Electron transport in the plasmonic regime ... - Wiley Online Library

Electron transport in the plasmonic regime: Silver nanoparticles in ZnO matrix

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Phys. Status Solidi B 252, No. 3, 558–565 (2015) / DOI 10.1002/pssb.201451467

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basic solid state physics

D. Ghosh1, B. Ghosh1, R. Bhunia1, S. Das1, S. Hussain1,2, R. Bhar1, and A. K. Pal*,1 1 2

Department of Instrumentation Science, USIC Building, Jadavpur University, Calcutta 700032, India UGC-DAE CSR, Kalpakkam Node, Kokilamedu 603104, India

Received 3 September 2014, revised 9 October 2014, accepted 21 October 2014 Published online 19 November 2014 Keywords Efros–Shklovskii model, electron transport, hopping energy, nanocrystalline silver, surface plasmon resonance * Corresponding

author: e-mail [email protected], Phone: þ91-33-2414-6321, Fax: þ91-33-2414-6584

Silver nanoparticles were embedded in zinc oxide matrix by using sputtering-cum-evaporation technique. Particle size and metal volume fraction were tailored by varying the amount of silver in the Al–ZnO matrix. Strong surface plasmon resonance peak in the optical absorbance spectra was observed at

500 nm. Electrical conductivity was measured in the temperature range of 80–220 K when illuminated at 500 nm, and in dark to explain the observed transport processes associated with this material.

ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Electron transport in low dimensional structures has been studied intensively for decades [1–5]. These systems usually had metal nanocrystallites embedded in dielectric matrices with the research mainly focussed on the optical properties of the nanoparticles embedded. Usually, these nanoparticles were of noble metals, primarily, gold and silver. Dependence of the shape and position of the surface resonance frequency on the nanocrystallite size and shape was studied critically and the experimental results were analyzed in the light of the existing theories. In such reports, the most favored dielectric matrix was either ZnO or SiO2. More recently, the use of plasmonic layer in photovoltaic solar cells for improving light trapping and hence cell performance was witnessed and reported [6–12]. Introduction of a surface plasmon layer in the cell structure offered an alternate way to improve the efficiency of thin-film solar cell structures [7–12]. Light-induced excitations of electrons on metal surfaces were also seen to provide the possibility of integrating electronics and optics on the nanoscale. The above reports depict the advantages of having the plasmonic metal nanoparticles in enhancing the optical and electrical performances of the PV devices. Despite the above advantageous contribution in photovoltaic technology of improving the cell performance by the plasmonic layer, the electron transport phenomena of these systems in the surface plasmon domain has not been

addressed critically till date. The nano-Ib metal-composite materials resemble a system similar to that of a disordered material. In such nanocrystalline-disordered system, there are two issues that distinguish their charge transport as compared to that for a bulk system. First is the granular or discrete nature of the electronic charge and the second, the preservation of phase coherence of the electron wave function over short dimensions. These mesoscopic systems can thus, be considered as disordered systems where hopping of charge carriers through localized states would spread in the gap region. A few reports have been published on the electron transport processes of similar disordered system consisting of semiconducting materials in nanocrystalline form embedded in a glassy matrix [13–17]. Recently, Saleh et al. [18], fabricated single and double plasmonic interfaces consisting of silver nanoparticles embedded in media with different dielectric constants including SiO2, SiNx, and Al: ZnO by a self-assembled dewetting technique and integrated to amorphous silicon films. In their report, the photocurrent showed an overall decrease for the samples with the interfaces. Significant enhancement of photocurrent was observed near the low energy edge of the bandgap (600– 700 nm). These results correlated well with the broadened extinction spectra of the interfaces and are interpreted in terms of enhanced absorption in that region. The only report available on the studies on electron transport in ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original Paper Phys. Status Solidi B 252, No. 3 (2015)

