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Localized surface plasmon resonances and its related defects in orthorhombic Cu3SnS4 nanocrystals
Accepted Manuscript Localized Surface Plasmon Resonances and Its Related Defects in Orthorhombic Cu3SnS4 Nanocrystals Yingwei Li, Wuding Ling, Qifeng Han, Tae Whan Kim, Wangzhou Shi PII: DOI: Reference:
S0925-8388(15)00468-5 http://dx.doi.org/10.1016/j.jallcom.2015.02.042 JALCOM 33416
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
25 November 2014 23 January 2015 6 February 2015
Please cite this article as: Y. Li, W. Ling, Q. Han, T.W. Kim, W. Shi, Localized Surface Plasmon Resonances and Its Related Defects in Orthorhombic Cu3SnS4 Nanocrystals, Journal of Alloys and Compounds (2015), doi: http:// dx.doi.org/10.1016/j.jallcom.2015.02.042
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Localized Surface Plasmon Resonances and Its Related Defects in Orthorhombic Cu3SnS4 Nanocrystals Yingwei Li,a,c Wuding Ling,a,c Qifeng Han,*,a Tae Whan Kimb and Wangzhou Shia a Key Laboratory of Optoelectronics Materials and Devices, Shanghai Normal University, Shanghai, China b Department of Electronics and Computer Engineering, National Research Laboratory for Nano Quantum Electronics Devices, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea c These authors contributed equally to this work.
Abstract
Orthorhombic Cu3SnS4 nanocrystals (NCs) possessing obvious localized surface plasmon resonances (LSPR) in the near-infrared region (NIR) frequencies are prepared. In Cu 3SnS4 NCs, *Corresponding Author at: Key Laboratory of Optoelectronics Materials and Devices, ShanghaiNormalUniversity, 200234 Shanghai, China. Tel: +86 13611757364
Email addresses: [email protected] (Yingwei Li), [email protected] (Wuding Ling), [email protected] (Qifeng Han), [email protected] (Tae Whan Kim), [email protected] (Wangzhou Shi)
valence state of Cu remains close to +1, indicating Cu+ occupies positions originally belong to Cu2+ in a similar way as Cu+ replaces Zn2+ to form CuZn antisites in Cu2ZnSnS4. The increase of Cu/Sn ratio results in slightly red-shifted and weakened NIR absorption. Moreover, LSPR is also verified in wurtzite Cu2SnS3 NCs, although the observed peak maximum is red-shifted comparing to the LSPR band of Cu3SnS4 NCs. But the calculated free carrier concentration for Cu2SnS3 by Drude model is on the same order of magnitude with Cu 3SnS4 in which sulfur vacancies acting as compensation centerscould reduce hole density to some extent. Whereas Cu2ZnSnS4 presents almost no absorption due to its relatively lower hole concentration.
Keywords: localized surface plasmon resonances; Cu3SnS4; nanocrystal; Cu2ZnSnS4; hole concentration
1. Introduction
Several semiconductor nanocrystals (NCs), such as binary copper chalcogenides [1-7], WO3-δ [8], aluminum-doped zinc oxide [9], tin-doped indium oxide (ITO) [10, 11], and GeTe [12], have been demonstrated to be capable of supporting localized surface plasmon resonances (LSPR) introduced by collective oscillations of free charge carriers. Wherein, binary copper chalcogenides, namely Cu2-xS, Cu2-xSe, Cu2-xS1-ySey, Cu2-xTe, etc., are more thoroughly scrutinized as they show relatively strong LSPR in near-infrared region (NIR) [3-7], originating from the Cu vacancies (VCu) which are considered as p-type dopant. Although the NCs with a small value of x show none or weak LSPR absorbance, the acceptor-related VCu can be enhanced over time as oxidation proceeds when exposed in air or treated with redox processes, resulting in an increase in LSPR frequency and intensity [4-7]. In addition to the redox reaction, tunable
LSPR could also be observed through altering size [3,13], stoichiometry [14,15], shape [16,17], crystal structures [15,18], or even capping ligands [3]. Two dipolar LSPR modes corresponding to in-plane and out-of-plane excitation are expected for anisotropic nanodisks [19-21] which are absent for quasi-sphere NCs.
