silver nanoclusters decked diamond thin film as a substrate for surface enhanced raman scattering

A typical polycrystalline diamond thin film surface decked with silver Ag nanoclusters is found to be suitable for surface-enhanced Raman scattering SERS.. [http://dx.doi.org/10.1063/1.4

Silver nanoclusters decked diamond thin film as a substrate for surface-enhanced Raman scattering

Harish Ojha, Upender Gangadharani, Satyavathi Ravulapalli, Mohan Kumar Kuntumalla, Vadali Venkata Satya Siva Srikanth, Narayana Rao Desai, and Chandrahas Bansal

Citation: AIP Advances 2, 032102 (2012); doi: 10.1063/1.4738367

View online: http://dx.doi.org/10.1063/1.4738367

View Table of Contents: http://aip.scitation.org/toc/adv/2/3

Published by the American Institute of Physics

2School of Physics, University of Hyderabad, Hyderabad 500046, India

(Received 19 February 2012; accepted 2 July 2012; published online 13 July 2012)

Modified diamond thin film surfaces are proven candidates for bio-sensoric ap-plications A typical polycrystalline diamond thin film surface decked with silver (Ag) nanoclusters is found to be suitable for surface-enhanced Raman scattering (SERS) Simple and time effective procedures are used to fabricate Ag nanoclus-ters/diamond/silicon structure Standard Rhodamine 6G probe molecules are used to access the SERS activity of the test surface The observed SERS activity is explained

in terms of predominant electromagnetic enhancement mechanism This work is ex-pected to benefit SERS based bio-sensing with diamond as the core sensing material

Copyright 2012 Author(s) This article is distributed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4738367]

Since the advent of surface-enhanced Raman scattering (SERS) there has been a continuous effort to develop stable and reproducible SERS active substrates that not only provide a large enhancement factor but are also useful in analytical applications like bio-sensing.1However, most

of the substrates lack long-term stability due to instability of metal nanostructures deposited on the substrate surface and/or the substrate itself.1 On the other hand, owing to its diverse intrinsic properties, diamond thin film has established itself as an exceptionally stable electro-chemical transducer.2Diamond thin film’s dielectric properties3are anticipated to help in the enhancement of surface plasmons resonance4and thereby enabling itself to detect bio-molecular interactions in real time based on SERS.4Moreover, over the past decade, diamond thin film has been developed as an attractive bio-sensoric material.5 In this work, a typical polycrystalline diamond thin film surface decked with silver (Ag) nanoclusters is shown as a suitable substrate for SERS Standard Rhodamine 6G (R6G) probe molecules are used to access the SERS activity of the test surface.6This work is expected to benefit SERS based bio-sensing using selective SERS metal (like Ag)/diamond/silicon (Si)/back contact (like Aluminum) structured metal-insulator-semiconductor devices

Polycrystalline diamond thin film considered in this study was deposited on p-type (100) Si

substrate using microwave plasma enhanced chemical vapor deposition technique Submicron sized diamond grains constituted the microstructure of the thin film The detailed deposition procedure and other characteristics of this diamond thin film have been previously reported.7,8The polycrystalline diamond thin film used in this work is a typical one and has been proven to have bio-sensoric activity.9,10Ag nanoclusters are deposited on the diamond thin film surface using a magnetron cluster deposition system (NANODEP 60 from Oxford Applied Research, UK) following a previously reported procedure.11 Ag nanoclusters deposition was carried out for only 2 min in order to avoid formation of any thick coating of Ag which may be unfavorable for SERS activity Field emission scanning electron microscope (Carl Zeiss, Ultra 55 system) operated at an accelerating voltage of

a Equally contributed to this work

b Electronic mail: vvsssse@uohyd.ernet.in

032102-2 Ojha et al. AIP Advances 2, 032102 (2012)

FIG 1 (a) Low magnification plane view secondary electron micrographs of Ag nanoclusters decked diamond thin film; (b) the corresponding high magnification micrograph As-deposited diamond thin film surface is shown in the inset.

