Au-controlled enhancement of photoluminescence of NiS nanostructures synthesized via a microwave-assisted hydrothermal technique

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Au-controlled enhancement of photoluminescence of NiS Nanostructures synthesized via a microwave-assisted Hydrothermal technique E.C. Linganiso, B.W. Mwakikunga, S.D. Mhlanga, N.J. Coville

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S0022-2313(14)00387-1 http://dx.doi.org/10.1016/j.jlumin.2014.06.053 LUMIN12780

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Received date: 15 January 2014 Revised date: 24 June 2014 Accepted date: 25 June 2014 Cite this article as: E.C. Linganiso, B.W. Mwakikunga, S.D. Mhlanga, N.J. Coville, Au-controlled enhancement of photoluminescence of NiS Nanostructures synthesized via a microwave-assisted Hydrothermal technique, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2014.06.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Au-controlled

Enhancement

of

Photoluminescence

of

NiS

Nanostructures Synthesized via a Microwave-Assisted Hydrothermal Technique E. C. Linganiso1,2, B. W. Mwakikunga1,5, S. D. Mhlanga4, N. J. Coville2,3 1

DST/CSIR Nanotech Innovation Centre, National Centre for Nano-structured Materials, Council for Scientific

and Industrial Research, P. O. Box 395, Pretoria, 0001, South Africa 2

Molecular Sciences Institute, School of Chemistry University of the Witwatersrand, Private Bag 3,

Johannesburg, Wits 2050, South Africa 3

DST/NRF Centre of Excellence in Strong Materials, University of the Witwatersrand, PO Wits, 2050,

Johannesburg, South Africa 4

Department of Applied Chemistry, University of Johannesburg, PO Box 17011, Doornfontein 2028

Johannesburg, South Africa 5

Department of Physics and Biochemical Sciences, The Polytechnic of the University of Malawi, Private Bag

303, Chichiri, Blantyre 0003, Malawi. Corresponding author: Bonex Mwakikunga ([email protected])



Abstract Nickel sulphide (NiS) nanostructures decorated with gold (Au) nanoparticles (NPs) were synthesized via a microwave-assisted hydrothermal technique. Binary phase NiS ( and ) crystalline nanostructures, bare, and decorated with Au NPs were obtained and confirmed by X-ray diffraction (XRD) studies. TEM analysis revealed that the NiS nanostructures were of various shapes. A quantum confinement effect was confirmed by the blue shift PL emissions and high optical energy band gap observed for the as-synthesized sample. A threefold light emission enhancement due to Au NP coatings was obtained when Au metal NP decoration concentrations was varied from 1 % to 10 %. These enhancements were attributed to the surface plasmon resonance (SPR) excitation of the surface decorated metal NPs which results in an increased rate of spontaneous emission. The PL enhancement factor was observed to vary at different NiS emissions as well as with the size of the Au NPs. The effect of metal NP decoration on the PL emission of NiS is to the best of our knowledge, presented for the first time.

1 

Keywords: nickel sulphide; enhanced PL emission; photoluminescence; surface plasmon resonance

