- Email: [email protected]
A study on the defects structure of colloidal ZnS using Ag surface plasmons
Chemical Physics Letters 528 (2012) 49–52
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
A study on the defects structure of colloidal ZnS using Ag surface plasmons Sung Il Ahn ⇑ Department of Engineering in Energy & Applied Chemistry, Center for Green Fusion Technology, Silla University, Busan 617-736, Republic of Korea
a r t i c l e
i n f o
Article history: Received 20 October 2011 In final form 19 January 2012 Available online 25 January 2012
a b s t r a c t The effects of Ag surface plasmons (SPs) on the defects of ZnS are investigated. The photoluminescence (PL) spectra of ZnS on an Ag film show well resolved, enhanced, and red-shifted emissions compared to reference samples. The enhancement of the PL by the Ag layer is attributed to an enhanced energy transfer between overlapping band energies of ZnS by Ag SP. Based on the results, it is suggested that the PL emissions from defects can be enhanced and distinguished by SPs, which provide a direction for the application of SPs to characterize inorganic materials. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Surface plasmons (SPs) from the interface of a metal and a luminescent material are known to increase the photoluminescence (PL) efficiency of the material in question by enhancing the spontaneous recombination, where the surface plasmon resonance energy matches well with the emission energy of the luminescent [1–5]. A common example in semiconductors is an enhanced UV emission from Ag/ZnO, where the SP energy at the Ag/ZnO interface is close to the band-edge emission of ZnO of 3.27 eV [6,7]. Such enhancement was also observed from ZnO on a metal layer due to the roughness of the metal layer at the metal/semiconductor interface [8]. In the case of CdSe/ZnS quantum dots, roughly 50-fold enhanced fluorescence efficiency was obtained by tuning the SPR to the quantum dot exciton emission band in a specific design of interconnected Ag nano-crystals (NCs) [9]. The SP phenomenon has been also applied to spectroscopic analysis of biological materials and sensors under sharp variations of the plasmon resonance conditions [10,11]. These instrumentations using SP have allowed research on single molecules placed in various media; however, the applications are restricted to organic based materials except in a few cases [12–15]. Inorganic solids contain various types of defects, which are difficult to distinguish and characterize by normal spectroscopic methods. ZnS is an example having various crystal defects such as zinc and sulfur vacancies, surface defects, and defects by interstitial ions [16–19]. Difficulties in the characterization of ZnS are caused by challenges related to distinguish defects using spectroscopic methods. Upon this background, the author is interested in determining whether emissions from a defect or defects among numerous other defects in a solid could be enhanced or modulated by SPs. If this is
⇑ Fax: +82 51 999 5335. E-mail address: [email protected] 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2012.01.041
possible, the defect structures could feasibly be simplified or characterized easily by the application of SPs. The aim of this work is to characterize defect structures of ZnS using SPs. In addition, the effects of SPs on the emissions from defects sites of ZnS are discussed. It is anticipated that the finding of this study can facilitate the application of SPs to the analysis of inorganic materials.
2. Experimental Among various ZnS synthesis methods, a modified thiourea route was chosen to form a film type ZnS emitter. This allows in situ study of the SPR between an as-grown ZnS film and Ag film. After several trials to deliberately introduce defects to ZnS, it is found that acetyl-acetone suppressed the growth of ZnS and stabilized the ZnS NCs with various defects due to its strong ligand action. For preparation of the ZnS precursor, 0.4 mol of Zn(CH3COO)22H2O (Aldrich, P98%) were dissolved in a mixture solvent comprising 70 ml of 2-methoxyethanol (Aldrich, 99.8%), 20 ml of acetyl-acetone (Aldrich, 99.5%), and 10 ml of distilled water. Thiourea (Aldrich, 99.0%), in quantities of 0.32, 0.4, 0.48, and 0.56 mol, respectively, was then added to the mixture, which was stirred for 30 min. Each mixture was reacted under a reflux condition for 40 min and concentrated to 1.0 mol based on the content of the zinc source. An Ag film was formed on a Si(0 0 1) substrate with a thickness of 70 nm by sputtering at room temperature. The ZnS precursors were coated on the Si wafer as a reference (designated as Rf1 for 0.8, Rf2 for 1.0, Rf3 for 1.2, and Rf4 for 1.4 S/Zn ratio, respectively) and an Ag film on a Si wafer (designated as Ag1 for 0.8, Ag2 for 1.0, Ag3 for 1.2, and Ag4 for 1.4 S/Zn ratio, respectively) using a spin coater at 1500 rpm for 15 s. Each sample was dried at 80 °C for 30 min. Each dried sample was treated at 100 °C for 15 min and its PL was then measured. Using the same sample to avoid measuring variation of the PL, the same process was carried out at 120, 140, 160, and
50
S.I. Ahn / Chemical Physics Letters 528 (2012) 49–52
Figure 1. Selected XRD patterns, SEM, TEM images of a ZnS sample, and photos of spin-coated ZnS films on a Si wafer; (a) photos of spin-coated ZnS films on Si and Ag/Si (note that the reflected color indicates the uniformity of the films). (b and c) TEM images showing the colloidal ZnS NCs. (d) A selected surface image of ZnS/Si. (e and f) Selected SEM images of thickness-controlled ZnS films from different ZnS precursor solutions (0.75 and 0.5 mol, respectively). (g) Selected XRD patterns of ZnS from at a S/Zn ratio of 1.2 at various temperatures.