discontinuous silver film deposited by DC magnetron sputtering has been reported by Mandal et al. [19]. In the report, temperature dependence of electrical conductivity was studied and it indicated the variable range hopping to be the predominant mode of electron transport in the films. But, studies on electrical conductivity of nanocrystalline Ib metal embedded in dielectric material composites have not been reported so far. Further, modulations of electron transport processes in the surface plasmon domain in these materials have not been reported at all. In this communication, we present our studies on the electron conduction processes of the composite materials composed of silver nanoparticles (n-Ag) embedded in Al– ZnO matrix in the plasmonic domain. This report is perhaps the first of its kind on this important scientific aspect of electron transport in plasmonic regime. The composite films studied here were prepared by simple sputtering-cumthermal evaporation technique. Data were analyzed to derive meaningful information on the electron transport processes operative in this composite film. 2 Experimental details Films of nanocrystalline silver embedded in Al–ZnO matrix were synthesized by using a multi-target sputtering system which could be evacuated to a vacuum level of 106 Torr by a turbo molecular pump. A 300 target of aluminium-doped ZnO was utilized to sputter ZnO at a system pressure of 102 Torr to realize Al–ZnO/n-Ag/Al–ZnO structure (henceforth referred as ZnO/n-Ag/ZnO structure). Nano silver (n-Ag) was evaporated by thermal evaporation on ZnO layer for the realization of a surface plasmon peak in the absorption spectra. The thickness of ZnO (50 nm) and n-Ag (4 nm) layers were pre-calibrated so that the ZnO/n-Ag/ZnO composite retained the surface plasmon characteristics. The total thicknesses of the composite films were in the range of 50–60 nm. A Carl Zeiss SUPRA1 55 field emission scanning electron microscopy (FESEM) was used to record the surface morphology at an operating voltage of 3–5 kV in secondary emission mode. X-ray diffraction (XRD) studies were carried out by using Rigaku MiniFlex XRD (0.154 nm Cu Ka line) to obtain the micro-structural information. Optical studies were performed by measuring transmittance and absorbance in the wavelength region l ¼ 200–900 nm using a spectrophotometer (Hitachi-U3410) at room temperature. The spectra were recorded with a resolution of l 0.07 nm along with a photometric accuracy of 0.3% for transmittance measurements. Electrical conductivity of the films was recorded at temperatures ranging from 80 to 220 K by using a Keithley source meter. For electrical contacts, we have used pre-fabricated thick silver pads (2 mm) on the cleaned glass substrate deposited by thermal evaporation using appropriate stainless steel mask. The ZnO/ n-Ag/ZnO structure was deposited using appropriate stainless steel mask and the active film area was 5 mm 5 mm. The size of the light spot was very nearly the same as that of the active area. The light spot was focussed from an www.pss-b.com

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Oriel 1/4 m monochromator using a 300 W xenon arc lamp as the emission source. A Janis cryostat, which could be evacuated to a level of 106 Torr by a turbo work station, was used for the above conductivity measurement. 3 Results and discussion Adhesion of nanocrystalline silver (n-Ag) embedded in ZnO films deposited on glass substrates were found to be very good. These composite films (ZnO/n-Ag/ZnO) synthesized here may be considered as a disordered system composed of ultra-fine metal (silver) particles embedded in a dielectric (ZnO) matrix. Large surface to volume ratio of the metal crystallites was a typical characteristic of the films. The distances between silver nanocrystallites were dependent on the metal particle loading in the insulating ZnO matrix. Thus, the properties of these disordered materials would be different from that of the bulk as long-range order and interaction effects influencing the electronic properties would be absent here [7–10]. The charge transport in such a system would be considered through a network of filler (n-Ag) particles within a dielectric matrix (ZnO). The nature of this network would entirely depend on the sizes of the n-Ag grains, their separation, and the volume fraction of the filler (n-Ag) in the specimen. At a lower volume fraction, the conducting particles would form small and isolated islands within the insulating matrix. In such a case, the conductivity becomes activated. With the increase in the volume fraction of the filler (n-Ag), the islands would tend to grow larger, numerous and quite comparable to the insulating medium (ZnO) separating two adjacent n-Ag. This would culminate in a reduction in the activation energy. In this case, the contribution to the electrical conductivity would come from the percolation along the n-Ag maze and tunneling of electron between the isolated islands (n-Ag sites). With the decrease in the number density of n-Ag nanocrystallites, i.e. the volume fraction of the filler, the density of percolation channels would get reduced. As the conductivity depends on the density of the percolation paths and the resistance offered by different paths, the conductivity would decrease in these systems. The basic interest of this study is to measure the electrical conductivity of ZnO/n-Ag/ZnO composite films in the plasmonic domain. For this, a careful selection of the particle size and the volume fraction of n-Ag are essential so that the ZnO/n-Ag/ZnO composite films retain the surface plasmon characteristic peak in the absorption spectra. A detailed study depicting the above issues in n-Ag/SiO2 system has been reported by Mandal et al. [4]. It was clearly indicated that there existed a critical particle size and metal concentration below which the surface plasmon resonance state was suppressed and gave rise to a sharp absorption edge corresponding to semiconducting optical properties. Keeping this in mind, we have confined our measurements on electrical conductivity to such ZnO/n-Ag/ZnO composite films, which showed a strong signature of surface plasmon resonance (SPR). ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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D. Ghosh et al.: Electron transport in the plasmonic regime