Introduction Metal chalcogenide nanomaterials (e.g. CdS, ZnO, ZnS, SnO2, SnS, CoS, CuS, NiS etc.) have been a topic of interest over the years owing to their novel properties resulting from the quantum confinement effect [1-12]. These electronic, mechanical and chemical and other properties have resulted in intensive investigations for new applications for these materials. In particular, NiS nano-materials have been widely researched for potential applications in catalysis, rechargeable batteries, hydrogen storage devices, optoelectronic devices and magneto-electronic devices [13-18]. These materials are relatively cheap to make with most fabrication reactions occurring at temperatures below 473 K and H2O being used as a solvent [19-22]. NiS exhibits two phases: the rhombohedral (-NiS) phase and the hexagonal (-NiS) phase. The hexagonal NiS phase is known to exhibit a metal to insulator (MIT) transition at 264 K. This MIT temperature occurs just below room temperature. This behaviour is found in very few materials e.g. VO2 at 241 K. At this MIT, changes in resistivity are accompanied by a corresponding change in transparency of the material. In the metallic state, the material is infrared reflecting; in the insulating state, the material is infrared transmitting. The MIT has been applied in ultrafast switches [23], laser protection [24], infrared detectors [25], protection against laser guided missiles in war time and defence [26] as well as a coating on window glass to keep homes warm in winter and cool in summer in a bid to save energy and reduce greenhouse gases and hence alleviate global warming [27]. Modifying the surface of nanostructures is normally applied to improve their properties as well as opening new fields for their applications. One of the ways this has been achieved is by decorating the surface of the nanomaterial with transition metal/metal oxide nanoparticles as well as other semiconductor quantum dots in order to add specific properties to the material e.g. optical, catalytic and electronic properties [27, 29]. Nanosized Au is well known for its size dependent optical properties [30]. Au has also been reported to show uncompromised optical and good catalytic properties when coated onto a substrate [31, 32]. Coating Au on a semiconductor substrate is also a good way of utilising its properties at a minimum cost. 2 

The microwave-assisted hydrothermal technique has become one of the desirable methods for nanoscale synthesis. It is a clean method of synthesis that is cheap and results are obtained fast with reported high purity and quite narrow size distribution of nanoparticles [33]. To our knowledge the coating of NiS with Au NPs, as well as the effect of these NPs on the NiS PL properties has not been described in the literature. In this paper we present interesting PL observations on properties of the NiS nanostructures when surface decorated with Au NPs and show how these materials affect the NiS properties. Enhancement of ultra-violet (UV) and near IR light emissions due to NP decoration is presented.

Experimental procedure Chemicals and Reagents All chemicals and reagents used in these experiments were purchased from Sigma Aldrich and used as received without any further purification.

NiS synthesis NiS nanostructures were synthesized as reported elsewhere [34]. Known amounts of Na2S.9H2O (4.00 g) and 2M NaOH were mixed in a glass beaker and a solution of NiCl2.6H2O (3.96 g) in 2M NaOH was poured into the mixture. The mixture was stirred for 5 minutes to ensure homogeneity and transferred into a 100 ml Teflon vessel. The reactants were then heated under microwave irradiation at 600 W for 30 minutes. The microwave irradiated sample was then cooled to room temperature. The black precipitate obtained was washed several times with distilled H2O, ethanol and acetone followed by drying in an oven at 363 K for 6 hours.

Au loading To load Au on the surface of NiS, a solution of HAuCl4 of known concentration was added to a NaOH solution containing NiS nanostructures and the Au loading was varied between 1 and 10%. The mixture was then transferred into an Anton Paar, multiwave 3000 microwave reactor which was operated at 600 W for 15 minutes and afterwards cooled to room temperatures. The product was then washed several times with H2O, ethanol and acetone followed by drying at 363 K for 2 hours. 3 

Characterization X-ray diffraction patterns of the samples were recorded using a Philips PW 1830 X-ray diffractometer with a Cu K ( = 0.154 nm). The morphology and crystallinity of the samples were analyzed using high resolution transmission electron microscopy (HR-TEM JEOL-JEM 2100). The overall qualitative elemental composition of the products was obtained by recording spectra using energy dispersive X-ray spectroscopy (EDS) that is attached to a SEM operated at an accelerating voltage of 15 kV. The photoluminescence (PL) spectra were recorded at room temperature by exciting the samples with the 300 nm deuterium lamp.

Results X-ray diffraction analysis XRD analysis of the as-synthesized product shown in Figure 1 confirms that a binary phase NiS was formed. All peaks were indexed to either a hexagonal phase of NiS (* represent NiS, JCPDS Card No. 02-1280) or a rhombohedral phase of NiS (# represent -NiS, JCPDS No. 12-0041). All the peaks indexed are clearly shown in Table 1. It can be seen from the spectra that -NiS diffraction peaks are more intense suggesting that the bulk material contains more -NiS nanostructures. New diffraction peaks due to Au coating were also observed confirming the presence of the Au NPs on the samples.