180 °C, sequentially. In addition, to ensure precise PL results, the measuring point was fixed using the right angle on a part of the Si wafer with ZnS film. ZnS precursor solutions with different concentrations of 0.75, 0.6, and 0.5 mol based on zinc source (designated as D1 for 1.0, D2 for 0.75, D3 for 0.6, and D4 for 0.5 mol, respectively) were additionally prepared and spin-coated on both substrates under the same coating conditions in order to investigate the effect of the thickness on the growth of ZnS at 140 °C. For characterizations, PL (Shimazu, UV-2450) spectra using an excitation wavelength of 325 nm from a Xenon lamp were recorded for each ZnS. X-ray diffraction (XRD, Rigaku D/max 2400) was used to investigate the crystallinity and structure type of ZnS NCs. The size and shape of ZnS was observed using transmission electron microscopy (TEM; JEOL Ltd., JEM-2100F (HR)) and scanning electron microscopy (SEM; FE-SEM S-4700, Hitachi Co.). The surface roughness of Ag film is characterized using atomic force microscopy (AFM; Scanning Probe Microscope, Seiko Co.).
10
0
b1 50
420
460
500
540
Wavelength (nm)
40
Ag1
Ag2
Ag3
Ag4
11
30
b2 50
Increasing sulfur
20
1 10
420
460
500
540
580
Wavelength (nm)
380
420
460
500
540
Wavelength (nm)
580
Rf3
Rf4
CB Vs
10
40
Ag1
Ag2
Ag3
Ag4
380
1 1 30 20
1 10
420
460
500
540
580
SS2
Wavelength (nm)
b3
0
0
20
0 380
580
PL intensity (a. u.)
380
PL intensity (a. u.)
0
PL intensity (a. u.)
Rf4
Rf2
2.39 eV
10
Rf3 20
c Rf1
2.85 eV
Increasing sulfur
Rf2
2.85 eV
Rf4
20
a3 Rf1
2.98 eV
Rf3
a2 30
3.88 eV
Rf2
The spin-coated samples have a thickness in the range of 1–0.5 lm with approximately 10% thickness variation based on SEM images of the cross-section of the ZnS film (selected SEM images are shown in Figure 1). The uniformity of the coated films can be estimated by the color of light reflected from the ZnS films, as shown in Figure 1a. Using TEM, attempts were made to define the NCs, but only colloidal aggregates of the NC were found. These most likely formed during the TEM sampling process, as shown in Figure 1b and c. A weak XRD peaks at around 28.3 and 31.9° in Figure 1e reveal that the as-synthesized ZnS with a S/Zn ratio of 1.2 and 1.4 at 140 °C has a cubic type ZnS structure mixed with ZnO. At a higher temperature of 180 °C, a reduction of the ZnO peak was observed. In the case of samples with low ratios of S/Zn at and below 140 °C, the specific peak for the cubic ZnS was not detected.
PL intensity (a. u.)
Rf1
PL intensity (a. u.)
PL intensity (a. u.)
a1 30
3. Results and discussion
VZn 40
Ag1
Ag2
SS1
Ag4
30
Ag3 1
VB
1 20
1
10 0
380
420
460
500
540
Wavelength (nm)
580
380
420
460
500
540
580
Wavelength (nm)
Figure 2. PL spectra of Rf and Ag samples as a function of the S/Zn ratio and heating temperature; Rf samples at 100 °C (a1), at 140 °C (a2), at 160 °C (a3) and Ag samples at 100 °C (b1), at 140 °C (b2), at 160 °C (b3). (c) The predicted schematic energy band diagram of the PL emissions (SS1; surface state 1, SS2; surface state 2, VZn; Zn vacancy, VS; S vacancy) [21]. Note that the notation ‘1’ in figures b1–b3 indicates the shifted emission depending on the S/Zn ratio.
