Varun Thakura,
Soumik Siddhantab,
C. Narayanaa and
S. M. Shivaprasad*a
aChemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P. O., Bangalore – 560064, India. E-mail: smsprasad@jncasr.ac.in; Fax: +91-80-22082947; Tel: +91-80-22082947
bDepartment of Mechanical Engineering, John Hopkins University, Baltimore, MD, USA
First published on 3rd December 2015
In the present experiment, two GaN nanowall network (NWN) samples with different porosity were grown on c-sapphire substrates using plasma assisted molecular beam epitaxy (PA-MBE). Ag nanoparticles were deposited on both the samples using a physical vapour deposition (PVD) system. Annealing the samples at different temperatures resulted in a change in Ag nanoparticle size due to diffusion and Ostwald ripening which had significant effect on the photoluminescence and SERS activity of GaN NWN. It was observed that the photoluminescence yield increased by more than five times in both cases at 200 °C. The SERS activity for thiophenol is higher in the as-deposited case for the sample with higher porosity, but after annealing to 200 °C the activity increased for the sample with lower porosity. It is also interesting to observe that the sample with higher porosity shows SERS signals even after being annealed to higher temperatures. Studies are also done for other analytes such as R6G and BSA. The results are discussed in terms of plasmonic effects of Ag nanoparticles on the excitonic emission from the GaN surface, which is also simulated using 2D-FDTD simulations.
For exploring exciton–plasmon interactions in metal–semiconductor hybrid systems, a broad range of semiconductors have been probed. Since Si is a semiconductor with an established technological application industry, it has attracted a lot of interest. There are reports of electroluminescence being obtained from silicon based devices11 which have been combined with metal nanostructures to exploit exciton–plasmon interactions. However, wide and direct band gap semiconductors are more commonly explored to be used in such structures. GaN is used for fabricating optoelectronic devices for high emission, such as light emitting diodes (LEDs) and laser diodes (LDs) since it is easy to alloy it with Al and In by band gap engineering to obtain devices emitting/absorbing across the solar spectrum and beyond.
GaN based devices have been coated with Ag to enhance internal quantum efficiency3,12 and current collection efficiency in photovoltaics.13 Since surface roughening of GaN is known to improve light extraction,14 it is interesting to study the effect of Ag nanoparticles on the emission properties in its nanostructured morphology. Further, GaN has also been used with Ag nanoparticles to fabricate sensors using SERS.15 Since the emission of GaN in the nanowall form is significantly higher than the traditional flat films16 and can be improved by surface modifications,17 it is desirable to combine the Ag adsorption and enhanced emission capabilities to fabricate multiple usage substrates.
We have previously reported an Ag–GaN system using a novel nanowall network structure of GaN which has shown excellent properties and application as a SERS substrate6 used to sense proteins of both positive and negative surface charges. It was shown that the multiple reflections of the incident radiation due to the morphology of the nanowalls resulted in the enhancement of SERS sensitivity. Although other templates have been explored for SERS application,18 dual use of GaN for enhancement of band edge emission as well as sensing is attractive.
In the present experiment, we extend our previous study by considering variation of porosity of the GaN nanowall structure and its effect on SERS. Further, the size and distance between the Ag nanoparticles is also varied by carrying out annealing at different temperatures, which is known to affect the LSPR frequency of Ag. Apart from the change in SERS signals, the change in plasmon frequency is also harnessed to maximize the band edge emission from GaN which is probed using photoluminescence. It is then interesting to use this substrate not only as a SERS sensor but also as a highly luminescent material whose properties can be tuned by controlling the size and distribution of Ag nanoparticles on its surface. The SERS activity for three different analytes is presented here.