Microscopy analysis TEM images revealed that the NiS nanostructures formed have a wide morphology distribution which consists of irregular shaped particles, layer-like structures as well as some nanohexagons and nanorods [Figure 2(a)]. The particle size of the nanostructures observed was less than 50 nm. Small spherical particles were observed on the surface of NiS for the Au coated NiS nanostructures shown in Figure 2 (b). The average diameter distribution for the Au NPs decorated on NiS surface from 1 to 10 % is shown in Figure 3. It can be seen from the Figure that the average particle size in all the samples is below 7 nm. The average particle size distribution slightly differs independent of the Au concentration [Figure 3 (g)].

4 

Elemental composition of the as-synthesized material was confirmed by EDS analysis which is shown in Figure 4. All expected peaks due to the presence of Ni, S, Au and Sn were obtained with the exception of oxygen peak resulting from the surface adsorbed oxygen which does not form part of the overall crystal structure.

Photoluminescence properties The room temperature PL properties of the materials were investigated. A broad UV emission peak located around 400 nm was observed in Figure 5(a). The broad peak is comprised of three peaks located at about 400, 426 and 446 nm. UV emission around 400 nm has been observed for both - and -NiS in the literature [19, 20]. The presence of more than one NiS phase could be accountable for the multi-emission peaks observed. It is well known that PL properties of nanomaterials are greatly affected by the size, shape as well as the presence of structural defects within the sample. The multi-peaks observed for the as-synthesized NiS could also be attributed to a wide size distribution range of the material as well as uneven morphology distribution in the sample. A second and much more pronounced PL emission was observed in the near IR region. Three peaks were evident with emission maxima located at 710, 754 and 784 nm respectively. Red emission from -NiS was observed by Pan et al. and was attributed to a frequency-doubling peak [19]. The multi peaks indicate the presence of structural defects as well as wide size distribution of sample. The band gap of a semiconducting NiS material has been reported to be around 0.22 eV [35]. Both the UV and the near IR emissions observed in our study appear at higher energy compared to the reported band gap of NiS. The appearance of the emissions at a higher energy is indicative of a quantum size effect of the NiS nanostructures (please refer to the section below on how band gap calculations were obtained). Since the reported optical band gap of NiS is too far in the IR region, the observed emissions could also be attributed to intra-band transitions that take place on the NiS band structure after excitation. This has been observed for our earlier report on NiS2 intra-band transition [7]. The additional multi peaks could also be due to S induced energy state transitions [7].

5 

Figure 5 (a) also shows PL spectra of the Au nanoparticle decorated NiS in the UV and near IR region. It can be noted for both the UV and the near IR emissions that Au enhances the emission of the NiS material with a slight blue shift in the peak positions. The enhancement factor as a function of Au concentration is shown in Figure 5 (b) for the emission peaks located at 400, 426, 446, 754 and 784 nm. It can be seen from the graph that the UV emission located at 446 nm is the most enhanced peak and an average of about threefold enhancement was obtained for all the peaks. The PL enhancement is due to SPR excitation of the surface Au NPs which results in increased rate of spontaneous emission. SPR process is also accountable for enhanced PL emission observed due to increased Au concentration. According to the plot in Figure 5 (b), there is a great enhancement on the 2% Au coated NiS sample followed by 7% and 10 % Au coated NiS samples respectively. Although these concentrations are different, the Au NP size distributions are higher compared to the other Au coated NiS samples. This suggests that the effect could mostly be related to particle size distribution. The enhancement observed at 2% Au loading compares well with some metal chalcogenide materials with reported PL enhancement found in the literature as shown in Table 2. A small PL enhancement was observed for the 1% and 5% Au NP coated NiS samples. This could be due to Au particle size as well as differenced in Au-NiS interaction. Our study reports for the first time in our information that Au NP coating can enhance emission properties of NiS nanostructures, and the enhancement can vary depending mostly on the average particle size of Au.