Recently, LSPR has been discovered in CuxInyS2 (CIS) quantum dots (QDs) by Rosental et al. [22,23], and inherent plasmonic modes are supposed to be raised by intrinsic crystalline vacancy defects. Within the Cu-Sn-S system, Su and coworkers use successive ionic layer absorption (SILAR) to fabricate Cu2SnS3, Cu5Sn2S7, and Cu3SnS4 thin films, and interestingly, NIR absorption peaks are revealed [24]. However, no appropriate explanation on the source of this kind of absorption is given so far.
In this article, we report orthorhombic Cu3SnS4 NCs with NIR absorption which can be attributed to LSPR; and to the best of our knowledge, this is the first time for this phenomenon to be discussed in Cu-Sn-S family. It is found that LSPR frequency red-shifts for a certain degree when the stoichiometry turns from Cu/Sn < 3 to Cu/Sn > 3. It is also noted that higher carrier density of Cu3SnS4 NCs plays a decisive role in presenting a relatively stronger NIR absorption, while the number of free carrier per NC is probably insufficient to support LSPR in Cu2ZnSnS4 (CZTS).
2. Experimental details Chemicals. CuCl2 (98%), Zn(CH3COO)2 (99.98%), SnCl2 (98%), and thiourea (99.0%) were purchased from Alfa Aesar. Oleylamine (70%) was purchased from Aldrich.
Synthesis of Cu3SnS4nanocrystals.The preparations of Cu3SnS4 NCs were conducted in a glove box filled with nitrogen. In a typical synthesis of Cu/Sn< 3 Cu 3SnS4, metal precursor was made by vigorously stirring 0.405 mmol CuCl2, 0.15 mmol SnCl2, and 3 ml oleylamine at 60 ºC for 2 h. In a separate pot, 0.75 mmolthiourea, and 2 ml oleylamine were sufficiently mixed at 90ºC for at least 1 h and then raised to 180 ºC. Precursors containing cation and anion sources were mixed and heated to 225ºC for reaction. The NCs were allowed to grow for 20 min before naturally cooled down to about 60 ºC. 10 ml of ethanol was added to precipitate the NCs followed by centrifugation at 8000 rpm for 3min. The supernatant was discarded and the process was repeated for 2 more times to remove any poorly coordinated ligand. The obtained NCs were then re-dispersed in organic solvent. Synthesis of Cu2SnS3 and Cu2ZnSnS4 nanocrystals. The cation precursor for Cu2SnS3 (Cu/Sn < 2) was prepared by mixing 0.285 mmol CuCl2, 0.15 mmol SnCl2, and 3 ml oleylamine. The cation precursor for Cu-poor Cu2ZnSnS4 (Cu/(Zn+Sn) < 2) included 0.27 mmol CuCl2, 0.225 mmol Zn(CH3COO)2, 0.15 mmol SnCl2, and 3 ml oleylamine, and the reaction was performed at 250 ºC for 30 min. Other conditions for preparation and post treatment were kept the same as the preparation of Cu3SnS4 NCs. See Table S1 for the mole ratios of the ingredients for all the samples. Measurement and Characterization. D8 Focus X-ray diffraction (XRD) and Raman spectra (RenishawinVia Raman spectrometer) with an incident laser of 514.5 nm were used to determine the crystalline structures of nanocrystals. After drop-casting from toluene dispersions onto Cu grids, high-resolution transmission electron microscopy (HRTEM) images were obtained on JEM-2100 microscope for the prepared samples. Chemical compositions of the NCs were acquired by energy dispersive spectrometers (EDS) equipped on field emission scanning electron
microscopy (Hitachi S-4000). NIR absorbance of the NCs dispersions were detected by Shimadzu UV-3600. Surface analysis was studied with the help of Perkin-Elmer PHI 5000C Xray photoelectron spectroscopy (XPS).