Ag was deposited on the GaN nanowall network samples held at room temperature using electron beam evaporation in a PVD system (SVT, USA) operating at a base pressure of 1 × 10−9 Torr while the pressure during evaporation was 2 × 10−6 Torr. The quartz crystal thickness monitor was utilized to estimate the amount of Ag deposited and the thickness in both the cases was 13 nm. Annealing of the samples was carried out in a tube furnace in a nitrogen environment to prevent oxidation. Prior to annealing, the furnace was purged with N2 gas for 20 minutes to evacuate air. The ramp rate of the furnace was 20 °C per minute and the duration of annealing in each case was 1 hour. For both the samples, annealing was carried out at 200 °C and 500 °C. The morphology of the samples was determined ex situ by FESEM (FEI, Netherlands). XPS studies (not shown here) confirm that the Ag nanoparticles are not oxidized during annealing.
The Raman and SERS spectra were recorded using thiophenol as the analyte in the same setup described earlier.19 Optical properties of all the samples were examined by Photoluminescence (PL, Horiba Jobin Yvon) using a Xe lamp source. The excitation wavelength was kept at 325 nm for all measurements using a filter.
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Fig. 1 (a), (b) FESEM images showing S1 and S2 GaN nanowall network with different morphologies. All scale bars pertain to 500 nm. (c) Photoluminescence spectra of both the samples. |
Fig. 2 compiles the plan view FESEM images of all the samples studied in the present experiment. The images of both the samples reveals the difference in the way that Ag gets deposited on the surface. Since S1 is relatively more porous and the walls are sharper at the apex, most of the Ag is deposited on the sides of the nanowalls. However, due to higher surface coverage of S2 and a thicker apex, most of the Ag is deposited on the flat surface at the top of the walls. In both the cases, the Ag average island sizes are 20 ± 5 nm for S1 and 30 ± 5 nm for S2. Annealing the samples to different temperatures results in an increase of the size of the Ag nanoparticles due to thermally assisted diffusion of the Ag and subsequent Ostwald ripening. The size of the particles for the 200 °C annealed sample for S1 and S2 was 30 and 45 nm, respectively. For the 500 °C annealing case in both the samples the particles grow to a size where they are trapped in the voids between the nanowalls forming large islands with average size 150 nm in both the samples. Since the voids are of different dimensions there is a distribution of large Ag island sizes from 70–300 nm.
Fig. 3 shows the PL spectra of S1 and S2 plotted alongside their respective Ag-deposited and annealed samples. The bare and as-deposited samples show similar spectra in both cases, but after annealing to 200 °C, the area under the PL curve increases by 5.3 times the as-deposited case for S1 and 5.7 times for S2. Annealing further to 500 °C reduces the PL for both the samples. In case of S1, the area under the curve is now 1.8 times that of as-deposited PL. However, for S2 the area under the curve is only 0.7 times than the as-deposited sample. The FWHM of the band edge emission for S1 and S2 remains almost unchanged, indicating that only the intensity is modulated appreciably as a function of annealing temperature.
Continuing from our previous report on the application of GaN NWN as a SERS active substrate,6 we have carried out SERS on all the samples using thiophenol as the analyte. Fig. 4 shows the SERS enhancement spectra of thiophenol for all the samples in the case of both S1 and S2. It is observed that S2 gave a higher SERS enhancement compared to S1 in the as deposited and 200 °C annealed samples. In the case of 500 °C annealed sample, there was no detectable signal for S2 though S1 still showed some peaks. S1 also shows a reduction in SERS enhancement with annealing of the sample while S2 showed an increase when the sample was annealed to 200 °C. Since the nanoparticles are at their closest in the as deposited configuration for both the samples, the SERS enhancement is expected to be the largest in both samples. The reduction in the SERS signal of S1 when annealed to 200 °C is due to Ag nanoparticles diffusing to form larger nanoparticles and consequently reducing the number of potential hot spot sites on the top surface. However, in S2, due to the flatness of the apex the SERS signal increases because of the reduction in the interparticle distance between the Ag nanoparticles which increases intensity of electromagnetic signals arising between them. The residual signal seen for S1 after being annealed to 500 °C is due to Ag nanoparticles diffusing down the nanowalls and retaining a reasonable interparticle distance, indicating that S1 retains its sensitivity over a much higher range of temperature compared to S2. In case of S2, the particles are too large as compared to S1 and hence no SERS signal is detected. The enhancement factor was calculated for the peak at 1086 cm−1 which corresponds to the in-plane breathing mode coupled to the ν(C–S) mode using the method given by Yu et al.22 Enhancement factor values for all the samples are provided in Table 1.
