Nanoscale Silicon as a Catalyst for Graphene Growth: Mechanistic

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Nanoscale Silicon as a Catalyst for Graphene Growth: Mechanistic Insight from In-Situ Raman Spectroscopy Keith Share, Rachel E. Carter, Pavel Nikolaev, Daylond Hooper, Landon Oakes, Adam P. Cohn, Rahul Rao, Alexander A. Puretzky, David B. Geohegan, Benji Maruyama, and Cary L. Pint J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03880 • Publication Date (Web): 08 Jun 2016 Downloaded from http://pubs.acs.org on June 22, 2016

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The Journal of Physical Chemistry

Nanoscale Silicon as a Catalyst for Graphene Growth: Mechanistic Insight from In-Situ Raman Spectroscopy Keith Share,1,2 Rachel E. Carter,2 Pavel Nikolaev,3 Daylond Hooper,3 Landon Oakes,1,2 Adam P. Cohn,2 Rahul Rao,3 Alexander A. Puretzky,4 David B. Geohegan,5 Benji Maruyama,3 Cary L. Pint*,1,2 1

Interdisciplinary Materials Science Program, 2Department of Mechanical Engineering,

Vanderbilt University, Nashville, Tennessee 37235, USA 3

Air Force Research Laboratory, Materials and Manufacturing Directorate, RXBN, WPAFB,

Ohio 45433, USA 4

Center for Nanophase Materials Sciences and Materials Science Division, Oak Ridge National

Laboratory, Oak Ridge, TN, USA

ACS Paragon Plus Environment

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ABSTRACT

Nanoscale carbons are typically synthesized by thermal decomposition of a hydrocarbon at the surface of a metal catalyst. Whereas the use of silicon as an alternative to metal catalyst could unlock new techniques to seamlessly couple carbon nanostructures and semiconductor materials, stable carbide formation renders bulk silicon incapable of the precipitation and growth of graphitic structures. Here, we provide evidence supported by comprehensive in-situ Raman experiments that indicates nanoscale grains of silicon in porous silicon (PSi) scaffolds act as catalysts for hydrocarbon decomposition and growth of few-layered graphene at temperatures as low as 700 K. Self-limiting growth kinetics of graphene with activation energies measured between 0.32 – 0.37 eV elucidates the formation of highly reactive surface-bound Si radicals that aid in the decomposition of hydrocarbons. Nucleation and growth of graphitic layers on PSi exhibits striking similarity to catalytic growth on nickel surfaces, involving temperature dependent surface and subsurface diffusion of carbon. This work elucidates how the nanoscale properties of silicon can be exploited to yield catalytic properties distinguished from bulk silicon, opening an important avenue to engineer catalytic interfaces combining the two most technologically-important materials for modern applications – silicon and nanoscale carbons.


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INTRODUCTION Graphene growth directly on semiconducting and insulating substrates can enable advanced applications in semiconductor electronics.1

Conventional growth of graphene or

carbon nanotubes from metals such as Cu2 and Ni3 involve the capability of the catalyst surface to decompose the hydrocarbon feedstock and precipitate a graphitic lattice in a flat or cylindrical geometry. Theoretical4 and experimental5 results have shown that growth due to bulk or surface diffusion processes are correlated to carbon solubility, and Cu has been lauded for monolayer graphene growth due to its highly catalytic surface and low carbon solubility.6 Since Ni exhibits a higher solubility of C, the bulk metal acts as a reservoir for carbon yielding temperaturedependent growth dominated by surface diffusion at low temperatures and bulk precipitation at higher temperatures.7-11 On another front, researchers have observed the thermal formation of carbon on silicon surfaces12-14 and attributed it to a low carbon solubility of silicon15 even though the diffusivity of C in Si is actually quite high. In fact even at temperatures below 1000°C, a near surface C concentration forms that is higher than predicted from the bulk phase diagram.16-17 Additionally, there have been studies on the graphitization of nonmetallic Si/SiO2 substrates, but the role of the surface and growth mechanism remains elusive.18-20 Despite the lack of evidence for the mechanistic underpinning of this process, the ability to produce graphene coatings on nanoscale silicon produced a range of high performance electrochemical energy storage and conversion systems.21-24 Of the many routes to study growth processes, Raman spectroscopy is arguably the most useful technique to probe the local bonding environment in silicon and carbon materials. Raman


