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Photoluminescent SiC tetrapods



Andrew P Magyar, I Aharonovich, Mor Baram, and Evelyn L Hu



Nano Lett., Just Accepted Manuscript • DOI: 10.1021/ nl304665y • Publication Date (Web): 22 Feb 2013



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Photoluminescent SiC tetrapods Andrew P. Magyar,∗,†,‡ Igor Aharonovich,†,¶ Mor Baram,† and Evelyn L. Hu† Harvard School of Engineering and Applied Sciences, Cambridge MA E-mail: [email protected]

KEYWORDS: SiC; tetrapod; seeded CVD; fluorescent nanoparticle; Abstract Recently, significant research efforts have been made to develop complex nanostructures to provide more sophisticated control over the optical and electronic properties of nanomaterials. However, there are only a handful of semiconductors that allow control over their geometry via simple chemical processes. Herein, we present a molecularly seeded synthesis of a complex nanostructure, SiC tetrapods, and report on their structural and optical properties. The SiC tetrapods exhibit narrow linewidth photoluminescence at wavelengths spanning the visible to near infrared spectral range. Synthesized from low-toxicity, earth abundant elements, these tetrapods are a compelling replacement for technologically important quantum optical materials that frequently require toxic metals such as Cd and Se. This previously unknown geometry of SiC nanostructures is a compelling platform for biolabeling, sensing, spintronics and optoelectronics.

Semiconductor nanocrystals have enabled or transformed research in technologically important areas including biolabeling, 1–3 solid state lighting, 4,5 and quantum information. 6,7 The desirable ∗ To

whom correspondence should be addressed School of Engineering and Applied Sciences, Cambridge MA ‡ Center for Nanoscale Systems, Harvard University, Cambridge MA ¶ School of Physics and Advanced Materials, University of Technology Sydney, Sydney, Australia † Harvard

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optical properties of quantum dots arise from the nanoscale confinement of excitons within semiconductor nanoparticles. The emission properties of quantum dots can be controlled by adjusting their size, altering the material composition, or by building core-shell type structures to improve exciton confinement. Increased structural complexity of a nanostructure can yield even greater control over its excitonic and optical properties. 8 Highly symmetric four-legged nanostructures known as tetrapods are an example of a more complex nanocrystal morphology that can exhibit properties that reach beyond those of a simple spherical quantum dot. 9 Specifically, the tetrapod structure allows for spatially indirect excitonic transitions with control over the carrier behavior and localization through manipulations of the tetrapod geometry. 10 Employing traditional colloidal techniques, tetrapods have been synthesized from numerous different materials, including ZnO, 11–13 CdS, 14 CdTe, 15,16 and Pd, 17 with remarkable control over the tetrapod geometry. SiC tetrapods have been proposed, 15 but never synthesized, in part because the synthesis of SiC using traditional colloidal synthesis methods is challenging. 18 Eliminating the need for a colloidal synthesis, we develop a technique employing microwave plasma enhanced chemical vapor deposition (PECVD) to grow SiC tetrapods from an adamantane seed embedded in a silica sol gel. SiC has exceptional material properties, noted specifically for its mechanical strength, high refractive index, and wide band gap. 19 As a result SiC has recently drawn interest for applications in photonics 20,21 and spintronics. 22 Recent work studying beaded SiC nano-rings exemplifies the rich optical properties possible from SiC nanomaterials. 23 The rich polymorphism of SiC in conjunction with the variation in band gaps and polarizabilities of its polytypes provides a rich parameter space for the design of tetrapods having complex excitonic structure and tunable optical behavior. The synthesized SiC tetrapods exhibit strong, visible, sub-band gap photoluminescence (PL), which is attributed to spatially indirect exciton transitions arising from a homomaterial heterojunction within the tetrapod. The arrangement of carbon atoms in cubic (3C) SiC is congruent with the tetrahedral structure of diamond. 19 Adamantane is the smallest molecule having the tetrahedral structure of diamond and can be used to seed the CVD growth of diamond films on silicon. 24–27 Indeed, it is suggested


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Figure 1: A schematic of the tetrapod synthesis and scanning electron micrographs of tetrapods. (a) Adamantane, the smallest diamondoid molecule, is embedded in a sol gel film deposited on SiO2 on Si. The adamantane serves as a seed for the growth of SiC tetrapods during microwaveenchanced PECVD. (b) Tetrapods as grown on the substrate, showing the different sizes and orientations of the nanocrystals. (c) A histogram showing the distribution of leg diameters and lengths. (d) A top-down view of a single tetrapod. The hexagonal symmetry of the legs is observed for the fourth leg of the tetrapod, which is normal to the plane of the image. 3 ACS Paragon Plus Environment