3. Results and discussion Characterization of Cu3SnS4nanocrystals.In Figure 1 (a), the major diffraction peaks can be well indexed to orthorhombic Cu3SnS4 (JCPDS no. 36-0217) as reported in otherwork [25]. The obtained NCs are re-dispersed in tetrachloroethylene (TCE) due to its extremely low absorption in infrared region, and an absorbance band is observed with peak maximum located at 1815 nm in Figure 1 (b). Before attributing the NIR absorption to LSPR, two questions must be clarified. Firstly, Cu2-xS phases which are widely acknowledged having LSPR might exist as an impurity with the major Cu3SnS4 phase. Additionally, some other ternary phases, e.g. Cu2SnS3, should also be distinguished as these compounds have similar XRD pattern with Cu3SnS4. Therefore, composition of the sample is required to be ascertained especially when the calculated potential phase space of Cu3SnS4 is adjacent to those of CuS, Cu2S and Cu2SnS3 [26]. Secondly, NIR extinction can also arise from other factors, e.g. scattering effect [13]. So, further measurements should be taken to assign the NIR absorbance bands to LSPR. As shown in Figure 1 (c), EDS result indicates that the composition is close to stoichiometric Cu3SnS4 except for sulfur deficiency, which is also reported for Cu 3SnS4 NCs elsewhere [25,27]. Cu content is deliberately controlled by the mole ratio of cation in precursor to result in Cu/Sn < 3 composition. Further referring to Raman spectrum which is a powerful tool to determine the phase of the product, a single intensified peak at 318 cm-1 (Figure 1(d)) convinces the formation
of orthorhombic Cu3SnS4 [28]. Cu2-xS can be excluded as little trace is detectable at ~475 cm-1. So it is certain to confirm that the phase presenting NIR absorption is Cu 3SnS4 without secondary phases. To the second question, one typical attribute of LSPR is that its resonance frequency depends on dielectric constant of the surrounding medium [13,29]. So three Cu3SnS4 NCs dispersions are made with different solvents: carbon tetrachloride (CCl4), tetrachloroethylene (C2Cl4), and carbon disulfide (CS2), and their refractive indices are 1.46, 1.51, and 1.63, respectively. Figure 1(e) illustrates that the absorbance band position red-shifts as the refractive index of the solvent increases, implying LSPR is the source of NIR absorption. Furthermore, the plasmonic sensitivity is estimated to be ~300 nm per refractive index unit (nm/RIU) for Cu3SnS4 NCs, which is comparable to those of other copper chalcogenides, e.g. Cu 2-xS QDs (~350 nm/RIU) [13], and CIS QDs (~250 nm/RIU) [22]. The non-linear change of the spectral shift is speculated to be caused by refractive effects of ligand shell [22]. TEM images display nearly monodispersed NCs with an average diameter of 14.7 ± 1.5 nm. In Figure 2 (c), the measured lattice spacing of 0.326 nm is corresponding to the (200) lattice planes of orthorhombic Cu3SnS4. The fast Fourier transform of an individual NC (Figure 2 (d)) is quite similar to the electron diffraction pattern for Cu3SnS4 nanosheet [25], manifesting an orthorhombic phase. One supposes the orthorhombic structure (Pmn21) of Cu2ZnSnS4 is based on a double wurtzite cell [30], and Chen et al. also indicate the wurtzite-stannite structure of I2II-IV-VI4 chalcogenides belongs to space group Pmn21 [31]. So, as for the crystal structure of orthorhombic Cu3SnS4, we believe it can be regarded as wurtzite-derived. Stoichiometry-dependence of LSPR for Cu3SnS4 NCs. In last section, we have attributed NIR absorption to LSPR for Cu/Sn < 3 Cu3SnS4 NCs. Niezgoda and coworkers claim a rapid crystal