Figure 1 FESEM pictures of: (a) n-Ag (inset shows the histogram), (b) ZnO, and (c) ZnO/n-Ag/ZnO films.

3.1 Microstructural studies Figure 1a–c shows the representative FESEM pictures of the n-Ag, ZnO, and ZnO/ n-Ag/ZnO film, respectively. All the films were deposited on glass substrates at room temperature. It may be mentioned here that the deposition conditions of all the above films were kept invariant while synthesizing the ZnO/n-Ag/ZnO composite films. The topography of n-Ag film (Fig. 1a) shows the existence of discrete silver nanocrystallites forming a film with a narrow size distribution. The ZnO film showed a compact texture with a rougher surface (Fig. 1b) with surface roughness 20 nm. In contrast, the ZnO/n-Ag/ZnO composite films showed a smoother surface (Fig. 1c) than that of virgin ZnO film. Some nanocrystals of silver are also visible on the surface. The XRD traces for the above ZnO film and ZnO/n-Ag/ ZnO composite films are shown in Fig. 2a and b. Both the traces indicated peaks at 2u 31.68, 2u 36.78, 2u 42.98, 2u 56.58, 2u 76.48, and 2u 80.68 for reflections from (100), (101), (204), (110), (202), and (104) planes of hexagonal ZnO. Intensity of peaks for reflections from (101) and (202) peaks located at 2u 36.78 and 2u 76.38 increased substantially with the overlayers of nano silver and another ZnO layer grown above the nano-silver layer. No characteristic peaks for silver was observed. 3.2 Optical studies Figure 3a and b shows the transmission and absorption spectra of ZnO, n-Ag, n-Ag/ ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ZnO, and ZnO/n-Ag/ZnO films, respectively. The transmission spectra indicated (Fig. 3a) that pure ZnO film had maximum transmittance (93%) and as such had quite low absorbance (Fig. 3b). With the addition of n-Ag film, transmittance decreased while the absorbance increased significantly due to SPR resonance peak (Fig. 3b). The intensity of the SPR peak was stronger for n-Ag film than for both the n-Ag/ZnO and ZnO/n-Ag/ZnO composite films. The SPR peak for n-Ag film appeared at 500 nm. The peak position indicated a blue shift when the silver nanocrystallites were embedded in ZnO matrix. This is due to modulation of the effective dielectric medium in n-Ag/ZnO and ZnO/n-Ag/ZnO composite films. The band gap of the ZnO, n-Ag/ZnO, and ZnO/n-Ag/ZnO composite films was estimated (Fig. 3c) from their (ahn)2 versus hn plot. It was found that the band gap of ZnO/n-Ag/ZnO composite films (3.19 eV), was lower than that obtained for pure ZnO film (3.3 eV). The fall in the absorbance of the n-Ag/ZnO and ZnO/n-Ag/ZnO composite films is not that sharp as has been observed in pure ZnO film (Fig. 3b). This is due to the presence of large number of density of states arising out of n-Ag inclusion in ZnO matrix. The position of the SPR peak in pure n-Ag sample is also different from that of the ZnO/ n-Ag/ZnO composite film. The optical absorbance spectra are dominated by a single absorption peak corresponding to dipolar interaction between the particles. Higher order multiple interaction among the nanoparticles would appear www.pss-b.com


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Figure 2 XRD trace of: (a) ZnO and (b) ZnO/n-Ag/ZnO composite films deposited at 300 K on glass substrate.