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that SiC is formed as an intermediary during the adamantane-seeded growth of diamond on Si. 24 In our work, we exploit these crystallographic similarities by using adamantane to seed the growth of SiC tetrapods: a schematic of this process is shown in Figure 1a. Briefly, adamantane embedded in a methyltrimethoxysilane (MTMS) sol gel matrix is grown by microwave PECVD in a methane and hydrogen plasma. The encapsulation of the adamantane with in the sol gel matrix prevents its instantaneous sublimation under the low pressure and elevated temperature in the reactor. Additionally, the MTMS provides a source of reactive silicon for the synthesis. The synthesis is detailed in the methods section. The samples are characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 1b shows an SEM image of the tetrapods. The growth yields approximately 25 tetrapods per µ m2 , with a higher tetrapod density near cracks in the sol gel film. The tetrapods exhibit some variation in size and symmetry and do not exhibit a preferred growth orientation. The tetrapod size distribution for three hours of growth is shown in Figure 1c. The legs are 40 ± 10 nm in diameter and 90 ± 20 nm long. The legs of the tetrapods have a hexagonal cross-section, as seen from the top-down image of a single tetrapod depicted in Figure 1d. The tetrapods only grow in the presence of the adamantane seed molecule, while substrates with MTMS but without adamantane do not show any nanoparticles after growth under the same conditions. Similarly, no tetrapods are observed by performing SEM analysis on the adamantane powder only. Therefore, we conclude that the adamantane is critical to the formation of the tetrapods, acting as a seed during the PECVD growth. The morphology of an individual tetrapod as observed by TEM is shown in Figure 2a. Two of the legs of the tetrapod are clearly visible, while the third and fourth legs overlap. A schematic of this tetrapod is shown as an inset in Figure 2a. Examining the lattice planes of tetrapod legs with varied orientations with respect to the beam enabled the measurement of d-spacings corresponding to three different lattice planes of 4H-SiC. A magnified view of the tetrapod shown in Figure 2a is shown in Figure 2b; with two different lattice planes measured from the different legs of the ¯ tetrapod: {0004}, aligned perpendicular to the direction of the bottom leg and {1102} at 35◦ with


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Figure 2: Transmission electron micrographs showing tetrapod crystal structure. (a) A single tetrapod, with two arms overlapping on the right hand side. (b) A magnification of the tetrapod in (a), showing the d-spacing for legs with two different orientations with respect to the electron beam. The insets show the d-spacings and the corresponding family of planes for 4H-SiC. (c) A leg of a tetrapod, with the TEM lattice fringes parallel to orientation of the leg. (d) The lattice fringes looking down the axis of a tetrapod leg. The inset shows the FFT pattern for these fringes.


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respect to the direction of the left leg, as shown in the inset. The leg of another tetrapod, with ¯ lattice planes, parallel to the direction of the leg, is lattice fringes corresponding to the {1010} shown in Figure 2c. Figure 2d shows the view of a different tetrapod looking down one of the ¯ and {1100} ¯ legs; from the fast Fourier transform (FFT) of the marked region, the {0110} planes are measured. The TEM data conclusively indicate that the legs of the tetrapods match the 4H-SiC crystal structure (wurtzite). To understand the formation of the SiC tetrapod nanocrystals, we considered tetrapod growth mechanisms reported in the literature. The morphology of the SiC tetrapods, together with the moderate energy difference between 3C (zinc blende) and 4H (wurtzite) phases of SiC suggests that the tetrapods likely grow from a cubic core similar to the model for CdTe tetrapods described by Manna et al. 15 Tetrapods can also grow by other mechanisms; legs can grow from an octahedrally twinned wurtzite core 11,13,16 or tetrapods can be formed due to a rhombohedral crystal structure. 17 Tetrapods growing from a zinc blende core are expected to exclusively form tetrapods, as the hexagonal legs grow equivalently from the four identical facets of the core. 14–16 With a twinned nucleus, the number of legs depends on the number and types of twins in the core, yielding a distribution of tetrapods with different numbers of legs. 16 The preponderance of tetrapods ( 97 %) grown by our technique have four legs, further suggesting that the tetrapods have a zinc blende core. The few tetrapods that have fewer than four legs may arise from impediments to growth of a particular leg, such an effect would be more prevalent in this synthesis than those in the literature because growth takes place on a surface rather than colloidally. Alternatively, tetrapods can break after/during growth, leaving behind single legs. Unfortunately, direct observation of the core structure via electron microscopy was not possible with our imaging conditions because this measurement requires a very specific orientation of the tetrapod to eliminate interference from the arms. In a colloidal synthesis of CdTe tetrapods, for instance, the morphology is controlled by the ratio of the precursors such as Cd and Te in solution. 15 Similarly, the preference for growing a particular polytype of SiC in a microwave plasma is strongly influenced by the Si:C ratio, with