51
S.I. Ahn / Chemical Physics Letters 528 (2012) 49–52
a2 25
D2 D4
15
Decreasing thickness
10 5 0
20 15 10 5
460
500
540
D1 D3
30
1 20
420
b2
460
500
540
D1 D3
40
Decreasing thickness
10
1
420
460
500
540
30
b3
D2 D4
1
420
c2 5 D3
D4
PL enhancement ratio
4
D2
3 2 1 0
460
500
540
420
460
500
540
Wavelength (nm)
580
540
580
D2 D4
1
30 20 10
580
1
380
D1 D3
4
420
460
500
540
580
Wavelength (nm)
c3 6
D2 D4
3 2 1 0
380
D3
Wavelength (nm)
5
500
0 380
580
460
D1
40
1
10
420
Wavelength (nm)
20
Wavelength (nm)
D1
5
380
580
PL enhancement ratio
380
D4
10
0
0
D2
D3
15
Wavelength (nm)
D2 D4
D1
0 380
580
PL intensity (a. u.)
b1 PL intensity (a. u.)
20
PL intensity (a. u.)
420
Wavelength (nm)
PL enhancement ratio
D2 D4
0 380
c1
a3 25 D1 D3
PL intensity (a. u.)
D1 D3
PL intensity (a. u.)
PL intensity (a. u.)
a1 20
D1 D3
5
D2 D4
4 3 2 1 0
380
420
460
500
540
Wavelength (nm)
580
380
420
460
500
540
580
Wavelength (nm)
Figure 3. PL spectra of Rf and Ag samples as a function of the thickness of ZnS treated at 140 °C (D1; 1 lm, D2; 0.80 lm, D3; 0.71 lm, and D4: 0.59 lm). Rf samples; a1 (Rf1; 0.8 S/Zn), a2 (Rf2; 1.0 S/Zn), and a3 (Rf3; 1.2 S/Zn), Ag samples; b1 (Ag1; 0.8 S/Zn), b2 (Ag2; 1.0 S/Zn), and b3 (Ag3; 1.2 S/Zn). PL enhancement ratios of Ag samples at every wavelength compared to each corresponding reference; c1 (Ag1/Rf1), c2 (Ag2/Rf2), and c3 (Ag3/Rf3). Note that the notation ‘1’ in figures b1–b3 indicates the shifted emission depending on the thickness of the ZnS.
The PL spectra of the Rf series in Figure 2a shows complicated emissions dependent on the S/Zn ratio and temperature. Regardless of the ratio of S/Zn, each sample shows both sulfur and zinc vacancies at around 430 and 470 nm, in agreement with previous reports [20,21]. As-formed ZnS using thiourea and acetyl-acetone is considered to have many sulfur and zinc vacancies caused by the low reactivity of thiourea and the strong ligand action of acetyl-acetone, respectively. PL emissions by electron traps and defects on the surface of ZnS are generally observed between these two vacancies related emissions [21]. The emissions above 500 nm are hard to define precisely, but several reports have assigned those to elemental S species (or dangled sulfur) on the surface of ZnS; for instance, a green PL band at 535 nm originates from ZnS nano-belts [19], an orange PL at 560 nm stems from ZnS colloids [22], and so on. Figure 2c shows a schematic energy band diagram indicating some major emissions from the ZnS film based on both previous research [21] and the PL results in this work. The PL spectra of ZnS samples on the Ag film in Figure 2b shows relatively well resolved emissions compared to those of the reference samples in Figure 2a. The complex PL emissions appear to arise from the oscillation of the major defects in the ZnS, as shown in Figure 2c, as the crystals grow. Interestingly, red-shifted
emissions are clearly observable depending on the S/Zn ratio as temperature increases and the thickness of the ZnS on the Ag decreases as shown in Figure 3b (see the area denoted as ‘1’ in Figures 2 and 3b). These tendencies indicate that the red-shift in this experiment is closely related to the growth of ZnS crystals. A previous report introduced a theoretical model to explain the red-shift for half-gold coated CdSe/ZnS quantum dots and provided evidence that the quadrapole interaction between the electron– hole pair and its image charges is the major contributing factor for the red-shift [23]. Recently, the Franz–Keldysh effect, which describes an electric field effect at the near band-edge absorption, has been used to interpret the red-shift phenomenon for Se-doped CdS nano-ribbons on Ag film [24]. The red-shift in this experiment is apparently caused by both of these effects and is reinforced as the ZnS grows and the band structure of ZnS thereupon develops. Every sample on the Ag film has higher PL intensity than that of the reference samples. Closely looking at the PL spectra of the Ag samples in Figure 2b, an increase of specific emissions with comparison to the Rf samples can be found. The PL enhancement itself is not surprising, as it is a common result in the SP research area, as noted in the introduction. However, the enhanced PL emissions at specific wavelengths, for example, 454 and 499 nm of Ag1, 464 and
52
S.I. Ahn / Chemical Physics Letters 528 (2012) 49–52