5 kV was used to study the surface morphology of the as-deposited and Ag nanoclusters decked diamond thin film Energy dispersive x-ray (EDX) analysis was carried out at an accelerating voltage

of 16 kV 10μl of 5×10−5 M R6G aqueous solution was dropped onto the test surface The wet test surface (with R6G) are kept undisturbed and allowed to dry naturally Room temperature SERS activity of R6G adsorbed on the test surface was recorded using 514.5 nm laser line equipped micro Raman spectrometer (Horiba Jobin Yvon LABRAM-HR-800) The typical laser power was kept

at 2.6 mW in order to avoid any thermal influences The laser beam (spot diameter∼ 2 μm) was

focused onto the test surface (with and without R6G) with a 50x objective lens with numerical aperture of 0.7 SERS signals are collected in the spectral range of 50 to 2000 cm−1with a spectral resolution of 1 cm−1in a back scattering geometry perpendicular to face of the (100) Si substrate with only 5 sec integration time Raman spectrometer was calibrated using the standard 521 cm−1 band of Si Ag nanoclusters (deposited using the same experimental conditions as in the case of diamond thin film) decked microscope glass slide and single crystalline silicon p-type (100) are also considered as test surfaces in this study for comparison purposes

The secondary electron micrographs of Ag nanoclusters decked diamond thin film surface are shown in Fig 1 Fig 1(a)shows the original morphology impression of the underlying diamond thin film It is also evident from the micrographs that the Ag nanoclusters are uniformly decked throughout the diamond thin film surface EDX analysis confirmed the presence of Ag on the surface SERS data obtained from R6G molecules adsorbed on as-deposited and Ag nanoclusters decked diamond thin film surfaces are shown in Fig.2 For convenience Raman spectra obtained from as-deposited (Fig 2(c)) and Ag nanoclusters decked (Fig.2(b)) diamond thin film surfaces are also included in Fig.2 Raman bands at 1135, 1332, 1483, and 1550 cm−1in Fig.2(c)are often seen in the Raman spectrum of a typical diamond thin film The broad 1332 cm−1Raman peak indicates the presence of small diamond crystallites in the diamond film.12 , 13The appearance of Raman band at

1135 cm−1(with an accompanied band at 1480 cm−1) is due to the presence of trans-polyacetylene segments in the grain boundary regions of the diamond thin film.14 Another broad Raman band at

1550 cm−1called the ‘G’ band, is attributed to sp2bonded graphite present in the grain boundaries between the diamond crystallites in the film.14 – 16Before discussing further about SERS signals from R6G molecules, it is important to note (Fig.2) that Ag nanoclusters decked diamond thin film did not result in any SERS signal from the underlying diamond phase Therefore, SERS data obtained from R6G molecules adsorbed Ag nanoclusters deposited diamond thin film surface have no interference from Raman signals corresponding to the underlying diamond phase

SERS signals corresponding to R6G molecules immobilized on as-deposited diamond thin film surface are not observed (Fig.2(a)); the reasons for this is the absence of any interaction be-tween R6G molecules and the test surface at the atomic level and too low concentration of R6G molecules on the test surface to scatter Raman signals On the other hand, SERS signals corre-sponding to R6G molecules adsorbed on Ag nanoclusters decked diamond thin film are highly amplified and can be observed (Fig.2(d)) at∼612, ∼772, ∼920, ∼1130, ∼1188, ∼1312, ∼1362,

∼1506, ∼1572, and ∼1649 cm−1.11 The reason for this is the creation of an apt condition for SERS It is evident that the presence of Ag nanoclusters has resulted in SERS signals from R6G molecules This is attributed to the surface plasmon resonance of the Ag nanoclusters Ag has been commonly used to study trace amounts of Raman active materials using SERS methodology.1

FIG 2 Raman spectra obtained from (a) R6G on diamond thin film (b) Ag deposited diamond thin film (c) as-deposited diamond thin film, and (d) R6G adsorbed on Ag deposited diamond thin film.