growth of sulfur planes hinders complete filling of all cationic sites, creating cation vacancies [22]. On the assumption that charge neutralization sets a line between plasmonic and nonplasmonic, the amount of cation vacancies, which causes non-neutralization in charge, is deemed to lead to p-type conductivity and LSPR in CIS QDs [23], similar to binary copper compounds. However, why LSPR is still obvious for Cu3SnS4 NCs having extra cation stoichiometry? Even though all the samples are sulfur deficient, it is still difficult to ascribe the LSPR to S vacancies (VS) because they have been reported as deep donor levels for copper chalcogenides [32,33]. Since Cu3SnS4 could be achieved by only supplanting zinc atoms with copper ones in CZTS, its upper valence band is very likely to be mainly composed of Cu 3d and S 3p, and the feature of VS might be inherited despite of no calculation details are available yet. Deep donor level could act as carrier recombination center, making V S definitely not the reason to produce free carriers and LSPR. An investigation on the cations in Cu3SnS4 is further conducted, and the element valence states are analyzed by XPS. If it was charge neutral compound, Cu3SnS4 should have both monovalent and bivalent copper. From Figure 3 (a), two peaks representing Cu 2p3/2 and Cu 2p1/2 are located at 932.2 eV and 952.2 eV, respectively. The peak splitting of 20 eV is corresponded to Cu +, while signal of Cu2+ at ~942 eV is absent [34], indicating only monovalent coppers exist in Cu3SnS4 NCs. The binding energies of Sn 3d5/2 and Sn 3d3/2 are 486.6 eV and 495.0 eV, in accordance with the value of Sn4+, and the S 2p peak which is in the range of 160~164 eV informs the existence of S2(Figure 3 (b),(c)). The results are generally identical with Yi et al. in observing only Cu + in Cu3SnS4 NCs [25].
Not only should Cu+ vacancies (VCu) be one kind of defects in Cu3SnS4 due to their low formation energy when Cu incorporation is lesser than stoichiometry, as indicated by XPS results, both Cu2+ vacancies and Cu2+ related antisites could also be created in Cu3SnS4 as well. Cu2+ related antisites ( Cu Cu ) are most probably be formed when Cu+ occupies the positions 2
originally belong to Cu2+ in a similar way as Cu+ replaces Zn2+ to form CuZn antisites in CZTS. As for CZTS, a significant amount of hole carriers can be produced when a high population of CuZn antisites is ionized [33]. In our case, domination of such monovalent Cu in cation sublattice could also supply more free carriers to sustain LSPR, and somehow explain the high self-doping hole density in Cu3SnS4. To further elucidate the influences of defects on LSPR, Cu 3SnS4 NCs with Cu/Sn ≈ 3, and Cu/Sn > 3 compositions are then prepared under the same condition except for different starting ratios of cation to offer an experimental control. Not only are the crystallite sizes of these NCs very close to that of the Cu/Sn < 3 sample as estimated by Scherrer analysis, their Raman spectra overlap with each other as well (Figure 1 (a), (d), Figure S1 (a), (b) and Figure S2 (a), (b)). A comparison of absorbance spectra is given in Figure 4.Three LSPR bands are observed except that they are red-shifted and dampened in intensity with the increase of copper compositions, indicating a decrease in free carrier concentration. Once again, XPS detects no Cu2+ in Cu3SnS4 with Cu/Sn > 3 as shown in Figure S3. On the other hand, EDS results (Figure 1 (c), Figure S1 (c) and Figure S2 (c)) for the three samples show that Cu content increases at the expanse of decreasing Sn when the composition turns from Cu/Sn < 3 to Cu/Sn > 3. So it is reasonable to speculate that when the amount of Cu rises, additional Cu would probably take Sn sites to form Cu Sn defects and fill part of VCu. Further considering LSPR is always prominent and the valence state of Cu remains intact when Cu incorporation is enhanced, influences on the
amount of Cu Cu antisites, a major contributor of free carrier, can be neglected. As deep acceptor 2
level defect CuSn increases and shallow acceptor VCu decreases, a smooth decline in hole concentration, acting as a detrimental factor for LSPR, is observed. In addition, the random change in sulfur composition proves again that the regular shift in LSPR cannot be a consequence of VS. Interband absorbance might also be collaterally affected. Zhao found in UV-vis absorption that interband of Cu2-xS NCs showed a blue shift comparing to bulk material and contributed it to both quantum size effect and Moss-Burstein effects [35]. On condition that the sizes of the three kinds of NCs are kept similar, the blue-shifted bandgaps are mainly ascribed to the latter. Figure 5 