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4H being the preferred phase in a carbon-rich environment. 28 Indeed, the presence of 4H-SiC is surprising, because growth of the 4H polytype typically requires temperatures of at least 1600 K, 28,29 while the temperature of growth in the PECVD chamber reaches a maximum temperature of 1200 K under our experimental conditions. Previous reports suggest that the microwave plasma can provide enough additional energy to promote the formation of hexagonal SiC. 28 The following model is proposed for the PECVD growth of SiC tetrapods. Cubic SiC is nucleated from adamantane in the Si-rich sol gel environment. As the growth continues the sol-gel is etched away revealing the 3C-SiC nanocrystal and exposing it to the methane-rich plasma. With most of the sol gel etched away, the growth environment is carbon-rich, which promotes a transition to the growth of 4H-SiC. The cubic and hexagonal polytypes of SiC have substantially different band gaps; 3C-SiC has a band gap of about 2.3 eV while 4H-SiC has a band gap of 3.2 eV. Subsequently, the heterojunction at the interface between the core and legs of the tetrapod together with the quantum confinement arising from the nanoscale geometry yields a structure with a complex and intriguing electronic structure. In previous works, the differences in band gap between polytypes of SiC have been exploited for ‘bandgap engineering.’ Such properties have been observed from molecular beam epitaxy (MBE) grown superlattices of structures of SiC, 30 where a heterostructure is created from different polytypes of the same material. In the tetrapod, the heterojunction created between the 3C core and 4H legs can create a confinement potential localized at the 3C core. The valence and conduction band offsets are both large enough to confine the exciton even at room temperature. 29,31–33 Additionally, the excitons in tetrapods are confined due to the nanoscale geometry of the structure. The tetrapod structure lends itself to a complex excitonic model. 8,34 For example, in CdTe tetrapods the electron is localized in the core and the hole is localized in the legs for the lowest energy exciton, but for the second lowest excitonic state both the electron and the hole are localized in the leg. Creating tetrapod heterostructures having a core of one material and legs of another provides even greater control over the emission properties of tetrapods. 10 The band structure of SiC tetrapods is even more complex than that of previously studied


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tetrapods. The difference in spontaneous polarization between the 4H and 3C regions in SiC tetrapods can create a strong internal electric field. The facets of the 3C core are expected to have different surface terminations leading to differential polarization of the 4H-SiC arms. The tetrapod is modeled using finite element methods (Comsol), with two legs having net polarization pointing into the core and two legs with net polarization pointing away from the core. In SiC quantum wells, the electric field influences the band structure creating a triangular quantum well leading to spatially indirect exciton transitions between electrons in the 3C region to holes in the 4H-SiC. 29,32,33 Applying the computed electric field, the band structure is determined for a line drawn between two legs of the tetrapod, passing through the core, as shown in Figure 3a. As in the quantum well structure, the band structure exhibits a triangular confinement potential, suggesting that spatially indirect excitons, labeled Ei in Figure 3a, may also occur in the tetrapod system. To verify our model, photoluminescence (PL) studies have been carried out on the tetrapods. A custom built confocal micro-photoluminesence setup is used to characterize the optical emission from the SiC tetrapods at room temperature. The tetrapods are excited at 532 nm (2.33 eV), below band gap for either polytype of SiC. The excitation source provides enough energy to excite an electron from the leg to the top of the 3C region, Ed , Figure 3a. The internal electric field can then sweep the carrier into the triangular well. Figure 3b shows representative PL spectra, recorded at room temperature from individual tetrapods. The emission maximum varies for different tetrapods, appearing between 550 nm and 800 nm and having a full width half maximum (FWHM) of ∼ 5 nm. The variation in PL is believed to arise from structural differences among the different tetrapods, specifically variation in arm size and symmetry, as shown in Figure 1b and c. In very high density regions of tetrapods only broadband luminescence is observed, likely because of signal averaging of luminescence from many tetrapods. Since the PL is sub-band for 3C-SiC, the emission is consistent with spatially indirect exciton transition from the 3C core to the 4H legs. In this model, changes in the core diameter and internal electric field dramatically influence the energy of the exciton, elucidating the large variation in emission wavelength between tetrapods. The PL FWHM of the SiC tetrapods are narrower


