Longitudinal optical phonon–plasmon couplingin luminescent 3C–SiC nanocrystal films
L. Z. Liu,1 J. Wang,1 X. L. Wu,1,* T. H. Li,1 and Paul K. Chu2
1Nanjing National Laboratory of Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China2Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
Silicon carbide (SiC) is one of the most attractive nanos-tructures owing to its potential applications in semicon-ductor technologies and biophysics [1–4]. Recently,various types of SiC nanostructures with different mor-phologies have been synthesized [5–12] and the asso-ciated phonon properties have been investigated [13,14].The variations in phonon frequency and damping (line-width) observed from many Raman spectra are believedto arise from quantum confinement and the bond-angledistortion induced by the local strain in the imperfectmicrostructure [15,16]. In fact, the complicated surfacestructures of SiC nanocrystals (NCs) affect the phononproperties.Previously in some Raman studies on different types of
SiC nanostructures, the transverse optical (TO) mode in-tensity is higher than that of the longitudinal optical (LO)mode [11,16]. This phenomenon is mainly ascribed to thesuppression effect on the LO mode as a result of the im-perfect microstructure or surrounding disordered atoms[17]. However, experiments have disclosed that the LOmode intensity is higher than that of the TO mode. Thisphenomenon has been explained by quantum confine-ment or defects induced by the inner stress during theNC growth [18], but the intensity and damping variationmechanisms associated with the TO and LO modes arenot clearly understood [19,20]. In this Letter, we fabricatea 3C–SiCNC solid filmwith tunable blue emission. TheNCsurfaces are passivated with a thin glycerol layer. The in-tensity of the LO mode is far higher than that of the TOmode, and the origin is determined by considering thecoupling between the LO phonon and plasmon.The preparation of the 3C–SiC NCs and glycerol-passi-
vated 3C–SiC solid films has been described previously[9,10]. Transmission electron microscopy [(TEM) JEOLJEM-2100] and x-ray photoelectron spectroscopy (XPS)were conducted to analyze the film samples [9–11]. TheRaman spectra are obtained on aT64000 triple Raman sys-tem at a backscattering geometry using the 514:5 nm lineof anAr-ion laser as the excitation source. The diameter ofthe beam spot is 5 μm, and the power illuminating the
sample is 5 mW. The resolution of the spectrometeris 0:1 cm−1.
Figure 1(a) depicts the TEM image of the glycerol-passivated 3C–-SiC NCs, which are almost sphericalthanks to surface-free energy minimization. The diam-eters of most of the NCs are in between 2.0 and 6:5 nm,although some NCs are larger than 10 nm (see circles).Figure 1(b) exhibits the high-resolution TEM image oftwo representative NCs with the lattice fringes corre-sponding to the f111g and f200g planes of 3C–SiC. TheC 1 s core-level XPS spectrum acquired from the glycer-ol-absorbed 3C–SiC NC film on a silver thin film is shownin Fig. 1(c). The strongest peak at 283:1 eV can be as-cribed to the SiC component [21,22], and the twoshoulders on the high-energy side at 284.6 and 286:4 eVcorrespond to CHn and O-CH3 [23], respectively. Thestrong shoulder peak (denoted as CS) on the low-energyside at 280:8 eV is related to alkoxide [24]. TheXPS resultsindicate that the NC surfaces are indeed bonded to glycer-ol molecules [10]. Raman spectra acquired from the non-passivated and passivated NCs with small and large sizes
Fig. 1. (Color online) (a) TEM image of the glycerol-passivated 3C–SiC NCs. Some large NCs are marked withred circles. (b) HRTEM image. (c) C 1 s core level XPS spec-trum acquired from the glycerol-passivated 3C–-SiC NC filmdeposited on a silver film substrate.
are shown in spectra (a)–(c) in Fig. 2, respectively.We cansee from spectra (a) and (b) obtained from small NC re-gionswith sizes of 2–6 nm that the TO and LOmodes haveenhanced intensities and shift from 788.2 to 790:4 cm−1
and from 941.6 to 964:8 cm−1, respectively. This behaviorcan be explained by the larger NC size. If the NC size islarger than 15 nm [region 1 in Fig. 1(a)], the TO and LOmodes further upshift to 797.0 and 972:6 cm−1 [spectrum(c)], respectively. These Raman peak positions approachthose of bulk 3C–-SiC. However, it is surprising that inspectrum (c), the TO mode intensity is evidently lowerbut the LO mode intensity is significantly enhanced andeven far larger than that of the TO mode. This LO modebehavior has been rarely observed from previous Ramanspectra [11,16,17]. Generally, the LO mode intensity is al-ways smaller than that of the TO mode. This result indi-cates that the influence of surface passivation on theLO mode property is strongly NC size dependent. In addi-tion, the weak Raman peak (marked as S) at 695:4 cm−1 isprobably related to amorphous SiC [14].According to the phonon-confinement model proposed
by Campbell and Fauchet [25], a smaller NC size and lar-ger disorder cause progressive relaxation of q ¼ 0 in theselection rule, and the optical phonons at q ≠ 0 also con-tribute to Raman scattering. This leads to a downshift andbroadening in the Raman spectra. Here, the lineshape ofthe first-order Raman spectrum is calculated by [25]