illustrates Moss-Burstein effects in which optical bandgaps are associated with free carrier concentration [1]. When hole density is reduced, the optical bandgaps for Cu/Sn ≈ 3 and Cu/Sn > 3 Cu3SnS4 NCs are estimated to be 1.47 eV and 1.5 eV, respectively (Figure 4 (b)). Both values are close to the report on orthorhombic Cu 3SnS4 NCs [25], but < 0.1 eV narrower than their Cu/Sn < 3 counterparts of which higher doping level is presumed. It should be noted that the acceptor-related VCu can be enhanced over time as oxidation proceeds, resulting in an increase in LSPR frequency and intensity of Cu2-xS or Cu2-xSe [4-7]. The NIR absorptions of Cu3SnS4 NCs, on the other side, are more stable in the ambient. A second test was performed after the same sample was preserved in air for two weeks. Similar to CIS NCs [22], the intensity, shape, and position of the two absorption curves for Cu3SnS4 NCs remain almost the same. So oxidation is not responsible to produce additional free carrier in Cu3SnS4, and self-doping is very likely completed when the phase is formed. Other Cu-Sn-S NCs presenting LSPR. Except for Cu3SnS4, absorption in NIR has also been observed in other ternary Cu-Sn-S materials [24]. Cu2SnS3 is studied as it is p-type conductive
and has relatively larger chemical potential space [26]. Wurtzite Cu2SnS3 NCs and CZTS NCs have been reported by the authors before [36], and herein, by adjusting stoichiometry, Cu2SnS3 NCs with Cu/Sn < 2 and CZTS NCs with Cu/(Zn+Sn) < 2 (Cu-poor) are prepared. The information of their characteristics is given in Figure S4 to Figure S6. NIR absorption is also observed in Cu2SnS3 NCs, and we can also attribute it to LSPR as the peaks shift with the change of organic solvent (Figure 6). But LSPR of wurtzite Cu2SnS3 is obviously reduced comparing to wurtzite-derived orthorhombic Cu3SnS4 as indicated in Figure 7 (a), and little absorption is found for CZTS NCs. With the assistance of Drude model [13,22], a relationship between plasma frequency (ωp) of the free carriers in the NCs and LSPR frequency (ωsp) is given in the following equation (1):
sp =
p 2 1 + 2 m
2
(1)
where γ is the full width at half maximum (FWHM) of the LSPR peak, and εm represents the dielectric constant of the surrounding medium (εm = 2.28 for TCE, omitting the effect from the ligands). As shown in Figure 7 (a), the LSPR peak for Cu 3SnS4 NCs locates at 0.68 eV and its FWHM is measured as 1.19 eV. Thus, ωp is estimated at 3.23 eV from equation (1). Dopant density further depends on ωp as:
ωp
N h e2 ε0mh
(2)
whereε0 is free space permittivity, and mh is hole effective masses. As mentioned above, Cu3SnS4 is quite analogical to CZTS of which mh is calculated to be 0.22m0 (m0 is electron mass) [37,38] except for Cu substitution on Zn, leading to a certain extent of dispersed and delocalized valance band maximum which facilitates a relatively lower effective
hole mass. So we speculate mh for Cu3SnS4 as approximately 0.2m0 and substitute it into equation (2), obtaining Nh = 4.0 × 1019 cm-3. Similarly, as the LSPR maximum and FWHM for Cu2SnS3 NCs are 0.56 eV and 0.70 eV, respectively, and mh is reported as 0.33m0[39], an estimation of Nh is given at 2.7 × 1019 cm-3, within an order of magnitude comparing to that of Cu2SnS3 films, the other study presenting NIR absorption [24]. Although higher than Cu2SnS3, Nh of Cu3SnS4 NCs is still lower than the reported data [24]. Based on the fact that Cu2SnS3 NCs are nearly stoichiometric like the sulfurized films, but Cu3SnS4 NCs are quite sulfur deficient, the existence of VS in Cu3SnS4 which act as carrier combination center can be regarded as a major factor to be blamed for reducing hole density. On the other hand, no LSPR is found for CZTS as it has a relatively lower hole concentration within the range of 10 15~1018 cm-3 [40-44]. According to the volume of the NCs which is acquired from the average dimensions shown in HRTEM images (Figure 2 and Figure S5), we further estimate the number of free carriers at 55 per Cu3SnS4 NC and 35 per Cu2SnS3 NC, respectively. The number of carriers for an individual CZTS NC might be too small to support a LSPR mode.