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Figure 3: Band structure and optical properties of the SiC. (a) A schematic of the band structure for the SiC tetrapod. The spontaneous polarization of the 4H-SiC results in electric fields in both the 4H and 3C regions creating a triangular quantum well-like confinement potential and enabling indirect exciton transitions (Ei ). (b) The PL spectra of several different tetrapods are shown. The PL peaks are all sub-band gap and attributed to the Ei transition. Variation in emission wavelength arises from structural differences between tetrapods. (c) PL spectra of a single tetrapod acquired at different pump powers. The inset shows the variation in peak amplitude (blue squares, left axis) and wavelength (red triangles, left axis) with excitation power. At high power a discrete jump in wavelength is observed. This spectral diffusion is detailed in Figure 4. (d) A micrograph showing fluorescence lifetime measurements for 3 different tetrapods. A large variation in lifetime, between 1.4 ns and 6.3 ns, is observed, possibly arising from the morphological differences among the tetrapods.


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than the room temperature emission from colloidal quantum dots or other tetrapods. Reported defect luminescence in SiC is predominantly in the IR and observed only at cryogenic temperature, consequently the observed photoluminescence from the tetrapods can be better understood based on a quantum confined heterojunction formed by the nanoscale structure of the tetrapod. Figure 3c shows emission spectra from a single tetrapod as a function of excitation laser power. The power dependence of the tetrapod photoluminescence exhibits sub-linear behavior, best fit to Pn with n = 0.7, where P is the excitation power (Figure 3c, inset). The sublinear character can indicate saturation of the tetrapod excited state or an alternative mechanism limiting radiative emission, such as Auger recombination. The saturation power is similar for different tetrapods, ∼ 2 mW, however the maximum emission rate varies between tetrapods. The fluorescence lifetime, recorded from individual tetrapods using a pulsed 532 nm excitation source, are shown in Figure 3d. The lifetime varies between tetrapods with values ranging from 1.4 ns (blue curve) to 6.3 ns (red curve). The variation in lifetime may be due to different non-radiative pathways among the different tetrapods or arise from tetrapod size variation. Spatially indirect transitions in tetrapods having a larger 3C region would exhibit longer radiative lifetimes. Spectral diffusion between two closely spaced, discrete emission states of the emission wavelength is measured at high excitation powers, as shown in Figure 4a. Interestingly, the emission occurs with near equal probability from either state, with an average time in each state of ∼ 5 s before spectrally diffusing. Faster spectral jumps below our resolution measurements are possible, such jumps would result in broadening of the emission line. The FWHM of the emission, Figure 4b, fluctuates independent of state, although negative spikes in the FWHM seem to correlate with transitions between states. Future investigation of the spectral diffusion could provide deeper insights into the complex electronic structure of these tetrapods. We have engineered, for the first time, a new nanoscale geometry of SiC - the tetrapod. This synthesis is carried out employing the smallest diamondoid molecule (adamantane) embedded in a solgel matrix and exposed to a microwave plasma CVD growth. The CVD synthesis does not preclude colloidal or biological applications; the tetrapods can be collected aqueously and retain


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Figure 4: Spectral diffusion of tetrapod PL. (a) The evolution of the fluorescence spectrum with time showing switching of the emission maximum between two discrete states. The plot is overlayed with example spectra of the two emission states. (b) The evolution of the peak emission wavelength (fit to a lorentizan) and the FWHM with time. No significant difference in FWHM is observed for the different states, however small dips in the FWHM may correlate with switching between states.


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their structural morphology and optical properties. Through detailed investigation of the structural and compositional properties of the SiC tetrapods, it is determined that the tetrapods have 4H arms and likely grow from a 3C core. We expect that through the controlled introduction of gaseous silane precursors into the reactor and with temperature regulation of the substrate, precise control over the SiC tetrapod structure and geometry will become possible. Remarkably, the SiC tetrapods exhibit room temperature PL having a line width of less than ∼ 5 nm, better than any commercial quantum dots or other known tetrapods. The photoluminescence is best explained by a quantum confined heterojunction created by the nanoscale geometry of the tetrapod. Moreover, the excited state lifetime of the tetrapods is relatively short (∼ 1-6 ns) allowing rapid modulations. Therefore, the SiC tetrapods exhibit tremendous promise as a functional nanomaterial, to be exploited in photonics, biosensing and optoelectronic applications.

Acknowledgement The authors thank D.R. Clarke, J. Joo, J. C. Lee, T.-L. Liu and K. J. Russell for discussion and D. C. Bell for TEM assistance on this work. The authors acknowledge the use of the NSF/NNIN facilities at Harvard University’s Center for Nanoscale Systems.

Supporting Information Available Detailed procedures describing tetrapod synthesis and analysis. This material is available free of charge via the Internet at http://pubs.acs.org/.

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