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modes provide fingerprints of carbon to track the thermodynamics and kinetics of in-situ carbon growth processes, such as for single-walled carbon nanotubes and graphene.25-27 In-situ monitoring enables the capability to finely distinguish between mechanistic processes occurring in the growth of carbon nanostructures. Here we employ in-situ Raman spectroscopy where laser excitation plays two simultaneous roles: to heat the PSi and to produce a Raman signal. The Raman signal is used to both measure temperature through the Stokes and anti-Stokes signature of silicon and characterize the real-time growth kinetics of carbon on the PSi surface. Our measurements reveal a self-limiting growth process whereby multilayer graphene forms on nanoscale grains of silicon at temperatures as low as 700 K. With hundreds of independent experiments, we isolate a mechanism for graphene growth on nanoscale silicon that bears striking resemblance to that observed on Ni catalysts. We calculate activation energies at three different pressures, which unambiguously support the idea that nanoscale silicon exhibits catalytic properties in a manner that bulk silicon cannot. EXPERIMENTAL SECTION Porous Silicon Etching P+ (100) Si wafers (0.01-0.02Ω*cm) were etched in a homemade etch cell utilizing a HF/H2O/ethanol electrolyte in a 1:1:2.3 ratio and a Pt counter electrode. 45 mA/cm2 etch current densities were applied for 180 seconds to produce uniform porous silicon films that were around 4 µm thick. To image porous silicon before and after growth, a FEI Tecnai Osiris TEM with Energy Dispersive X-ray Dispersive Spectroscopy was used.


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In-situ Raman Studies Graphene growth is carried out using a laser induced CVD within a high-vacuum chamber coupled to a Raman microscope with a 532 nm laser. Each piece of porous silicon was used for multiple experiments. The spots on the PSi were separated by a distance much larger than the spot size in order to avoid carbon contamination. Ethylene (research grade, 99.995% pure), hydrogen (research grade, 99.9999% pure), and Ar (research grade, 99.99997% pure) were flowed through the chamber at partial pressures of 15 torr, 34.8 torr, and the remainder as Ar. The total pressure was adjusted while keeping the gas flows the same for total pressures of 150 torr, 300 torr, and 700 torr. Water vapor was monitored and maintained between 5-8 ppm. The temperature was changed by increasing or decreasing the laser power and was calculated by

ħ

comparing the anti-Stokes and Stokes ratios using = exp (

). Raman spectra between -

1,000 and 3,000 cm-1 were collected at 5 sec intervals during the growth experiments. Graphene growth and nucleation was detected by the appearance of carbon D and G peaks around 1350 cm-1 and 1600 cm-1 respectively. Post growth Raman analysis: All peaks were baselined and normalized to the larger Stokes Si peak from the cool areas of the PSi to enable fair comparison between experiments. This peak never disappears due to carbon coverage. The reduced thermal conductivity of PSi means the Raman spectra is sampling areas with different temperatures. In order to eliminate the cold region, 2 methods were used. The spectrum of the cold PSi (the initial spectrum in each experiment) was subtracted from the hot spectrum and the remaining peak was fit with 2 Lorentzians, one for the hotter region and one for the cooler region. See the SI for more details. The anti-Stokes to Stokes ratio of the hotter peak was used for the temperature calculations. The