Optical band gap energies for the phosphors The optical band gap for all the samples were calculated from the UV-Vis-IR spectrophotometer measurements by using the Tauc’s equation. Figure 6 (a) and (b) shows the (DE)2 vs (E) plot where the extrapolations were made on the straight line of the plots until the energy was reached where absorption coefficient (D) is zero. The calculated band gaps for all the samples are listed in Table 3. All the band gaps were shown to be higher energy than the reported optical band gap of the dominating D-NiS phase (0.2 eV) [39]. This can be attributed the quantum confinement effect occurring as a result of decreased particle size. Salavati-Niasari and Sobhani also reported on the optical properties of mixed phase NiS, where they obtained a UV-blue emission which they attributed to quantum confinement. 6 

They also reported that bulk NiS shows absorption at 2.1 eV [9]. Salavati-Niasari et al also reported on UV absorption of D-NiS which was blue shift compared to the 2.1 eV they reported for bulk NiS. Sadjadi et al prepared bulk NiS which consisted of the binary NiS phase and their reported optical band gap was 3.53 eV [40]. Although the contradiction, we consider the theoretical band gap of 0.2 eV. Our calculated band gaps appear to be in the green-NIR region. It can also be noted that more than one peak appears for the samples with Au loading. This can be attributed to Au influence on the binary phase nanoparticles involved as confirmed by XRD analysis.

Conclusions In summary, NiS nanostructures were successfully synthesized by a microwave-assisted hydrothermal technique, and further decorated with Au as confirmed by XRD, TEM and EDS analysis. The NiS was shown to emit light in the UV and near IR regions. Au NP decoration was shown to enhance the PL emissions of NiS. A threefold average PL enhancement was obtained for Au NP decorated NiS nanostructures. Improvements in terms of morphology and metal NP size control could improve the results, as well as studies on the NP-NiS surface interactions. Au NP coating on the surface of NiS nanostructures should expand optoelectronic application range of the NiS material.

Acknowledgements The authors would like to acknowledge the financial support received from the Council for Scientific and Industrial Research (CSIR), South Africa (Project No. HGER27S). We would also like to acknowledge the contribution from the NCNSM characterization facility: Dr James Wesley-Smith, Mrs Tutuzwa Xuma and Ms Charity Maepa.

7 

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Shahri,

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Bazarganipour,

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2923.

List of figures Figure 1: XRD spectra of the as-synthesized NiS and Au decorated NiS. Symbol (*) indexed peaks correspond to D-NiS and (#) indexed peaks correspond toENiS phase. Figure 2: Typical TEM images of the as-synthesized NiS (a) and Au/NiS (b). Figure 3: Average diameter distribution of Au NPs decorated on NiS calculated using imageJ. 1% to 10 % loadings are shown from (a) to (f) respectively. The average diameter distribution change for the different Au NP percentages is shown in (g). Figure 4: EDS spectra for the 10 %Au decorated NiS and as-synthesized NiS. Figure 5: Shows photoluminescence spectra of the prepared samples excited at  = 300 nm. As-synthesized NiS nm and Au NP decorated NiS PL emissions showing both UV and red emissions are shown in Figure 5(a) while NiS emission peak enhancement factor vs Au concentration for the UV and red emission peaks located around 400, 426, 446, 754 and 784 nm is shown in Figure 5(b). The enhancement factor was obtained by dividing the peak intensities of the Au/NiS samples with that of the bare NiS intensity. Figure 6: Band gap estimation for (a) bare NiS and (b) Au-NiS samples.

List of tables Table 1: Peak list for the NiS phases and Au phase obtained from the XRD pattern. Table 2: PL emission for Au NP decorated semiconducting nanostructures. Table 3: optical band gap of bare and Au NP decorated NiS calculated from UV-Vis-IR spectrophotometry absorption analysis.

