Blue photoluminescence (PL) from hydrogenated amorphous carbon (a-C:H) films has been successfully enhanced via surface plasmons (SPs). a-C:H films with different thickness were deposited on Ag interlayers, of which the nanostructure was tuned from nanoparticles (NPs) to continuous films via processing conditions control. The PL enhancement factor was found to increase with the Ag NP growth and the surface roughness of the continuous Ag interlayer. A PL enhancement factor of more than 9 times has been successfully achieved when the 43 nm-thick a-C:H film coupled to an Ag interlayer with the peak surface roughness. a-C:H films with SP-enhanced PL have therefore been demonstrated to be promising for light-emitting applications.
© 2011 OSA
Hydrogenated amorphous carbon (a-C:H) has received great attention in the past decades due to its wide applications in micro-electronics and optoelectronics [1–6]. By manipulating the growth condition, a-C:H can be deposited with characteristics ranging from soft and polymeric, to hard and diamond-like [7,8]. The soft and polymeric a-C:H films with strong light emission has been widely studied due to its promising functionality as an active layer in light emitting devices [6,8]. In addition, uniform a-C:H films with sound optical and electrical properties have been deposited over a large area by plasma enhanced chemical vapor deposition (PECVD) [1,5–8]. Therefore, a-C:H has been considered as a promising candidate for the large-area solid-state lighting and flat-panel display devices. However, the limited light-emission efficiency of a-C:H has generally become the bottle-neck for its more extensive utilization. The extensive research for an appropriate strategy to overcome such technical challenges is still ongoing worldwide.
SPs are the collective oscillations of free electrons in a metal, which occur at the interfaces between metals and dielectrics . In addition to the propagating SPs (PSPs) on a plane surface, the SPs confined in metal nanoparticles (NPs) embedded in a dielectric matrix are localized SPs (LSPs) [10,11]. Since 1990, the concept of the SP-enhanced light emission has been proposed [12,13] and subsequently been studied to enhance the luminescence efficiency of light emitting materials and devices, such as InGaN/GaN [11–14], silicon quantum dot (Si-QD)  and ZnO films [15–20]. However, to the best of our knowledge, SPs have not been utilized to enhance the light emission from a-C:H yet, and such method can be an effective way for luminescence enhancement.
Therefore, in this study, we present a systematic investigation into blue light emission enhancement of a-C:H films via the SPs. The SPs generated at the Ag interlayers were tuned from LSPs to PSPs through the modification of Ag nanostructure by control of process conditions. The PL enhancement and quenching of the a-C:H/Ag composite film was studied, and the mechanism of such phenomenon was also discussed.
2. Experimental details
Fourteen series of Ag films with different sputtering time (10 s–700 s) were prepared on 2 × 1 cm2 silica substrates by DC magnetron sputtering in Ar ambience. The Ag film thickness increased monotonically with the sputtering time. The Ag film deposited for 700 s is of ~120 nm in thickness. The subsequent rapid thermal annealing (RTA) was carried out at 500 °C for 120 s in N2 ambience. The annealed Ag films were labeled as ‘S’-‘sputtering time’. Subsequently, the a-C:H deposition was carried out in an RF-PECVD system at room temperature using C2H4 (flow rate = 20 sccm) and H2 (flow rate = 18 sccm) as the gas sources. The chamber pressure and the plasma power were set at 50 Pa and 50 W, respectively. Three groups of a-C:H films (PL peak at ~445 nm) were deposited on the Ag interlayers for 15 min, 30 min and 60 min (denoted as C15, C30 and C60), respectively. a-C:H control samples of different thickness were also prepared under the same condition for each group to compare the PL of a-C:H films with Ag interlayers. Each sample was labeled as ‘C’-‘a-C:H deposition time’-‘S’-‘Ag sputtering time’. The labels of the Ag interlayers, the a-C:H/Ag composite films and the control samples were summarized in Table 1 .