513 nm of Ag2, 483 nm of Ag3, and 464 and 498 nm of Ag4 at 120 °C (or slightly other positions at a different temperature), has important implications. Specially, this demonstrates that emissions from defects can be enhanced and distinguished by SPs, thus potentially providing a means of analyzing inorganic materials. Figure 3 shows the thickness dependency of the PL from each sample at 140 °C. Decreasing the thickness in both samples reduces the PL and the emission peaks. Interestingly, the center positions of the PL from the Rf samples do not change much, while the center positions of PL from the Ag samples are red-shifted apart from those with higher sulfur contents, as shown in Figure 3b3. In particular, comparing the PL spectrum of Rf1-D4 in Figure 3a1 to that of Ag1-D4 in Figure 3b1, a difference of about 24 nm can be observed in the peak position. This result also indicates that the SPs can influence how the band overlaps via either the Franz–Keldysh effect or the quadrapole interaction as mentioned above. The PL intensity generally increases with increasing emitter thickness before luminescence saturation at a certain excitation power. A part of the PL influenced by the SPs is also dependent on the thickness of the ZnS film; however, the PL enhancement by the SPs should be reduced as the thickness is increased. These two effects are combined and result in a non-linear enhancement of the PL intensity depending on the thickness, which provides an evidence of the SP effect on the PL enhancement in this experiment. Notably, the PL enhancement of D2 in Figure 3c3 provides a clear evidence of the SP effect on the PL enhancement in this experiment. 4. Conclusions A previous report suggests that the band-edge or excitonic emission can be effectively overlapped with the expendable absorption of the surface states to the absorption edge of the inter-band transition or to the exciton absorption, resulted in energy transfer to the surface state [18]. The Ag film can influence the band overlapping via either the Franz–Keldysh effect or the quadrapole interaction, likely in the form of red-shifting. Although we could not estimate whether the energy transfer rate is also influenced by the SPs, it is clear that the emission from surface states of ZnS is supplemented by the Ag film. Consequently, the enhancement mechanism of the PL by the Ag layer in this experiment appears to involve an increase of the radiative recombination
rate due to the Ag SPs as generally sited in SP related research, and by enhanced energy transfer between the absorption bands of the inter-band states (or excitons) and the surface states. This result implies that emissions from defects can be enhanced and distinguished by SPs, thus potentially providing a means of analyzing inorganic materials. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20090073189). References [1] K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, A. Scherer, Nat. Mater. 3 (2004) 601. [2] W. Chao, R. Wu, C. Tsai, T. Wu, J. Appl. Phys. 107 (2010) 013101. [3] T. Neal, K. Okamoto, A. Scherer, M. Liu, A. Jen, Appl. Phys. Lett. 89 (2006) 221106. [4] N. Hecker, R. Hopfel, N. Sawaki, T. Maier, G. Strasser, Appl. Phys. Lett. 75 (1999) 1577. [5] P. Hobson, S. Wedge, J. Wasey, I. Sage, W. Barnes, Adv. Mater. 14 (2002) 1393. [6] W. Ni, J. An, C. Lai, H. Ong, J. Appl. Phys. 100 (2006) 026103. [7] C. Lai, J. An, H. Ong, Appl. Phys. Lett. 86 (2005) 251105. [8] J. You, X. Zhang, Y. Fan, S. Qu, N. Chen, Appl. Phys. Lett. 91 (2007) 231907. [9] J. Song, T. Atay, S. Shi, H. Urabe, A. Nurmikko, Nano Lett. 5 (2005) 1557. [10] S. Nie, S. Emory, Science 275 (1997) 1102. [11] J. Anker, W. Hall, O. Lyandres, N. Shah, J. Zhao, R. Duyne, Nat. Mater. 7 (2008) 442. [12] A.W. Schell, G. Kewes, T. Hanke, A. Leitenstorfer, R. Bratschitsch, O. Benson, T. Aichele, Opt. Express 19 (2011) 7914. [13] H.B. Zeng, W.P. Cai, P.S. Liu, X.X. Xu, H.J. Zhou, C. Klingshirn, H. Kalt, ACS Nano 2 (2008) 1661. [14] M. Achermann, J. Phys. Chem. Lett. 1 (2010) 2837. [15] J.T. Choy et al., Nat. Photonics 5 (2011) 738. [16] M. Kaschke, N. Ernsting, H. Weller, U. Muller, Chem. Phys. Lett. 168 (1990) 543. [17] R. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. [18] S. Kar, S. Chaudhuri, J. Phys. Chem B 109 (2005) 3298. [19] C. Ye, X. Fang, G. Li, L. Zhang, Appl. Phys. Lett. 85 (2004) 3035. [20] K. Era, S. Shionoya, Y. Washizawa, J. Phys. Chem. Solids 29 (1968) 1827. [21] K. Manzoor, S. Vadera, N. Kumar, T. Kutty, Mater. Chem. Phys. 82 (2003) 718. [22] D. Dunstan, A. Hagfeldt, M. Almgren, H. Siegbahn, E. Mukhtar, J. Phys. Chem. 94 (1990) 6797. [23] K. Zhao, J. Choi, Y.-H. Lo, Appl. Phys. Lett. 88 (2006) 243104. [24] Z. Fang, S. Huang, Y. Lu, A. Pan, F. Lin, X. Zhu, Phys. Rev. B 82 (2010) 085403.