IðωÞ ∝Z
1
0
expð− q2D2
16π2Þ½ω − ωðqÞ�2 þ ðΓ0=2Þ2
d3q;
where q is expressed in units of 2π=a0 and a0 is the 3C–SiClattice constant. The parameter Γ0 is the FWHM of theRaman peak of the TO (LO) phonon of bulk crystalline3C-SiC, and ωðqÞ describes the phonon dispersion in
the Brillouin zone along the f111g direction in bulk crys-talline 3C–SiC [26]. Thedispersion equations of LOandTOphonons can be expressed as
ωðqÞ ¼ 972:8þ 12:0q − 325:3q2 þ 275:7q3 − 363:2q4
þ 269:2q5;
ωðqÞ ¼ 796:2 − 9:35q − 68:4q2 þ 47:73q3:
In Fig. 3, we show the calculated Raman spectra ofNCs with different diameters D of 5, 6, 8, 10, and 15 nm.The Raman spectrum of a 50 nmNC is shown in the inset.The TO phonon peak of a small NC shows a small posi-tion change, but the LO phonon peak exhibits an obviousdownshift and asymmetrical broadening. According tothe calculated Raman lineshape, it can be inferred thatas the NC size increases, the relative intensity of the LOmode increases, gradually approaching the TO modeintensity. This is understandable because the energychange of the LO phonon in the whole Brillouin zone isfour times greater than that of the TO phonon. The LOphonon scattering can therefore be spread out over amuch larger range of frequencies by varying the NC size.Meanwhile, the LO phonon is affected by the size confine-ment effect more than the TO phonon. Hence, it is logicalthat the LO mode intensity can be enhanced significantlyas the NC becomes larger. When the NC is larger than10 nm [13], the amorphous composition effect on theLO mode lineshape becomes weaker. If we consider onlythe size effect, the LO mode intensity should still be smal-ler than the TO mode intensity.
By considering the deformation potential in thesurface layer caused by the electro-optic mechanism,the Raman spectrum can be calculated by [27,28]
IðωÞ ¼ d2SdωdΩ ∝ Im
�−
AðωÞεðωÞ
�;
AðωÞ ¼ C2B2χphðε∞ þ 4πχfcÞ − 2CBε∞χph −
ε2∞
4π ;
Fig. 2. (Color online) (a) Raman spectrum acquired from the3C–-SiC NC solid film without NC passivation treatment. (b), (c)Raman spectra acquired from two glycerol-passivated 3C–SiCNC solid-film regions that contain (b) small and (c) large NCsizes. The calculated Raman spectra are given in red.
Fig. 3. (Color online) Calculated Raman spectra of several ty-pical 3C–SiC NC sizes. The inset shows the calculated Ramanspectrum of a 50 nm NC.
where the Faust–Henry coefficient CFH has been modi-fied into
C ¼ CFH
�εðrÞ∞mω2
T ½ωðrÞ2Lm − ωðrÞ2Tm�f∞ω2Tmðω2
L − ω2T Þ
�1=2
;
which depends on the TO and LO mode frequencies ωTmand ωLm and the high-frequency dielectric constant ε
∞mof the sphere-medium surface layer (here ε
∞m is relatedto the glycerol-bonded surface layer and NC size [17]).B ¼ ω2
TO=ðω2LO − ω2
TOÞ, and the total dielectric functionεðωÞ is considered to be due to contributions fromphonons and plasmons:
εðωÞ ¼ ε∞
�1þ ω2
LO − ω2TO
ω2TO − ω2
− iωΓ −
ω2p
ωðωþ iγÞ�:
Here ωp is the plasma frequency, χph is the free-carriersusceptibility, χfc is the ionic susceptibility, ωLO andωTO are the LO and TO phonon frequencies, and Γ and γare the phonon and plasmon damping constants, respec-tively. In the calculation, ωTO ¼ 797:6 cm−1, ωLO ¼972:6 cm−1, ε
∞¼ 6:78, ωp ¼ 17:5 cm−1, and γ ¼ 67 cm−1
[19,20]. Γ and C are adjustable parameters related tothe deformation potential induced by glycerol-passivatedsurface layer and the lattice imperfection near thesurface. By considering the coupled LO phonon–plasmoneffect, the measured Raman spectra of the glycerol-passivated NCs can be fitted well with Γ ¼ 80 cm−1, C ¼5:68 for spectrum 2(c) and Γ ¼ 30 cm−1, C ¼ 0:98 forspectrum 2(b). This result is understandable because lar-ger NCs have better crystalline structure and thus astronger LO phonon–plasmon coupling (larger Γ and Cvalues). Consequently, the LO mode behavior can be in-ferred to arise from the combination of quantum confine-ment and the coupled LO phonon–plasmon effectinduced by the glycerol-passivated surface layer.In conclusion, we have revealed that when the glycer-
ol-passivated 3C-SiC NCs are small (<10 nm), the quan-tum confinement effect dominates the LO phononscattering and the coupled LO phonon–plasmon effectsare correspondingly negligible. Hence, the Raman spec-tra of small SiC nanostructures always show smaller LOmode intensities. When the NCs are larger than 10 nm,the two effects must be considered together. This leadsto the largely enhanced LO phonon intensity.
The authors sincerely thank Dr. W. S. Yan in the Syn-chrotron Radiation Laboratory, University of Science andTechnology of China for performing the XPS mea-surement. This work was jointly supported by theNational Basic Research Program of China (grant2011CB922102), the National Natural Science Founda-tion of China (NSFC) (grants 60876058 and 60976063),and the Jiangsu Science Foundation (BK2008020). Partial
support was also received from the Hong Kong ResearchGrant Council General Research Fund (grant CityU112608).
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