The surface morphology and the cross-section structure of the samples were characterized by field-emission scanning electron microscopy (S4800, Hitachi). The Ag and a-C:H thickness were obtained by measuring the SEM cross-section images of the corresponding samples. The PL spectra were examined with a fluorescence spectrometer (FLS920, Edinburgh Instruments Ltd.). The samples were excited by 325 nm UV xenon lamp at an incidence angle of 45 degree. To avoid reflecting the excitation light into the detector, all the samples were maintained at 60 degree to the detection plane which is defined by the Xe lamp, the samples and the detector. The transmission spectra of all samples were measured using a UV-Vis spectrometer (TU-1901, Beijing Purkinje General Instrument CO., Ltd). Then the extinction was calculated as 1 minus transmission . The root-mean-squared roughness (Rq) and the atomic force microscopy (AFM) images of the annealed Ag films were obtained in non-contact mode by an AFM (SPI-3800N, Seiko Instruments Inc.). The refractive index and extinction coefficient (k) of a-C:H film were obtained from an ellipsometer (M-2000DI, J.A.Woollam Co., Inc.) to calculate the dielectric constant.
3. Results and discussion
The nanostructure of the annealed Ag interlayers was found to vary significantly with the sputtering time, as shown in Fig. 1 . The Ag interlayers deposited for the periods from 10 s to 30 s present as Ag NPs. The average size of Ag NPs for S10, S20 and S30 are ~20 nm, ~35 nm and ~68 nm, respectively. When the Ag sputtering time is increased to 40 s and 60 s, the Ag interlayers composed of NPs become semi-continuous. The silica substrates are fully covered by continuous Ag films when the sputtering time is 80 s or longer. LSPs are the SPs confined in metal NPs embedded in a dielectric matrix [10,11]. S10, S20 and S30, showing randomly distributed NPs, could be referred to as a LSP system. PSPs are the SPs propagating on a planar interface [10,11]. When the sputtering time is 40 s and 60s, the Ag NPs form semi-continuous films, which can be referred to as an intermediate system containing both LSPs and PSPs. Ag interlayers deposited for more than 80 s present continuous films with little pinholes and rough surfaces, which support PSPs.
Three groups of a-C:H films with different thickness were subsequently deposited on the as-prepared Ag interlayers directly. As there was no distinguishable difference among the extinction spectra of C15, C30 and C60 were observed, only the spectra of C60 series are presented in Fig. 2 . The Ag NPs formed in S10 and S20 are of spherical shape with a narrow size distribution, and thus the extinction spectra of these two samples are symmetrical and their full width at half maximum (FWHM) is smaller than those of the others. With the sputtering time increased to 30 s, the Ag NPs generally merged to form the irregular shapes with increased size distribution, leading to a red-shifted and broader extinction spectrum. Due to the varied structural characteristic and the surface roughness of Ag interlayers, the SP modes of different resonance energies lead to broad extinction spectra for C60S40~C60S700 .
Figure 3 shows the PL spectra of the naked silica substrate, the substrate with a-C:H film (C15S0), and the substrates with Ag films (S10-S700). The PL intensity of the substrate was an order of magnitude lower than that of C15S0. C15S0 was the thinnest control sample with lower PL intensity than those of C30S0 and C60S0. Furthermore, when the silica substrates were covered with the a-C:H films, PL from the substrates could be even lower due to the absorption of excitation light by the upper a-C:H films. Therefore, the influence of PL from the substrate can be neglected. As shown in Fig. 3, no ‘PL’ was observed from all the annealed Ag films. The phenomenon indicated that the scattering or the reflection of the excitation light by Ag films were not detected. Consequently, the PL measurement would not be influenced by the scattering and the reflection.
The smoothed PL spectra of all the samples are shown in Fig. 4(a) . The a-C:H films are of ~21 nm, ~43 nm and ~92 nm in thickness for C15, C30 and C60, respectively. The PL intensity ratios of three control samples follow IC15S0:IC30S0:IC60S0 = 1:1.67:2.92. Although the PL intensity of the control samples increased with the film thickness, the peak shape and position remain unchanged. It has been suggested that a-C:H films contain sp3 and sp2 hybridized carbon atoms and a large amount of hydrogen [8,22,23]. The PL originates from the geminate recombination of electron hole pairs within sp2 bonded clusters in an sp3 bonded amorphous matrix . In our previous study, we have confirmed the structure and PL mechanism of the a-C:H films . As shown in Fig. 4(a), due to the influence of LSPs and PSPs generated at the Ag interlayers, the intensity, the peak position and the FWHM of SP-mediated PL spectra vary in comparison to the corresponding control samples. The insets of Fig. 4(a) show the PL images (captured via a conventional digital camera) of the samples with largest PL intensity in each time series (C15S460, C30S460 and C60S460), exhibiting bright blue PL visible to the naked eye under the room light.