4. Conclusions LSPR in NIR is observed in orthorhombic Cu3SnS4 NCs for the first time as proved by the dependence of resonance frequency on medium’s dielectric constant. The formation of Cu Cu 2 antisites is used to elucidate the high carrier concentration in Cu 3SnS4. More CuSn defects
which are detrimental for LSPR will also be produced when cationic composition turns from Cu/Sn < 3 to Cu/Sn > 3, explaining the slightly red-shifted and weakened LSPR bands. A further research is performed on the absorption differences between Cu 3SnS4 and other analogical semiconductors, i.e. Cu2SnS3 and CZTS. As a result of calculation, Nh of the two ternary
compounds are on the same order, both over the magnitude to support LSPR. Little absorption in NIR is detectable in CZTS NCs due to its lowerhole density.
Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC) (No.61006010),
Innovation Program of Shanghai
Municipal
Education Commission
(No.14YZ079), Shanghai Municipal Natural Science Foundation(14ZR1430500), also partially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013-016467).
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Figure captions Figure 1 (a) XRD pattern for orthorhombic Cu3SnS4 NCs and the standard data (JCPDS no. 360217) is plotted below for reference. (b) Absorbance spectrum for Cu 3SnS4 dispersion in TCE. The optical bandgap estimated by plotting (αE)2 as a function of the photon energy with α being the absorption coefficient is shown in the inset. (c) EDS and (d) Raman spectra of the sample. (e) Absorbance spectra for NCs dispersed in different solvents, and the inset shows the dependence of LSPR maximum on the solvent refractive index. Figure 2 (a), (b) TEM, (c) HRTEM images for orthorhombic Cu3SnS4 NCs, and (d) their corresponding fast Fourier transform. Figure 3 XPS analyses of the three elements in Cu 3SnS4 with Cu/Sn< 3. Figure 4 (a) Absorbance spectra for Cu/Sn> 3, Cu/Sn ≈ 3, and Cu/Sn< 3 Cu3SnS4 NCs dispersions in TCE. (b) The optical band gap estimation. Figure 5 Band structures of plasmonic p-type Cu3SnS4nanocrystals. Intrinsic bandgap (W), and optical bandgap (Eg) which is defined as the difference between the bottom of the conduction band and the highest occupied state in the valence band become larger when doping level is increased. Figure 6 Absorbance spectra of wurtzite Cu2SnS3 NCs dispersed in different solvents, and the inset shows the dependence of LSPR maximum on the solvent refractive index. Figure 7 (a) Absorbance spectra for wurtzite Cu2SnS3, CZTS, as well as orthorhombic Cu3SnS4 NCs dispersions in TCE. (b) Estimations of band gap energy for different NCs. WurtziteCu 2SnS3
NCs has an optical bandgap of 1.45 eV, and Eg of wurtzite CZTS (1.54 eV) is equal to the previous reports on CZTS films or nanoparticles.
Highlights
Orthorhombic Cu3SnS4 nanocrystals were obtained via hot-injection method.
The ternary nanocrystals with localized surface plasmon resonances are reported.
Localized surface plasmon resonances are caused by high carrier concentration.
Cu+ occupy the positions originally belong to Cu2+ to form Cu Cu antisite defects.
CuSn defects are detrimental for localized surface plasmon resonances.
2
S0925-8388(15)00468-5 http://dx.doi.org/10.1016/j.jallcom.2015.02.042 JALCOM 33416
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
25 November 2014 23 January 2015 6 February 2015
Please cite this article as: Y. Li, W. Ling, Q. Han, T.W. Kim, W. Shi, Localized Surface Plasmon Resonances and Its Related Defects in Orthorhombic Cu3SnS4 Nanocrystals, Journal of Alloys and Compounds (2015), doi: http:// dx.doi.org/10.1016/j.jallcom.2015.02.042
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Localized Surface Plasmon Resonances and Its Related Defects in Orthorhombic Cu3SnS4 Nanocrystals Yingwei Li,a,c Wuding Ling,a,c Qifeng Han,*,a Tae Whan Kimb and Wangzhou Shia a Key Laboratory of Optoelectronics Materials and Devices, Shanghai Normal University, Shanghai, China b Department of Electronics and Computer Engineering, National Research Laboratory for Nano Quantum Electronics Devices, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea c These authors contributed equally to this work.
