Nitte, and Karmic Manipal. Published several international and national

Journals. He is also consultant and advisor to many industries/students. Shounak De is Presently working as a Lecturer at Manipal Institute of Technology, a constituent institution of Manipal University, Manipal. He obtained his M.Tech Degree from Manipal Institute of Technology in the year 2002.He obtained B.E Degree from T.C.E, Gadag, Visveswariah University in the year 1998.His interested areas are Nanomaterials, Microelectronics Devices, and Amorphous Semiconductors. Presented /published as a author/co-author for 10 research papers in national /international conferences.

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