10 

Highlights  x x x x

BinaryphaseNiSdecoratedwithgoldnanoparticles QuantumconfinementeffectconfirmedbyPLanalysis PLenhancementdependingmoreonparticlesizedistribution EffectofgoldonNiSPListothebestofourknowledgereportedforthefirsttime

11 

NiS peak 2Tpositions (D*, E#) 30.3* 32.3# 34.8* 35.8# 37.5#

Au peak 2Tpositions

38.5 40.5# 44.9 46.2* 48.9# 50.2# 52.7# 53.6* Table 1

Material 2%Au-NiS Au-CdS Au-SiO2 Au-CdTe

Enhancement factor 4.3 5 15 3

Reference Current work [36] [37] [38]

Table 2

Bare-NiS (eV)

1% Au/NiS (eV)

2% Au/NiS (eV)

3% Au/NiS (eV)

5% Au/NiS (eV)

7% Au/NiS (eV)

2.1

1.0 1.6

1.3 1.7

1.1 1.8

1.1 1.7

1.1 1.7

 

10% Au/NiS (eV) 1.2 1.8

Table 3

2% Au/NiS (eV)

1.3 1.7

1% Au/NiS (eV)

1.0 1.6

Bare-NiS (eV)

2.1 1.8

1.1

(eV)

3% Au/NiS

1.7

1.1

(eV)

5% Au/NiS

1.7

1.1

(eV)

7% Au/NiS

1.8

1.2

(eV)

10% Au/NiS

#

300

100*

30

#

101*

35

021

2% Au/NiS 5% Au/NiS 10% Au/NiS

45

Au(200)

Au(111)

2T (Degrees)

40

#

131

Bare NiS 1% Au/NiS 3% Au/NiS 7% Au/NiS

#

220

#

221

#

50

410

#

401

Figure 1

110*

102*

55

Counts (a.u.) Counts (a.u.) 0

50

100

150

200

250

0

50

100

150

200

250

1

2

2

3

4

4

5

6

5.60

6

5.79

8

7

10

5%

14

16

18

20

3

4

5

6

7

8

9

0

9 10 11 12 13 14 15 16 17

12

d (nm)

8

d (nm)

1%

Counts (a.u.) Counts (a.u.)

Figure 3

Average Diameter Distribution (nm)

4

6

8

8

10

12

12

d (nm)

10

14

14

16

18

18

7%

16

20

20

8

100

150

200

250

0

50

100

150

200

250

300

2

6

Au coating (%)

4

d (nm)

0 2

6.79

6

400 350

0

4

2%

50

2

6.68

50

100

150

200

250

0

50

100

150

200

250

Counts (a.u.) Counts (a.u.)

2

2

4

4

10

6

6.44

6

4.74

8

8

12

d (nm)

10

12

14

d (nm)

10

16

14

18

20

10 %

18

16

3%

22

20

Figure 4

-2 2 2

-2 2 2

Figure 6

0

10

20

30

40

50

60

70

80

90

(DE) (eV cm )

(DE) (eV cm ) 0.0

0

10

20

30

40

50

60

70

0.5

1.5

1.0

2.0

2.5

2.1 eV

1.5

2.0

Bare NiS 1% Au 2% Au 3% Au 5% Au 7% Au 10% Au

Bare NiS

E (eV)

2.5

3.0

3.5

E (eV)

3.0

(a)

3.5

(b)

4.0

4.0

4.5

4.5

5.0

5.0

Figure 2

Figure 5

0

50000

100000

150000

200000

250000

300000

350000

Enhancement Factor

PL Intensity (a.u.)

0

2

4

6

8

350

0

400

400

650

4

400 nm 426 nm 446 nm 754 nm 784 nm

6

8

(b)

710

700

Wavelength (nm)

500

(a)

Au concentration (% w/w)

2

450

426

446

Bare NiS 1% Au/NiS 2% Au/NiS 3% Au/NiS 5% Au/NiS 7% Au/NiS 10% Au/NiS

750

754

10

800

784