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carbon peaks were fit with a single Lorentzian. The area of this peak was calculated using the trapezoidal rule. This peak area was plotted against time to obtain growth curves for the graphene. These are then fitted with exponential decaying equations in order to calculate the growth rate and lifetime (see main text for details). The peak areas and D/G ratio for each temperature is the average of the last 10 data points. In Situ Reflectivity Reflectivity of a HeNe laser on porous silicon was measured via the response of a Keithley power meter in an experimental set up previously described.28 The porous silicon was heated in Ar (1000 sccm) and H2 (200 sccm) from room temperature to 600°C. It took around 10 minutes to reach the temperature set point. C2H2 (10 sccm) was turned on at 950 seconds for 10 minutes. RESULTS AND DISCUSSION Figure 1a illustrates the experimental system utilized to simultaneously measure the substrate temperature, heat the PSi, and grow multilayered graphene. PSi has a lower thermal conductivity than bulk Si that facilitates heating (using a 532 nm laser) up to high temperatures simply by increasing laser power. After the introduction of a gas mixture containing ethylene (C2H4), hydrogen (H2), and argon, the Raman signatures of graphene, namely the in-plane stretching G peak and disorder-induced D peak are observed. Control experiments verify that temperature conditions leading to carbon growth on PSi yield absolutely no carbon formation on bulk Si.


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The Raman spectrum of Si is fit with two Lorentzians representing PSi and bulk Si because the PSi peak is broader and lower in energy than crystalline Si.29-30 Additionally, the full width at half maximum (FWHM) of the PSi peak is ~ 40 cm-1, characteristic of nm-sized crystallites and in good agreement with transmission electron microscope (TEM) analysis.31 Although there is a 2D peak at higher wavenumbers, the defective nature of the graphene combined with a fluorescence at higher wavenumbers reduce the usefulness of this peak (Figure S1).32 Based on the ratio of the Stokes and anti-Stokes peaks for the PSi, substrate temperature can be identified as discussed further in the supporting information (Figure S2). To characterize the in-situ growth of graphene on porous silicon, three different pressures (150 torr, 300 torr, and 700 torr) are studied using different laser powers, generating between 20-80 different temperatures at each pressure. Spectra are collected every 5 seconds with experiments lasting between 400-900 seconds. Figure 1b shows representative Raman spectra for an experiment at 300 torr and 1086 K (813 oC). The 0 sec spectrum measured at low temperature (371 K) exhibits a low-intensity anti-Stokes Si peak and no carbon peaks. By 16 seconds the temperature has increased to 837 K with defined D and G peaks. The growth of the carbon peaks follow an exponential decay curve and begin to saturate over time, indicative of a self-limiting catalytic reaction (Figure 1c). Note that the narrow FWHM of the D and G peaks correspond to graphitic carbon and not amorphous carbon.33 A broad peak at 977 cm-1 corresponds to a multi-phonon mode in Si34 but no SiC peaks (970 cm-1) are observed in any experiments. This is attributed to the nanoscale grain size of PSi, evident from the PSi Raman peak FWHM and TEM images, that catalyze hydrocarbon decomposition and have a highly strained SiC-Si interface that favors carbon precipitation over carbide nucleation.35-36


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A TEM image of the multilayer graphene grown on PSi is shown in Figure 2a (SEM image in Figure S3). Interlayer spacing (002) of 0.389 nm is measured, which is intermediate between highly crystalline few-layered graphene and reduced graphene oxide materials, possibly due to the lattice mismatch between Si and graphene.