The integrated PL enhancement/quenching factor, defined as the ratio of the integrated PL intensity of the SP-mediated sample to that of the control sample, has been illustrated in Fig. 4(b) as a function of Ag sputtering time. As shown in Fig. 4(a)&(b), the PL of C30S10, C15S10, C15S20 and C15S30 are quenched, while those of all the other samples are enhanced. The PL enhancement via SPs is the result of two sequential processes. Firstly, when the excited dipole energies of the light-emitting layer and the SP energy of the metal are similar, the excited dipole energies can be transferred into SP modes of the metal. As indicated in Fig. 2 and Fig. 4(a), the SP resonance band of the Ag interlayers overlaps the emission band of a-C:H at ~445 nm. Secondly, the SPs localized in metal NPs, which have zero momentum, can radiate into light directly and efficiently . The scattering component of the NP extinction is a measure of the extent to which the plasmons can radiate into the far-field . As shown in Fig. 1, for the LSP system (S10, S20 and S30), the longer sputtering time led to Ag NPs with larger dimension. In addition, the plasmonic scattering efficiency increases with the size of Ag NPs [25,26]. Therefore, the PL enhancement factor raised with the increasing sputtering time. For small metal particles, absorption is more dominant in the total extinction, and thus the optical absorption by metal NPs is responsible for the quenching [24,27].
However, that may not be the only quenching mechanism for the LSP system, otherwise the PL from thicker a-C:H films should also be quenched. Another unneglectable PL quenching effect occurs at the very short distance near the interface of light-emitting layer and metal layer [19,28]. While the effect of PL enhancement is found to decay away over longer distance . Therefore, the PL enhancement/quenching factor rises with the increase of a-C:H film thickness, since the enhancement effect become dominant. Such quenching is currently under debate whether it is due to the non-radiative energy transfer induced damping , or the electron transfer from the light-emitting layer to the metal layer .
For Ag interlayers which support PSPs (S40-S700), the enhancement factors of all C15, C30 and C60 series initially increase with the Ag sputtering time, till reaching the peak values at the same point where the Ag sputtering time is 460 s, and then decrease afterwards. It is suggested that for the SPs propagating on a planar interface, the nanostructure or the roughness of the metal layer allows SPs of high momentum to scatter, lose their momentum, and couple to radiated light . Accordingly, the scattering of SPs increases with the surface roughness of the continuous Ag interlayers . To confirm the relationship between the surface roughness and the PL enhancement factor in this case, the root-mean-squared roughness (Rq) of several Ag interlayers from PSP system was characterized using AFM. As shown in Fig. 5 , the surface roughness increases with the Ag sputtering time, reaches the maximum for S460 (Rq(S460) = 14.9 nm), then decreases as the sputtering time prolonged. Therefore, the PL enhancement factors of all three series reach their maxima on S460 due to the peak roughness of the Ag interlayer. For the samples deposited on other Ag interlayers, the reduction of surface roughness attenuates the scattering of SPs, and more SP energy is thermally dissipated . The insets of Fig. 4(b) show the PL images corresponding to several SP-mediated samples of C30, presenting variation of the PL intensity visible to the naked eye under the room light.
Another interesting phenomenon is that all enhancement factors of C30 are higher than those of C15 in the intermediate and PSP system (S40-S700). This is because the aforementioned PL quenching at a very short distance from the a-C:H/Ag interface, leading to the decrease in part of PL enhancement. The quenching effect of C15 weighs more than that of C30. However, when the film thickness reaches ~92 nm (C60), the enhancement factors also decrease. The explanation is that only electron–hole pairs located within the near-field of the Ag surface can couple to the SP mode. Assuming the metal surface is flat, the penetration depth (Z) of the SP fringing field into the semiconductor can be estimated by Eq. (1) [19,31]:32]. The result indicates that the evanescent field in the a-C:H film decays significantly above this distance. Consequently the most part of the a-C:H films (C60) with 92 nm thickness cannot be coupled with the SPs of Ag interlayers. As a result, the enhancement factors of C60 are lower than those of C30.