Sample name | Enhancement factor |
---|---|
Sample 1 as deposited | 9.97 × 104 |
Sample 1 200 °C | 9.57 × 104 |
Sample 1 500 °C | 1.06 × 105 |
Sample 2 as deposited | 4.73 × 105 |
Sample 2 200 °C | 5.51 × 105 |
In addition to thiophenol, we have performed SERS studies on two biologically relevant analyte molecules, namely Rhodamine 6G and bovine serum albumin (BSA). While R6G is used extensively to label, detect and image biomolecules like nucleotides and proteins,23 the protein BSA constitutes a class of serum albumins used to study drug–ligand interactions.24 The SERS modes of these molecules could be discerned at low (micro and nano) molar concentrations which makes the SERS substrate useful for potential biological applications. The SERS band assignments for R6G and BSA were made according to those reported in the literature.25–27 R6G gives strong SERS signals due to presence of highly polarizable groups (Fig. 5). On the other hand, proteins are complex and bulky molecules which have low Raman scattering cross section. The SERS substrate was able to enhance the BSA Raman signals efficiently and we could observe SERS modes corresponding to the aromatic amino acids phenylalanine, tyrosine, tryptophan and histidine as well as modes from the peptide backbone and aliphatic side chains. We could also observe the amide modes which are a combination of CO stretching combined with N–H bending vibrations.28 The amide modes (mostly the amide I mode at around 1650 cm−1) are indicative of the secondary structures of proteins and often used to elucidate different structural aspects of the proteins (Fig. 6).29
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Fig. 5 SERS spectra of bovine serum albumin (10−6 M, black) and Rhodamine 6G (10−6 M and 10−9 M in red and blue respectively). |
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Fig. 6 Zoomed in FESEM images of the as-deposited configuration of (a) sample 1 and (b) sample 2 with corresponding 2D-FDTD simulations using similar interparticle distances. |
In order to estimate strength of electromagnetic fields on the surface of the nanoparticle, 2D FDTD simulations have been carried out using the SEM images as a guide for the distribution of Ag nanoparticles on the GaN surface. A representative FDTD calculation done for the as-deposited case is shown in Fig. 5. A higher electromagnetic field strength around the nanoparticles in S2 in comparison to S1 is observed. For S1, the nanoparticles have an interparticle spacing of around ≈50 nm, which gives a much lower electromagnetic field strength compared to S2, where the interparticle spacing reduces to ≈10 nm or lower. The corresponding |E|2 values for the two samples are 219.49 and 60.53 (V m−1)2 respectively. Hence, S2 shows an electromagnetic field 3.6 times stronger than S1. Since in S1 the majority of Ag is deposited on the r-plane sidewalls,30 it is evident that nanoparticles when deposited on the top surface (c-plane) exhibit higher field strengths around them.
It should also be noted that the GaN nanowalls promote light trapping in the form of multiple reflections of the incident light which contributes to the enhancement of electromagetic field in the vicinity of the deposited Ag nanoparticles.6 The interplay of the enhanced reflections combined with the hotspots generated by the Ag nanoparticle aggregates increases the efficacy of the Ag–GaN hybrid SERS substrates.
Thus, we have demonstrated a tunable Ag–GaN NWN hybrid system which shows an enhancement in the band edge emission because of surface plasmon coupling to the excitonic emission. In addition, the system can be used as a SERS substrate at various temperatures with differing sensitivity depending on the surface coverage of the film. The two applications shown in this report vary in their sensitivity with temperature, and thus the use of a single hybrid substrate to realize high performance optoelectronic and sensing properties.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra24906f |
This journal is © The Royal Society of Chemistry 2015 |