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
A study on the defects structure of colloidal ZnS using Ag surface plasmons Sung Il Ahn ⇑ Department of Engineering in Energy & Applied Chemistry, Center for Green Fusion Technology, Silla University, Busan 617-736, Republic of Korea
a r t i c l e
i n f o
Article history: Received 20 October 2011 In final form 19 January 2012 Available online 25 January 2012
a b s t r a c t The effects of Ag surface plasmons (SPs) on the defects of ZnS are investigated. The photoluminescence (PL) spectra of ZnS on an Ag film show well resolved, enhanced, and red-shifted emissions compared to reference samples. The enhancement of the PL by the Ag layer is attributed to an enhanced energy transfer between overlapping band energies of ZnS by Ag SP. Based on the results, it is suggested that the PL emissions from defects can be enhanced and distinguished by SPs, which provide a direction for the application of SPs to characterize inorganic materials. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Surface plasmons (SPs) from the interface of a metal and a luminescent material are known to increase the photoluminescence (PL) efficiency of the material in question by enhancing the spontaneous recombination, where the surface plasmon resonance energy matches well with the emission energy of the luminescent [1–5]. A common example in semiconductors is an enhanced UV emission from Ag/ZnO, where the SP energy at the Ag/ZnO interface is close to the band-edge emission of ZnO of 3.27 eV [6,7]. Such enhancement was also observed from ZnO on a metal layer due to the roughness of the metal layer at the metal/semiconductor interface [8]. In the case of CdSe/ZnS quantum dots, roughly 50-fold enhanced fluorescence efficiency was obtained by tuning the SPR to the quantum dot exciton emission band in a specific design of interconnected Ag nano-crystals (NCs) [9]. The SP phenomenon has been also applied to spectroscopic analysis of biological materials and sensors under sharp variations of the plasmon resonance conditions [10,11]. These instrumentations using SP have allowed research on single molecules placed in various media; however, the applications are restricted to organic based materials except in a few cases [12–15]. Inorganic solids contain various types of defects, which are difficult to distinguish and characterize by normal spectroscopic methods. ZnS is an example having various crystal defects such as zinc and sulfur vacancies, surface defects, and defects by interstitial ions [16–19]. Difficulties in the characterization of ZnS are caused by challenges related to distinguish defects using spectroscopic methods. Upon this background, the author is interested in determining whether emissions from a defect or defects among numerous other defects in a solid could be enhanced or modulated by SPs. If this is
⇑ Fax: +82 51 999 5335. E-mail address: [email protected] 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2012.01.041
possible, the defect structures could feasibly be simplified or characterized easily by the application of SPs. The aim of this work is to characterize defect structures of ZnS using SPs. In addition, the effects of SPs on the emissions from defects sites of ZnS are discussed. It is anticipated that the finding of this study can facilitate the application of SPs to the analysis of inorganic materials.
2. Experimental Among various ZnS synthesis methods, a modified thiourea route was chosen to form a film type ZnS emitter. This allows in situ study of the SPR between an as-grown ZnS film and Ag film. After several trials to deliberately introduce defects to ZnS, it is found that acetyl-acetone suppressed the growth of ZnS and stabilized the ZnS NCs with various defects due to its strong ligand action. For preparation of the ZnS precursor, 0.4 mol of Zn(CH3COO)22H2O (Aldrich, P98%) were dissolved in a mixture solvent comprising 70 ml of 2-methoxyethanol (Aldrich, 99.8%), 20 ml of acetyl-acetone (Aldrich, 99.5%), and 10 ml of distilled water. Thiourea (Aldrich, 99.0%), in quantities of 0.32, 0.4, 0.48, and 0.56 mol, respectively, was then added to the mixture, which was stirred for 30 min. Each mixture was reacted under a reflux condition for 40 min and concentrated to 1.0 mol based on the content of the zinc source. An Ag film was formed on a Si(0 0 1) substrate with a thickness of 70 nm by sputtering at room temperature. The ZnS precursors were coated on the Si wafer as a reference (designated as Rf1 for 0.8, Rf2 for 1.0, Rf3 for 1.2, and Rf4 for 1.4 S/Zn ratio, respectively) and an Ag film on a Si wafer (designated as Ag1 for 0.8, Ag2 for 1.0, Ag3 for 1.2, and Ag4 for 1.4 S/Zn ratio, respectively) using a spin coater at 1500 rpm for 15 s. Each sample was dried at 80 °C for 30 min. Each dried sample was treated at 100 °C for 15 min and its PL was then measured. Using the same sample to avoid measuring variation of the PL, the same process was carried out at 120, 140, 160, and
50
S.I. Ahn / Chemical Physics Letters 528 (2012) 49–52
Figure 1. Selected XRD patterns, SEM, TEM images of a ZnS sample, and photos of spin-coated ZnS films on a Si wafer; (a) photos of spin-coated ZnS films on Si and Ag/Si (note that the reflected color indicates the uniformity of the films). (b and c) TEM images showing the colloidal ZnS NCs. (d) A selected surface image of ZnS/Si. (e and f) Selected SEM images of thickness-controlled ZnS films from different ZnS precursor solutions (0.75 and 0.5 mol, respectively). (g) Selected XRD patterns of ZnS from at a S/Zn ratio of 1.2 at various temperatures.