The wavelength-dependent PL enhancement factor (abbreviated as WPLEF) is defined as the ratio of the PL intensity of the SP-enhanced sample to that of the control sample at a specific wavelength, as shown in Fig. 6 . It was interesting to find that the WPLEF curves of C60 were similar to the corresponding extinction spectra, especially for the LSP system (C60S10/20/30). The phenomenon was also observed from the SP-mediated amorphous silicon carbide (a-Si1-xCx:H) films, a detailed report on the a-Si1-xCx:H films will be published elsewhere. The WPLEF curves and the extinction spectra (solid lines) were normalized and plotted in comparison in Fig. 7 . For the PSP system, the WPLEF curves and extinction spectra both raise at the longer wavelength (only C60S60 is shown in Fig. 7(d)). For the LSP system, the curves of these two spectra resemble each other in shape but with shifts of dozens of nanometer. Such red shift depends upon the Ag NP size. The longer Ag sputtering time led to bigger NP size and a more remarkable shift. The peak shifts are 8 nm, 22 nm and 36 nm for S10, S20 and S30, respectively. The observed spectral shifts between the C60 WPLEF curves and the extinction spectra are believed to be correspondent to the difference between the near- and far-field measured plasmon resonance peak positions. The measure of the plasmonic response of a metallic NP or nanostructure commonly used involves its far-field quantities, such as absorption, scattering, and extinction, and its near-field properties, such as the intensity and spatial distribution of its electromagnetic field enhancements . The localized plasmons of metallic NPs and nanostructures display an universal phenomenon: upon optical excitation, the maximum near-field enhancements occur at lower energies than the maximum of the corresponding far-field spectrum . Such red shift of the near-field peak energies with respect to the far-field peak energies is known to depend upon the particle size, with larger particles displaying a more marked shift [34,35]. The phenomenon above is found to be consistent with the red shift of WPLEF curves. Therefore, the WPLEF curves are attributed to the near-field effect in our case. The shape resemblance and the peak shift between the WPLEF curves and the extinction spectra suggest that the obtained PL enhancement originated from the coupling between electron-hole pairs in a-C:H films and the LSPs and PSPs generated at the Ag/a-C:H interface.
However, the relationship between the WPLEF curves and the extinction spectra of both C15 and C30 series is different from the phenomenon depicted above. The PL quenching influences the WPLEF curves of thin a-C:H films significantly, which could be neglected for thick films of C60. The WPLEF curves of thin films can actually provide more detailed information about the near- and far-field of SPs, which is under our current investigation.
The blue PL of a-C:H films was enhanced, for the first time, by coupling through SPs. The dependence of both LSPs and PSPs mediated PL on the thickness of a-C:H films and Ag interlayers was systematically investigated. It was found that the PL enhancement increased with Ag NP size and the surface roughness of the continuous Ag interlayer due to the increase of the SPs scattering. The PL quenching was also observed in some samples, which is potentially attributed to the absorption of small Ag NPs or the electron transfer at the Ag/a-C:H interface. The relationship between the SPs far-field extinction and the near-field PL enhancement has also been discussed. The near-field effect of SPs results in the dependence of the PL enhancement of a-C:H films on the film thickness. A maximum PL enhancement factor of more than 9 times was successfully obtained when the 43 nm-thick of a-C:H was deposited on the Ag interlayers with the highest surface roughness. a-C:H films with SP-enhanced PL have therefore been demonstrated to be a promising material for light-emitting application of large-area solid-state lighting.
This work was supported by the National Basic Research Program of China (973 Program) “2007CB613403” and Foundation of the scientific research base development (Engineering Research Center of the Education Ministry for the Surface and Structure Modification of Inorganic functional Materials) “KYJD09014”.
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