Figure 3 (a) Transmittance and (b) absorbance versus wavelength (l) plots for: n-Ag, ZnO, n-Ag/ZnO, and ZnO/n-Ag/ZnO composite films. (c) Plots of (ahn)2 versus hn for ZnO and n-Ag/ZnO and ZnO/n-Ag/ZnO composite films.

as a splitting of the single absorption peak into several peaks and such new features in the absorbance spectra (not observed in this case) could be ruled out [20]. 3.3 Electrical measurement Variation of electrical conductivities (ln s) of ZnO/n-Ag/ZnO composite films in DC mode with temperature (1000/T) and measured in dark and when exposed to 500 nm radiation have been shown in Fig. 4a. It is apparent that the conductivity of the film increased when measured at 500 nm radiation (Fig. 4a). A www.pss-b.com

plot of the variation in current as a function of temperature measured under identical condition is shown in the inset of Fig. 4a. From the figure, increase in current is clearly observed when illuminated with 500 nm radiation reaffirming the increase in electrical conductivity of the composite film. One may still argue that there could be possibile contributitions from either ZnO or n-Ag layers modulating the observed increase in current when irradiated with 500 nm radiation. For this, we have measured the variation in current as a function of temperature for both the ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 4 (a) Variation of ln(s) with 1000/T for a representative ZnO/n-Ag/ZnO films (s in mho cm1 and T in K) measured under dark (black squares) and 500 nm radiation (red circles). Inset shows the variation of current as a function of temperature measured under identical condition. (b) Plot of ln[d(ln s)/d(ln T)] versus ln(T) for a representative film of ZnO/n-Ag/ZnO (s in mho cm1 and T in K) measured under dark (black squares) and 500 nm radiation (red circles). Inset shows the variation of current as a function of temperature for Al–ZnO film measured under identical condition. (c) Plots of ln(s) versus 1000/T0.5 for a representative film of ZnO/n-Ag/ZnO measured under dark (black squares) and 500 nm radiation (red circles).

ZnO layer and n-Ag layer under identical condition. It may be noted here that the resistance of n-Ag layer (4 nm) used in this study was extremely high and as such current versus temperature plot could not obtained. The current versus temperature plot of the Al–ZnO measured in dark and when illuminated with 500 nm radiation is shown in the inset of Fig. 4b. One may notice that Al–ZnO layer did not show any increement in current with the exposure to 500 nm radiation. The above results conclusively indicate that the observed increement of current is due to plasmonic effect induced by the n-Ag. Both the variations in conductivity indicated Arrhenius type of activation. The activation energy for the composite films in the lower temperature range was quite low (0.05 to 0.10 eV) while that in the higher temperature range was 0.26 to 0.37 eV. The above values of activation energies are quite small which generally signify the electron transport processes to be governed by tunnelling in this disordered system. The difference in sizes of the grains and their irregular shapes would be the obvious sources of disorder in these ZnO/n-Ag/ZnO composite films. Changes in the intergrain or grain-boundary region in the system would also contribute a considerable amount to disorder. In such a disordered system, long-range Coulomb interaction would ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

play a dominant role in the electron transport processes and hence, localized states and a “Coulomb gap” could be conceived in the fundamental gap region of the low dimensional disordered system [21]. Electron transport process in our nanocrystalline ZnO/nAg/ZnO system has been studied at relatively low temperature. In the past, Demichelis et al. [22] showed that the variable range hopping (VRH) process favored an electron to jump from one localized state to another having an overlap of the wave functions. Absorption or emission of phonons compensates the difference in eigen energies in such cases. Thus, the electron transport in such a system in low temperature regime could be attributed to thermally activated hopping between localized states near the Fermi level. The conductivity for such a system, in general, may be given as [23–26] To p ; ð1Þ s ¼ s o exp T where so, the pre-exponential factor, may either be independent of T or a slow varying function of T while To is a constant given by To ¼ e2/e0era, where e0 and er are the free space permittivity and the dielectric constant of www.pss-b.com