180 °C, sequentially. In addition, to ensure precise PL results, the measuring point was fixed using the right angle on a part of the Si wafer with ZnS film. ZnS precursor solutions with different concentrations of 0.75, 0.6, and 0.5 mol based on zinc source (designated as D1 for 1.0, D2 for 0.75, D3 for 0.6, and D4 for 0.5 mol, respectively) were additionally prepared and spin-coated on both substrates under the same coating conditions in order to investigate the effect of the thickness on the growth of ZnS at 140 °C. For characterizations, PL (Shimazu, UV-2450) spectra using an excitation wavelength of 325 nm from a Xenon lamp were recorded for each ZnS. X-ray diffraction (XRD, Rigaku D/max 2400) was used to investigate the crystallinity and structure type of ZnS NCs. The size and shape of ZnS was observed using transmission electron microscopy (TEM; JEOL Ltd., JEM-2100F (HR)) and scanning electron microscopy (SEM; FE-SEM S-4700, Hitachi Co.). The surface roughness of Ag film is characterized using atomic force microscopy (AFM; Scanning Probe Microscope, Seiko Co.).
10
0
b1 50
420
460
500
540
Wavelength (nm)
40
Ag1
Ag2
Ag3
Ag4
11
30
b2 50
Increasing sulfur
20
1 10
420
460
500
540
580
Wavelength (nm)
380
420
460
500
540
Wavelength (nm)
580
Rf3
Rf4
CB Vs
10
40
Ag1
Ag2
Ag3
Ag4
380
1 1 30 20
1 10
420
460
500
540
580
SS2
Wavelength (nm)
b3
0
0
20
0 380
580
PL intensity (a. u.)
380
PL intensity (a. u.)
0
PL intensity (a. u.)
Rf4
Rf2
2.39 eV
10
Rf3 20
c Rf1
2.85 eV
Increasing sulfur
Rf2
2.85 eV
Rf4
20
a3 Rf1
2.98 eV
Rf3
a2 30
3.88 eV
Rf2
The spin-coated samples have a thickness in the range of 1–0.5 lm with approximately 10% thickness variation based on SEM images of the cross-section of the ZnS film (selected SEM images are shown in Figure 1). The uniformity of the coated films can be estimated by the color of light reflected from the ZnS films, as shown in Figure 1a. Using TEM, attempts were made to define the NCs, but only colloidal aggregates of the NC were found. These most likely formed during the TEM sampling process, as shown in Figure 1b and c. A weak XRD peaks at around 28.3 and 31.9° in Figure 1e reveal that the as-synthesized ZnS with a S/Zn ratio of 1.2 and 1.4 at 140 °C has a cubic type ZnS structure mixed with ZnO. At a higher temperature of 180 °C, a reduction of the ZnO peak was observed. In the case of samples with low ratios of S/Zn at and below 140 °C, the specific peak for the cubic ZnS was not detected.
PL intensity (a. u.)
Rf1
PL intensity (a. u.)
PL intensity (a. u.)
a1 30
3. Results and discussion
VZn 40
Ag1
Ag2
SS1
Ag4
30
Ag3 1
VB
1 20
1
10 0
380
420
460
500
540
Wavelength (nm)
580
380
420
460
500
540
580
Wavelength (nm)
Figure 2. PL spectra of Rf and Ag samples as a function of the S/Zn ratio and heating temperature; Rf samples at 100 °C (a1), at 140 °C (a2), at 160 °C (a3) and Ag samples at 100 °C (b1), at 140 °C (b2), at 160 °C (b3). (c) The predicted schematic energy band diagram of the PL emissions (SS1; surface state 1, SS2; surface state 2, VZn; Zn vacancy, VS; S vacancy) [21]. Note that the notation ‘1’ in figures b1–b3 indicates the shifted emission depending on the S/Zn ratio.
51
S.I. Ahn / Chemical Physics Letters 528 (2012) 49–52
a2 25
D2 D4
15
Decreasing thickness
10 5 0
20 15 10 5
460
500
540
D1 D3
30
1 20
420
b2
460
500
540
D1 D3
40
Decreasing thickness
10
1
420
460
500
540
30
b3
D2 D4
1
420
c2 5 D3
D4
PL enhancement ratio
4
D2
3 2 1 0
460
500
540
420
460
500
540
Wavelength (nm)
580
540
580
D2 D4
1
30 20 10
580
1
380
D1 D3
4
420
460
500
540
580
Wavelength (nm)
c3 6
D2 D4
3 2 1 0
380
D3
Wavelength (nm)
5
500
0 380
580
460
D1
40
1
10
420
Wavelength (nm)
20
Wavelength (nm)
D1
5
380
580
PL enhancement ratio
380
D4
10
0
0
D2
D3
15
Wavelength (nm)
D2 D4
D1
0 380
580
PL intensity (a. u.)
b1 PL intensity (a. u.)
