Phase Transformation and Evolution of Localized Surface Plasmon

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Phase Transformation and Evolution of Localized Surface Plasmon Resonance in Cu2−xS Thin Films Deposited at 60 °C A. Dennyson Savariraj,† Hee-Je Kim,† Senthil Karuppanan,‡ and Kandasamy Prabakar*,† †

Department of Electrical and Computer Engineering, Pusan National University, San 30, Jangjeong-Dong, Gumjeong-Ku, Busan 46241, South Korea ‡ Department of Physics, Bannari Amman Institute of Technology, Sathyamangalam 638 401, Tamil Nadu, India S Supporting Information *

ABSTRACT: Cu2−xS (0 ≤ x ≤ 1) thin films deposited at low temperatures ( 0) films develop LSPR in the NIR region.10,27,39 So, the advantages of our methods over others are that (i) phases and size/shape can be tuned by simply varying the deposition time and hence the LSPR frequency, (ii) deposition temperature is very low, and deposition can be made on flexible substrates, which is an essential factor in flexible photovoltaic devices, and (iii) integration into electronics circuits is feasible at low cost. Elemental composition measured from EDS analysis is shown in Figure S1, and the atomic percentage of the Cu:S ratio is given in Table S1. The EDS analysis shows that the Cu:S ratio of CTAB-2 is 63.76:36.24, while CTAB-3 has 41.99:58.01, where the Cu content is drastically decreased and holds almost same amount of Cu as CTAB-4 and CTAB-5. EDS analysis confirms that CTAB-2 is a different phase compared to rest of the films. The XPS analysis was used to infer the composition and ionization state of the compounds. The samples were not subjected to any heat treatment. In order to find the nature of bonding of the elements, the individual Cu 2p and S 2p peaks were scanned at a higher rate of resolution. XPS data were fitted with Gaussian−Lorentzian (30% Gaussian) functions and Shirley type background using Casa XPS software. Four constraints were applied to deconvolute the S doublets (2p3/2:2p1/2) such as the spin orbit splitting of 1.2 eV, the peak area ratio of 2:1, equal full width at half-maximum for the corresponding oxidation states in the four samples, and the binding energies for sulfide (S2−), disulfide (S22−), copper deficient nonstoichiometric sulfide (Sx2−), and sulfate (SO4)2− were fixed respectively at 161, 161.8, 163, and 168.5 eV.40,41 The presence of elemental sulfur (S0) has been excluded from fitting due to high vapor pressure; S0 would be evaporated if done at room temperature and at very low pressure (high vacuum).40 Hence, the presence of elemental sulfur can be measured only at low temperature. The binding energy positions should not be directly compared with other elements, for example, the binding energy of S2− in covellite (CuS) is different from that of FeS.42,43 The binding energy values depend on the structure of the materials.44 The fwhm depends on the X-ray source and the pass energy used.45 In our case, the pass energy was 40 eV and hence would normally have higher fwhm than 10 or 20 eV pass energies used. Figure 4 shows the core level XPS spectra for sulfur. The composition of the individual elements is given in Table 1. It is very interesting to

Figure 4. Core level XPS spectra for S of the Cu2−xS thin films on FTO substrate.

Table 1. Composition and Oxidation States of S from XPS Spectra of the Cu2−xS Thin Films on FTO Substrate sulfur (%) sample

161 (eV) (S2−)

161.8 (eV) (S22−)

163 (eV) (Sx2−)

168.5 (eV) (SO4)2−

CTAB-2 CTAB-3 CTAB-4 CTAB-5

33.25 19.98 16.41 14.94

31.09 44.62 47.91 54.74

29.39 34.40 35.68 30.33

6.27

note that S2− decreased with deposition time, whereas disulfide (S22− corresponds to S−S bonds) showed the opposite trend. Covellite (CuS) consists of alternating layers of Cu+−S22−−Cu+ and CuS (Cu2+ bond with S2−) linked by disulfide.46,47 Twothirds of sulfur atoms are present as disulfide (S22−), and onethird of sulfur atoms occupy the center of a triangle of copper, and the remaining copper atoms located at the center of a tetrahedral arrangement of sulfur atoms agrees with our XPS measurement. On the other hand, djurleite has almost similar to low chalcocite crystal structure with little high copper vacancy (∼3.1%), hence showing decreased minimum absorption compared to other films.48,49 Figure 5 shows the core level Cu spectra with two distinct peaks at 932.5 and 952.4 eV representing the binding energies of Cu 2p3/2 and 2p1/2 states, respectively. Even though it is not fitted, the asymmetry in the peak corresponds to two elements.50 The valence of copper in CuS is still debatable.16,51 The holes are left behind at the top of the valence band any time copper vacancies are created, which implies that the valency of chalcogenide is mainly altered. Even though (Cu+)3(S22−)(S−) or (Cu+)3(S2−)(S2−) with a valence of 1 in CuS, where sulfur has valence of −1, has been reported, our studies indicate the presence of both Cu(I) and Cu(II) with the formulation of (Cu2+)(S22−)(Cu+)2(S2−) and might have a valence between 1 and 1.5.52 D