Figures 2b,c,e provide quantitative

information about the effect of temperature on graphene growth at 300 torr from in-situ Raman peak fitting. The total carbon peak area (D+G area) over time at 735, 904, and 1060 K (Figure 2b) shows increased graphene formation at higher temperature and a continued slow increase in graphene growth at long durations. D and G peak area plotted against temperature (Figure 2c, bottom) indicates a transition near ~ 700 K that is associated with the onset of catalytic carbon growth. Above the onset temperature, the final D/G area ratio (a measure of defect density) exhibits a quick and then slow increase with growth temperature (Figure 2c, top) with the high defect content attributed to the growth on small silicon crystallites. The trends observed at the other pressures are similar (Figure S4). In-situ reflectivity in Ar and H2 (Figure 2d) shows drops in reflectivity around 60 and 130 seconds attributed to surface rearrangement and roughening and a large increase around 210 seconds near 525 K due to dehydrogenation of the Si surface, leaving exposed and reactive dangling Si bonds that can act as catalytic sites.37-39 Although some oxygen could be present on the PSi, the surface of PSi post etching is H terminated which slowly become oxygen terminated when exposed to air.40-41 PSi samples were stored in an Ar glove box and air exposure was limited. Unlike previous reports of graphene growth at higher temperatures (> 1300 K) where SiO2 is the active surface, this step precedes growth at temperatures comparable to catalytic metals, implying that Si itself is acting as the catalyst.22 Upon C2H2 introduction, the exponential


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decay of the reflectivity is similar to that observed through in-situ Raman experiments, indicating similar growth mechanisms (Figure S5). Time evolution of the D/G area ratio at different temperatures reveals a similar trend observed with Ni catalysts, favoring surface dominated growth at low temperatures and subsurface diffusion and precipitation at higher temperatures.10-11 At lower temperatures the initial D/G area ratio is large (low crystallinity) and quickly decreases, with the opposite trend at higher temperatures. (Figure S6a). The initially large D/G area is attributed to conformal surface growth due to the abundance of reactive PSi nucleation sites and decreases as grains are fully covered with carbon at longer durations (Figure S6b). At higher temperatures the evolution of the D/G area ratio is consistent with a subsurface precipitation mechanism where carbon rapidly diffuses into Si until the solubility limit is reached and precipitation occurs on planar Si faces. In this case, faster growth of the D peak compared to the G peak (Figure S6c) implies the initial formation of pristine graphene islands that become more defective as growth extends over the grain boundaries. To quantify this effect, the initial D/G area divided by the final D/G area is plotted versus temperature (Figure 2e) where values greater than 1 indicate a decrease in D/G ratio over time while values less than 1 signify an increase in D/G ratio over time. Larger values at low temperatures indicate surface nucleation as the growth mechanism whereas smaller values at higher temperatures indicate a precipitation mechanism. Higher temperatures exhibit more subsurface carbon and show a larger increase in the D+G peak area (Figure S7). Although both mechanisms likely occur at all temperatures, our data suggests a transition between the two around 950-975 K as seen by the noticeable change in slope in Figure 2e. To further gain insight into the growth kinetics, the in-situ D+G carbon peak is fit with a decaying exponential indicative of a self-limiting catalytic reaction described by = + +


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exp where is the initial growth rate and is the lifetime (Figure 3b). The growth rate

gives information about the kinetics of the reaction while the lifetime is related to catalyst deactivation processes.42 The logarithm of the growth rates and lifetimes are plotted versus 1/T (Figure 3a) for each pressure, with the slopes of the linear fits representing the activation energy (Ea) of a thermally activated process (Table 1). Similar trends emerge for each pressure with higher temperatures promoting faster growth rates, more graphene growth, and shorter lifetimes. This implies that when the graphene grows more quickly it covers the catalytic surface sites preventing additional carbon decomposition, resulting in shorter growth times.

Activation

energies calculated from the Arrhenius plots are ~0.3 eV, which is similar to the diffusion energy of carbon in Si.36 The activation energy calculated for graphene growth on PSi is compared to experimental and theoretical Ea for other relevant processes such as graphene growth on Cu and Ni, diffusion of carbon in Si, and pyrolysis43 in Figure 3c. Even though the pressure, catalysts, and kinetically limiting processes vary between the different studies, it is apparent that the Ea of graphene on PSi is much lower than the pyrolysis energy of hydrocarbons44 and is similar to the values found for graphene growth on metal catalysts. The activation energy for different steps of carbon diffusion in Si such as diffusion of carbon interstitials, Si vacancies, and Si interstitials are all below 1.0 eV, again pointing towards a diffusion limited growth.36 This proves that a gas phase reaction, such as the pyrolysis of C2H4 (Ea ~6.5 eV),45 is catalytically driven by the PSi. We propose the limiting step in graphene growth on PSi to be a diffusional process, either surface diffusion of carbon clusters or diffusion of subsurface carbon in Si, both of which have similar activation energies. A schematic of the proposed mechanism of graphene growth on PSi is shown in Figure 4a.