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the material, respectively. e refers to the electronic charge and a1 represents the electron localization length. The value of the exponent p would indicate the nature of the hopping process and could be obtained [27] from the plots of ln[W(T)] versus ln(T) where W(T) ¼ d[ln s(T)]/d[ln(T)]. Figure 4b shows such plots for the ZnO/n-Ag/ZnO composite films measured in dark and under illumination (500 nm). The values of p obtained from the slope of the above plots were in the range of 0.43–0.53. This value of p suggests that the dominant electron transport mechanism in the ZnO/n-Ag/ ZnO composite system for the entire low temperature range of measurement (80–220 K) would predominantly be the variable range hopping within the Coulomb gap (Efros– Shklovskii hopping or E–S hopping). In this temperature range, the nature of the Coulomb gap would be “soft.” Thus, the experimental data could be represented well by E–S model with p ¼ 0.5. Figure 4c shows the plots of ln(s) versus T0.5 for a representative film measured in dark and under illumination (500 nm). During electron transport in a system with “soft” Coulomb gap (p ¼ 0.5), the density of states (DOS) would exhibit a parabolic variation. This can be expressed as [23–27] gðEÞ ¼ goES ðE E f Þ2 ;

DES ¼

goES ¼

ð25 e6 Þ

¼

(i) (ii) (iii) (iv)

Ropt ¼

To T

ð3Þ

1=2 ;

ð4Þ

kT 0 ðpgoES Þ1=3 ; 10:5

ð5Þ

W opt ¼ 0:5k ðT 0 TÞ1=2 ;

ð6Þ

am ¼

ð7Þ

Wopt > kT or To > T Ropt d DES > Wopt DES > kT.

In the domain of E–S hopping, one would expect large goES and a wide Coulomb gap (DES > kT). At a very small goES, the width of the Coulomb gap would become smaller than kT rendering it irrelevant for conductivity. In such a case, the conductivity of the disordered system may arrive at Mott’s law region. It may be observed from Table 1 that both the hopping energy (Wopt) and the width of the Coloumb gap (DES) increased in the plasmonic region. The width of the Coulomb gap would also correspond to the width of the peak of the density of states. This observation may basically be due to the addition of hot electrons in the SPR region. As observed from Fig. 3b, the nature of the surface plasmon peak associated with these samples is broad and Gaussian in nature. This would constitute additional density of states in the higher energy domain for the sample when exposed to radiation (500 nm in this case). The energy of these hot electrons would also have a parabolic distribution of energies. This would culminate in the broadening of the Coulomb gap to accommodate the above additional density of states of the hot electrons, which has been reflected in Table 1. This increment of 3 meV in Wopt was observed when the sample was exposed to radiation of 500 nm. This increment would arise from the photon-assisted increase in energy of the hot electrons in the SPR state modulating the density of states. Now, using (2), (3), and (7), one may obtain: goM 1=2 : ð8Þ DES ¼ goES

Here er is the relative dielectric constant of the material, eo is the free space permittivity and e is the electronic charge, ao ¼ 3/p, ko is static dielectric constant and goES is the density of states for E–S model. E–S hopping within the soft gap is characterized by a few important hopping parameters like hopping distance (Ropt), tunnelling exponent (am), optimum hopping energy (Wopt), and coulomb gap (DES) and can be expressed as [28–30] 0:25 a1 m

goM 1=2 : k3o

3.4 Justification of the applicability of the E–S model The following criteria must be satisfied for the VRH to become the most dominant transport mechanism within the Coulomb gap:

ð2Þ

ao k3o : e6

2 ðT 0 TÞ1=2

¼ e3

Calculated values of the activation energies (Ea) and the hopping parameters evaluated from the experimental measurements for ZnO/n-Ag/ZnO films carried out in dark and under 500 nm radiation are shown in Table 1.

where 38 p2 e3r e30

k

Table 1 Different hopping parameters calculated from E–S model. sample no.

condition

Ea (eV)

p

TES (K)

am (m1) x107

am1 (m) 108

Ropt (nm)

Wopt (eV)

DES (eV)

(goESL/goESD) from DES

(goESL/goESD) from TES

S-1

dark light dark light

0.262/0.111 0.281/0.118 0.312/0.052 0.374/0.052

0.48 0.51 0.52 0.53

803 950 832 1053

5.42 6.41 5.61 7.11

1.84 1.57 1.78 1.41

7.5 6.9 7.4 6.6

0.021 0.024 0.022 0.025

0.015 0.017 0.017 0.019

1.28

1.65

1.25

2.02

S-2

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Thus, denoting the measurements made in dark and under illumination with the suffix “D” and “L,” respectively, one may write:

goM goESD

DESD ¼ DESL ¼

DESL DESD

goM goESL

2

1=2 ;

ð9Þ

;