Raman scattering resonance region depends upon the plasmon energy of the metal particles used

to create an apt condition for SERS For Ag particles, the resonance region is in the range 1300–1500 cm−1 The presence of two very high intensity SERS signals corresponding to R6G molecules at 1312 and 1362 cm−1, which fall in the resonance region of Ag confirms that R6G molecules are adsorbed on the test surface Additionally, the enhancement of R6G vibrational modes at∼612 and ∼1649 cm−1suggests that the enhancement originates due to the charge transfer from the Fermi level of the Ag metal to the lowest unoccupied molecular orbital of the adsorbed molecules, which changes the effective polarizability of the molecules, in-turn resulting in the ob-served SERS activity.11 In the present context, this electromagnetic effect can originate from the localized surface plasmon resonance of the Ag nanocluster aggregates17 on the diamond films These aggregates can generate a huge electromagnetic field at the junction sites If there is Raman signal enhancement mainly through any chemical mechanism,1R6G molecules immobilized on the as-deposited diamond thin film could have been detected using Raman scattering

Fig.3shows high quality Raman spectra (only for comparison purposes) obtained from R6G molecules adsorbed on Ag nanoclusters decked on diamond film, p-type (100) Si surface and on as-deposited diamond film It is evident that Si also shows SERS activity However, it should be noted that Si cannot replace diamond/Si structure for any further bio-sensing device fabrication The comparison of the spectra reveals the existence of different SERS enhancements A simple quantitative intensity comparison is shown in Fig.3for a selected Raman band at∼1650 cm−1 It can be clearly observed that the Raman signal enhancement in the case of Ag nanoclusters decked diamond film is better than that in Si Furthermore, reproducibility (refers to the ability to produce SERS signal at various locations of the SERS active surface with as minimum as possible intensity variation) of the SERS activity of Ag nanoclusters decked diamond thin film has been checked SERS spectra obtained from R6G molecules adsorbed at several positions on Ag nanoclusters decked diamond film are shown in Fig.4 The intensities of Raman bands at∼613, ∼773, ∼1186, ∼1362,

∼1509, ∼1533, ∼1576, and ∼1649 cm−1are quantified using Loretzian fit The results showed that the intensity variations of all the Raman bands are less than 5%, which is an acceptable value This

in turn shows that the test surface is homogeneous Here, it is also very important to mention that the fabricated Ag nanoclusters decked diamond thin film showed the same reproducibility even after leaving it in ambient conditions for days together This shows that neither Ag nanoclusters degraded nor the underlying diamond In another simple experiment, to check the reusability of the SERS substrate under consideration, the substrate was ultrasonicated in methanol to remove the adsorbed R6G molecules Then, freshly prepared R6G solution was dropped and allowed to dry naturally following the procedures described in the beginning Raman signals with the similar intensity from R6G could be again detected It was found that the fabricated SERS substrate is reusable for at least two times Thereafter, the Raman signal intensity would reduce

032102-4 Ojha et al. AIP Advances 2, 032102 (2012)

FIG 3 Raman spectra obtained from R6G molecules on (a) Ag nanoclusters decked diamond thin film, (b) Ag nanoclusters decked Si, and (c) as-deposited thin diamond film.

FIG 4 Reproducible SERS spectra obtained from R6G molecules at different locations on Ag nanoclusters decked thin diamond film.

In conclusion, a stable Ag nanoclusters decked diamond film demonstrates an excellent and reproducible SERS activity which is explained in terms of the well-known electromagnetic enhance-ment mechanism The diamond film used in the present experienhance-ments is a typical one and can be deposited on a variety of substrates with ease Similarly, Ag nanoclusters deposition procedure used

in this work is not only very easy but also not time consuming Even though the R6G molecules’ concentration used in the present study is good enough for practical applications, the detection limits have to be further determined The influence of Ag nanoclusters size, morphology, distribution etc.,

on any further enhancement and detection limits is the subject of future investigation

The authors wish to thank Ms Deepthi and Mr Lakshmi Narayana (Technical staff at School of Physics, University of Hyderabad) for helping us with Raman measurements and scanning electron microscopy, respectively The authors R Sathyavathi and G Upender acknowledge DST and UGC for providing financial support through Women Scientist Scheme (SR/WOS-A/PS-06/2009) and

Dr D S Kothari post-doctoral fellowship programme, respectively The diamond thin film was deposited by VVSSS during his employment at Institute of Materials Engineering, University of Siegen, Germany

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