20
PL intensity (a. u.)
420
Wavelength (nm)
PL enhancement ratio
D2 D4
0 380
c1
a3 25 D1 D3
PL intensity (a. u.)
D1 D3
PL intensity (a. u.)
PL intensity (a. u.)
a1 20
D1 D3
5
D2 D4
4 3 2 1 0
380
420
460
500
540
Wavelength (nm)
580
380
420
460
500
540
580
Wavelength (nm)
Figure 3. PL spectra of Rf and Ag samples as a function of the thickness of ZnS treated at 140 °C (D1; 1 lm, D2; 0.80 lm, D3; 0.71 lm, and D4: 0.59 lm). Rf samples; a1 (Rf1; 0.8 S/Zn), a2 (Rf2; 1.0 S/Zn), and a3 (Rf3; 1.2 S/Zn), Ag samples; b1 (Ag1; 0.8 S/Zn), b2 (Ag2; 1.0 S/Zn), and b3 (Ag3; 1.2 S/Zn). PL enhancement ratios of Ag samples at every wavelength compared to each corresponding reference; c1 (Ag1/Rf1), c2 (Ag2/Rf2), and c3 (Ag3/Rf3). Note that the notation ‘1’ in figures b1–b3 indicates the shifted emission depending on the thickness of the ZnS.
The PL spectra of the Rf series in Figure 2a shows complicated emissions dependent on the S/Zn ratio and temperature. Regardless of the ratio of S/Zn, each sample shows both sulfur and zinc vacancies at around 430 and 470 nm, in agreement with previous reports [20,21]. As-formed ZnS using thiourea and acetyl-acetone is considered to have many sulfur and zinc vacancies caused by the low reactivity of thiourea and the strong ligand action of acetyl-acetone, respectively. PL emissions by electron traps and defects on the surface of ZnS are generally observed between these two vacancies related emissions [21]. The emissions above 500 nm are hard to define precisely, but several reports have assigned those to elemental S species (or dangled sulfur) on the surface of ZnS; for instance, a green PL band at 535 nm originates from ZnS nano-belts [19], an orange PL at 560 nm stems from ZnS colloids [22], and so on. Figure 2c shows a schematic energy band diagram indicating some major emissions from the ZnS film based on both previous research [21] and the PL results in this work. The PL spectra of ZnS samples on the Ag film in Figure 2b shows relatively well resolved emissions compared to those of the reference samples in Figure 2a. The complex PL emissions appear to arise from the oscillation of the major defects in the ZnS, as shown in Figure 2c, as the crystals grow. Interestingly, red-shifted
emissions are clearly observable depending on the S/Zn ratio as temperature increases and the thickness of the ZnS on the Ag decreases as shown in Figure 3b (see the area denoted as ‘1’ in Figures 2 and 3b). These tendencies indicate that the red-shift in this experiment is closely related to the growth of ZnS crystals. A previous report introduced a theoretical model to explain the red-shift for half-gold coated CdSe/ZnS quantum dots and provided evidence that the quadrapole interaction between the electron– hole pair and its image charges is the major contributing factor for the red-shift [23]. Recently, the Franz–Keldysh effect, which describes an electric field effect at the near band-edge absorption, has been used to interpret the red-shift phenomenon for Se-doped CdS nano-ribbons on Ag film [24]. The red-shift in this experiment is apparently caused by both of these effects and is reinforced as the ZnS grows and the band structure of ZnS thereupon develops. Every sample on the Ag film has higher PL intensity than that of the reference samples. Closely looking at the PL spectra of the Ag samples in Figure 2b, an increase of specific emissions with comparison to the Rf samples can be found. The PL enhancement itself is not surprising, as it is a common result in the SP research area, as noted in the introduction. However, the enhanced PL emissions at specific wavelengths, for example, 454 and 499 nm of Ag1, 464 and
52
S.I. Ahn / Chemical Physics Letters 528 (2012) 49–52