ð10Þ

1=2

goESL : goESD

¼

ð11Þ

Similarly, one may write [27] for the characteristic expression of To for the E–S hopping: T ES ¼

2:8e2 k B ðgoES Þ1=3 aB

T ESD ¼

;

2:8e2 kB ðgoESD Þ1=3 aB

ð12Þ

;

ð13Þ

and T ESL ¼

2:8e2 kB ðgoESL Þ1=3 aB

;

ð14Þ

such that

T ESL T ESD

3

¼

goESL : goESD

ð15Þ

Using the experimental value of Coulomb gaps and To (i.e., TES), we have computed the possible increment in density of states in the plasmonic domain. An increase in the density of states by 1.28 times when these nano-silver/ZnO composites are exposed to radiation corresponding to the SPR position could be observed. Using the experimental values of TES obtained from the measurement in dark and when excited at 500 nm radiation, one can also see that an increment of 1.6 in the DOS value in the plasmonic domain is indicated. It is known that the product of the density of states and probability distribution function indicates the number of occupied states per unit volume at a given energy for a system in thermal equilibrium. Thus, the increase in the density of states in the plasmonic regime would also culminate in an increase in the probability of occupation of the DOS. This would be reflected in an increase in the hopping probability of the charge carriers from one grain to the other resulting in an increase in the current collected by the collecting grids in case of a photovoltaic device (solar cells) [12]. The values of the above parameters for two representative ZnO/n-Ag/ZnO films are shown in Table 1. These composite films differed only in slight difference in grain size distribution of n-Ag layer deposited on identical ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 5 (a) Modulation of current as a function of the wavelength of radiation of the ZnO/n-Ag/ZnO composite film. SPR spectra of the same film are also included. Inset shows the variation of current as a function of wavelength of radiation of the Al–ZnO.

ZnO layers. The nature of the plasmonic peaks differed slightly in their value of full width at half maxima but not in their position. The 1/T0.5 law appeared to be reasonably well obeyed over a large temperature range for the system under study. This equally applies to this system when exposed to SPR state. It will be prudent to examine the selectivity of the modulation of electron transport in this system when exposed to different radiations, i.e., spectral response of conductivity. For this, measurements were performed on these samples by recording the modulation of current as a function of wavelength of radiation impinging on the sample while keeping the voltage invariant. Figure 5 shows the results of the above experiment measured at two different temperatures (100 and 300 K). It may be observed that maximum change in the current is observed at 500 nm, which corresponds to the peak position of the SPR. It may also be noted that the above spectral response matches well with the nature (peak position and distribution of the absorbance) of the SPR peak, which is also shown in the same figure. It may be noted that the nature of the spectral response recorded at two different temperatures implies that in variable range hopping the concept of a thermal distribution of electron energies, which is independent of phonon temperature, would have no impact in modulating the conduction processes in these disordered materials. Thus, the spectral response would reflect the effect of SPR on the variable range hopping processes as applied in this report. Further to reaffirm that Al–ZnO layer did not contribute to the observed enhancement of conductivity, spectral response measurements were also performed on Al–ZnO samples by recording the possible modulation of current as a function of wavelength of radiation impinging on the sample while keeping the voltage invariant (inset of Fig. 5). The no increment of current was observed for ZnO sample www.pss-b.com


reaffirming the origing of the enhanced conductivity is due to plasmonic layer only. 4 Conclusions Films of silver nanoparticles-embedded zinc oxide were deposited by sputtering-cum-evaporation technique on glass substrates. Strong surface plasmon resonance peak in the optical absorbance spectra was observed at 500 nm. Electrical conductivity was measured in the temperature range of 80–220 K in dark and when illuminated at 500 nm radiation to explain the observed transport processes associated with this material. The effect of the surface plasmon peak would constitute additional density of states in the higher energy domain. This would culminate in the broadening of the Coulomb gap. Both the hopping energy (Wopt) and width of Coloumb gap (DES) increased in the plasmonic region. This increment would arise from the photon-assisted increase in energy of the hot electrons in the SPR state. Acknowledgements The authors wish to thank Board of Research in Nuclear Sciences (BRNS), Government of India for the financial assistance to carry out this research programme. R.B. wishes to thank the Jadavpur University while D.G. and B.G. wish to thank the Department of Science and Technology, Government of India, and UGC-DAE consortium, respectively to support their fellowship.

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