The growth process contains three temperature dependent steps: gas decomposition,


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nucleation, and growth. First, heating the PSi causes dehydrogenation of the surface, leaving reactive Si bonds that can drive the decomposition of the carbon precursor gas and create surface carbon species. At low temperatures, slow diffusion of carbon into the PSi yields surface-driven graphene growth that evolves from being defective to more crystalline as the Si grains are covered. At higher temperatures, in-diffusion of carbon leads to precipitation when the solubility limit is reached, as opposed to carbide formation which occurs in bulk silicon. In this case, even when the catalytic surface is covered, more graphene layers form because of continued subsurface precipitation. TEM images and corresponding energy-dispersive X-ray spectroscopy (EDS) maps (Figure 4b) at the early stages of graphene growth at 1123 K show carbon nucleation directly on the Si (111) and (200) lattice (Figure Sds). Figure 4c reveals many smaller graphene domains during the defect driven nucleation at 823 K. CONCLUSION These results prove that graphene growth on PSi is a catalytic process unique to nanoscale silicon materials with low activation energy of 0.32 – 0.37 eV. Most notably, the observations in this work for nanoscale silicon exhibit a striking resemblance to the mechanisms and temperatures where carbon growth on metal catalysts occurs. This unlocks a vast body of knowledge connecting the results of this work to other metal catalyzed growth,46 enabling advanced techniques such as preheating precursors to form polyaromatics47 as pathways to improve carbon quality from levels observed in this study. This work also implies that nanoscale silicon is a candidate for carbon nanotube growth, and could pave routes toward metal-free silicon interconnects for semiconductor electronics.

In a broader context, tuning catalytic

properties of silicon by exploiting the size-dependent thermodynamic phase diagram can open a new window to engineer materials for advanced applications.


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ASSOCIATED CONTENT Supporting Information is available free of charge on the ACS Publications website and contains: (i) Silicon peak fitting and temperature calculations, (ii) SEM of carbonized PSi, carbon growth data for 150 and 700 torr, (iii) normalized reflectivity and Raman growth, (iv) high and low temperature growth and nucleation mechanisms, (v) carbon precipitation, and (vi) TEM and electron diffraction pattern of the Si lattice during the early stages of graphene growth. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *(C.L.P) [email protected], +1-615-322-3720 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by NSF award CMMI 1334269 and an ASEE Summer Faculty Fellowship carried out at AFRL/RXBN.

K.S. and A.P.C. are supported by NSF GFRP

fellowships grant # 1445197. Characterization science for this work was in part sponsored by the Materials Sciences and Engineering (MSE) Division, Office of Basic Energy Sciences, U.S. Department of Energy.