513 nm of Ag2, 483 nm of Ag3, and 464 and 498 nm of Ag4 at 120 °C (or slightly other positions at a different temperature), has important implications. Specially, this demonstrates that emissions from defects can be enhanced and distinguished by SPs, thus potentially providing a means of analyzing inorganic materials. Figure 3 shows the thickness dependency of the PL from each sample at 140 °C. Decreasing the thickness in both samples reduces the PL and the emission peaks. Interestingly, the center positions of the PL from the Rf samples do not change much, while the center positions of PL from the Ag samples are red-shifted apart from those with higher sulfur contents, as shown in Figure 3b3. In particular, comparing the PL spectrum of Rf1-D4 in Figure 3a1 to that of Ag1-D4 in Figure 3b1, a difference of about 24 nm can be observed in the peak position. This result also indicates that the SPs can influence how the band overlaps via either the Franz–Keldysh effect or the quadrapole interaction as mentioned above. The PL intensity generally increases with increasing emitter thickness before luminescence saturation at a certain excitation power. A part of the PL influenced by the SPs is also dependent on the thickness of the ZnS film; however, the PL enhancement by the SPs should be reduced as the thickness is increased. These two effects are combined and result in a non-linear enhancement of the PL intensity depending on the thickness, which provides an evidence of the SP effect on the PL enhancement in this experiment. Notably, the PL enhancement of D2 in Figure 3c3 provides a clear evidence of the SP effect on the PL enhancement in this experiment. 4. Conclusions A previous report suggests that the band-edge or excitonic emission can be effectively overlapped with the expendable absorption of the surface states to the absorption edge of the inter-band transition or to the exciton absorption, resulted in energy transfer to the surface state [18]. The Ag film can influence the band overlapping via either the Franz–Keldysh effect or the quadrapole interaction, likely in the form of red-shifting. Although we could not estimate whether the energy transfer rate is also influenced by the SPs, it is clear that the emission from surface states of ZnS is supplemented by the Ag film. Consequently, the enhancement mechanism of the PL by the Ag layer in this experiment appears to involve an increase of the radiative recombination
rate due to the Ag SPs as generally sited in SP related research, and by enhanced energy transfer between the absorption bands of the inter-band states (or excitons) and the surface states. This result implies that emissions from defects can be enhanced and distinguished by SPs, thus potentially providing a means of analyzing inorganic materials. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20090073189). References [1] K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, A. Scherer, Nat. Mater. 3 (2004) 601. [2] W. Chao, R. Wu, C. Tsai, T. Wu, J. Appl. Phys. 107 (2010) 013101. [3] T. Neal, K. Okamoto, A. Scherer, M. Liu, A. Jen, Appl. Phys. Lett. 89 (2006) 221106. [4] N. Hecker, R. Hopfel, N. Sawaki, T. Maier, G. Strasser, Appl. Phys. Lett. 75 (1999) 1577. [5] P. Hobson, S. Wedge, J. Wasey, I. Sage, W. Barnes, Adv. Mater. 14 (2002) 1393. [6] W. Ni, J. An, C. Lai, H. Ong, J. Appl. Phys. 100 (2006) 026103. [7] C. Lai, J. An, H. Ong, Appl. Phys. Lett. 86 (2005) 251105. [8] J. You, X. Zhang, Y. Fan, S. Qu, N. Chen, Appl. Phys. Lett. 91 (2007) 231907. [9] J. Song, T. Atay, S. Shi, H. Urabe, A. Nurmikko, Nano Lett. 5 (2005) 1557. [10] S. Nie, S. Emory, Science 275 (1997) 1102. [11] J. Anker, W. Hall, O. Lyandres, N. Shah, J. Zhao, R. Duyne, Nat. Mater. 7 (2008) 442. [12] A.W. Schell, G. Kewes, T. Hanke, A. Leitenstorfer, R. Bratschitsch, O. Benson, T. Aichele, Opt. Express 19 (2011) 7914. [13] H.B. Zeng, W.P. Cai, P.S. Liu, X.X. Xu, H.J. Zhou, C. Klingshirn, H. Kalt, ACS Nano 2 (2008) 1661. [14] M. Achermann, J. Phys. Chem. Lett. 1 (2010) 2837. [15] J.T. Choy et al., Nat. Photonics 5 (2011) 738. [16] M. Kaschke, N. Ernsting, H. Weller, U. Muller, Chem. Phys. Lett. 168 (1990) 543. [17] R. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. [18] S. Kar, S. Chaudhuri, J. Phys. Chem B 109 (2005) 3298. [19] C. Ye, X. Fang, G. Li, L. Zhang, Appl. Phys. Lett. 85 (2004) 3035. [20] K. Era, S. Shionoya, Y. Washizawa, J. Phys. Chem. Solids 29 (1968) 1827. [21] K. Manzoor, S. Vadera, N. Kumar, T. Kutty, Mater. Chem. Phys. 82 (2003) 718. [22] D. Dunstan, A. Hagfeldt, M. Almgren, H. Siegbahn, E. Mukhtar, J. Phys. Chem. 94 (1990) 6797. [23] K. Zhao, J. Choi, Y.-H. Lo, Appl. Phys. Lett. 88 (2006) 243104. [24] Z. Fang, S. Huang, Y. Lu, A. Pan, F. Lin, X. Zhu, Phys. Rev. B 82 (2010) 085403.