DOI: 10.1021/acs.jpcc.7b07332 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

for comparison, Na2S2O3 for disulfide and Na2SO4 for sulfate were included. All the samples show sulfate (S6+) peaks at 2481.7 eV, and the intensity decreases with deposition time. XANES spectra were measured ten months after the films were deposited while XPS was measured within a month’s time. We could clearly see that XPS shows the presence of sulfates only for CTAB-2 samples with almost negligible amount for the rest of the samples, and hence, we could infer that all the samples were prone to oxidation over a period of time. A low lying pre-edge feature observed at 2469.8 eV could be due to the presence of S2− when a metal 3d hole delocalizes with the S 3p orbitals.57,58 In addition, an intense peak observed at 2471.1 eV is evidence toward the presence of S22−, which clearly matches with the disulfide peak of our reference sample Na2S2O3. The unoccupied states of S 3p character emerge from the electron density given by the 3d orbitals of Cu in a partially filled d-shell, through orbital hybridization.59 These data clearly show the significantly higher amount of disulfide (S22−) and lesser of monosulfide (S2−) in the spectrum of Cu2−xS that could be correlated to the covellite bonding as verified by XRD and XPS analysis. Even though we have not quantitatively calculated the mono- and disulfide, it is vividly seen that disulfide is much higher than monosulfide originating from two types of Cu bond formation, in which two-thirds of tetrahedral bound to disulfide and one-third of Cu center forming trigonal geometry bound to monosulfide agrees with our XPS measurement.

Figure 5. XPS spectrum of the Cu(2p) core level of the Cu2−xS thin films on FTO substrate.

It is a little difficult to compare the structural changes observed in XRD with XPS, since the latter is surface sensitive with maximum penetration depth of 10 nm whereas X-ray represents the whole crystal system with penetration depth of more than 100 nm. The surface oxidation is minimized with increased deposition that could possibly be because of reduced tangling bonds (surface atoms have made complete bond with neighbors) in hexagonal close packed covellite structure.46 These observations are in good agreement with the composition and phase evolution of metal chalcogenides, since the nature of ligands or anions determines the relative stability of Cu(I) and Cu(II) counts, such as those containing donor atoms like sulfur reacting with Cu(II) to form Cu(I) complexes.53 The p orbitals of the chalcogenide and the 4s and 4p orbitals of Cu contribute toward the formation of the top of the valence band and the bottom of the conduction band, respectively. Each Cu atom contributes with each 4s electron along with six p electrons toward bonding. In a perfectly stoichiometric Cu2X (X = S, Se, Te) compound, the valence band is totally filled.22 XANES spectroscopy of sulfur K-edge and pre-edge resonances is extremely sensitive to the oxidation state of sulfur and shifts to higher energy position of about 12 eV between sulfides (S2−) and sulfates (S6+) due to screening effects and hence we could probe the electronic and molecular structure of molecules (Sn2−) and its chemical bonds with different electronegativity elements.54−56 The K edge absorption of sulfur arises from the transition of S 1s core electrons to unoccupied orbitals above the Fermi level. Figure 6 shows the XANES spectra of the samples deposited at different time, and

5. CONCLUSION We have presented a novel and systematic study of localized surface plasmon resonance (LSPR) occurring in Cu2−xS thin films grown on FTO. Our report shows that Cu2−xS thin films synthesized through simple chemical bath deposition do exhibit LSPR due to increased Cu vacancies by increasing the deposition time and surfactant assisted structural morphology. As the deposition time is increased, the decrease in the surface free energy thermodynamically drives the reaction toward the stacking of the nanorods and nanoflakes to form uniform arrangements with the phase transformation from djurleite to covellite via Ostwald ripening mechanism. This in turn tunes the surface morphology, which eventually alters the quantum confinement of charge carriers. The occurrence of (LSPR) in a thin film at low temperature, without any dopant and only by varying the deposition time, opens hopes for exploring several potential optoelectronic and biological applications in the near future.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07332. EDS analysis and elemental composition (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Tel: 051- 510-7334. Fax: +82-51-513-0212. ORCID

Kandasamy Prabakar: 0000-0001-7582-0765

Figure 6. XANES sulfur K-edge spectra of the Cu2−xS thin films deposited on FTO; for comparison, standard samples of pure Na2S2O3 for disulfide and Na2SO4 for sulfate were included.

Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acs.jpcc.7b07332 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work was supported for 2 years by Pusan National University Research Grant.

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