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36. Pinacho, R. a.; Castrillo, P.; Jaraiz, M.; Martin-Bragado, I.; Barbolla, J.; Gossmann, H.-J.; Gilmer, G.-H.; Benton, J.-L., Carbon in Silicon: Modeling of Diffusion and Clustering Mechanisms. J. Appl. Phys. 2002, 92, 1582-1587. 37. Sailor, M. J., Fundamentals of Porous Silicon Preparation. Porous Silicon in Practice: Preparation, Characterization and Applications 2012, 1-42. 38. Tsai, C.; Li, K. H.; Sarathy, J.; Shih, S.; Campbell, J.; Hance, B.; White, J., Thermal Treatment Studies of the Photoluminescence Intensity of Porous Silicon. Appl. Phys. Lett. 1991, 59, 2814-2816. 39. Tsybeskov, L.; Fauchet, P., Correlation between Photoluminescence and Surface Species in Porous Silicon: Low‐Temperature Annealing. Appl. Phys. Lett. 1994, 64, 1983-1985. 40. Mawhinney, D. B.; Glass, J. A.; Yates, J. T., Ftir Study of the Oxidation of Porous Silicon. J. Phys. Chem. B 1997, 101, 1202-1206. 41. Gräf, D.; Grundner, M.; Schulz, R.; Mühlhoff, L., Oxidation of Hf‐Treated Si Wafer Surfaces in Air. J. Appl. Phys. 1990, 68, 5155-5161. 42. Picher, M.; Anglaret, E.; Arenal, R.; Jourdain, V., Self-Deactivation of Single-Walled Carbon Nanotube Growth Studied by in Situ Raman Measurements. Nano Lett. 2009, 9, 542547. 43. Homer, J. B.; Kistiakowsky, G., Oxidation and Pyrolysis of Ethylene in Shock Waves. J. Chem. Phys. 1967, 47, 5290-5295. 44. Kruse, T.; Roth, P., Kinetics of C2 Reactions During High-Temperature Pyrolysis of Acetylene. J. Phys. Chem. A 1997, 101, 2138-2146. 45. Skinner, G. B.; Sokoloski, E. M., Shock Tube Experiments on the Pyrolysis of Ethylene. J. Phys. Chem. 1960, 64, 1028-1031. 46. Hofmann, S.; Braeuninger-Weimer, P.; Weatherup, R. S., Cvd Enabled Graphene Manufacture and Technology. J. Phys. Chem. Lett. 2015, 6, 2714-2721. 47. Somekh, M.; Shawat, E.; Nessim, G. D., Fully Reproducible, Low-Temperature Synthesis of High-Quality, Few-Layer Graphene on Nickel Via Preheating of Gas Precursors Using Atmospheric Pressure Chemical Vapor Deposition. J. Mater. Chem. A 2014, 2, 1975019758.


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FIGURES

Figure 1. a. Schematic representing the adaptive rapid experimentation and spectroscopy (ARES) system used for in-situ studies of graphene growth by comparing the Stokes and antiStokes intensity ratio to calculate temperature and monitoring the growth of the D and G carbon peaks. b. Selective Raman spectrum taken at 300 torr and 1086 K showing the evolution of the silicon Stokes and anti-Stokes peaks and the carbon peaks. c. All of the Raman spectrum of the carbon peaks taken during one experiment at 300 torr and 1086 K.


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Figure 2. a. TEM image of the early stages of graphene growth on PSi, scale bar = 5 nm. b. The total carbon peak area as a function of time for 3 different temperatures. c. Bottom, The final areas of the G and D peaks over the temperature range studied. Top, Final D/G intensity ratio as a function of temperature. d. In-situ reflectivity of graphene growth on PSi using a ramp temperature process. e. Initial D/G area / final D/G area at different temperatures.


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Figure 3. a. Arrhenius plots of the D+G peak growth rate () and lifetime () vs 1/T. The closed data points are the growth rate values (, left axis) while the open data points are the lifetime values (, right axis) b. Representative exponential decay fit for a sample at 300 torr and 1060 K. c. Activation energies of related processes.


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Figure 4. a. Schematic representing the growth mechanism for graphene on PSi showing the differences between the high temperature precipitation and low temperature surface nucleation mechanisms. b. left, TEM (5 nm scale bar) image and corresponding EDS map (inset, 10 nm scale bar) for the early stages of graphene growth at 1123 K on the (111) and (200) Si planes. The box in the EDS map indicates the location of the TEM image. c. EDS map of graphene nucleation at 823 K (5 nm scale bar).


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TABLES.

Ea (eV)

150 torr

300 torr

700 torr

0.37

0.32

0.32

Table 1: Activation energies measured through in-situ Raman spectroscopy experiments for graphene growth on PSi at three independent pressures.


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TABLE OF CONTENTS GRAPHIC


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Nanoscale Silicon as a Catalyst for Graphene Growth: Mechanistic

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