Abstract
Orthorhombic Cu3SnS4 nanocrystals (NCs) possessing obvious localized surface plasmon resonances (LSPR) in the near-infrared region (NIR) frequencies are prepared. In Cu 3SnS4 NCs, *Corresponding Author at: Key Laboratory of Optoelectronics Materials and Devices, ShanghaiNormalUniversity, 200234 Shanghai, China. Tel: +86 13611757364
Email addresses: [email protected] (Yingwei Li), [email protected] (Wuding Ling), [email protected] (Qifeng Han), [email protected] (Tae Whan Kim), [email protected] (Wangzhou Shi)
valence state of Cu remains close to +1, indicating Cu+ occupies positions originally belong to Cu2+ in a similar way as Cu+ replaces Zn2+ to form CuZn antisites in Cu2ZnSnS4. The increase of Cu/Sn ratio results in slightly red-shifted and weakened NIR absorption. Moreover, LSPR is also verified in wurtzite Cu2SnS3 NCs, although the observed peak maximum is red-shifted comparing to the LSPR band of Cu3SnS4 NCs. But the calculated free carrier concentration for Cu2SnS3 by Drude model is on the same order of magnitude with Cu 3SnS4 in which sulfur vacancies acting as compensation centerscould reduce hole density to some extent. Whereas Cu2ZnSnS4 presents almost no absorption due to its relatively lower hole concentration.
Keywords: localized surface plasmon resonances; Cu3SnS4; nanocrystal; Cu2ZnSnS4; hole concentration
1. Introduction
Several semiconductor nanocrystals (NCs), such as binary copper chalcogenides [1-7], WO3-δ [8], aluminum-doped zinc oxide [9], tin-doped indium oxide (ITO) [10, 11], and GeTe [12], have been demonstrated to be capable of supporting localized surface plasmon resonances (LSPR) introduced by collective oscillations of free charge carriers. Wherein, binary copper chalcogenides, namely Cu2-xS, Cu2-xSe, Cu2-xS1-ySey, Cu2-xTe, etc., are more thoroughly scrutinized as they show relatively strong LSPR in near-infrared region (NIR) [3-7], originating from the Cu vacancies (VCu) which are considered as p-type dopant. Although the NCs with a small value of x show none or weak LSPR absorbance, the acceptor-related VCu can be enhanced over time as oxidation proceeds when exposed in air or treated with redox processes, resulting in an increase in LSPR frequency and intensity [4-7]. In addition to the redox reaction, tunable
LSPR could also be observed through altering size [3,13], stoichiometry [14,15], shape [16,17], crystal structures [15,18], or even capping ligands [3]. Two dipolar LSPR modes corresponding to in-plane and out-of-plane excitation are expected for anisotropic nanodisks [19-21] which are absent for quasi-sphere NCs.
Recently, LSPR has been discovered in CuxInyS2 (CIS) quantum dots (QDs) by Rosental et al. [22,23], and inherent plasmonic modes are supposed to be raised by intrinsic crystalline vacancy defects. Within the Cu-Sn-S system, Su and coworkers use successive ionic layer absorption (SILAR) to fabricate Cu2SnS3, Cu5Sn2S7, and Cu3SnS4 thin films, and interestingly, NIR absorption peaks are revealed [24]. However, no appropriate explanation on the source of this kind of absorption is given so far.
In this article, we report orthorhombic Cu3SnS4 NCs with NIR absorption which can be attributed to LSPR; and to the best of our knowledge, this is the first time for this phenomenon to be discussed in Cu-Sn-S family. It is found that LSPR frequency red-shifts for a certain degree when the stoichiometry turns from Cu/Sn < 3 to Cu/Sn > 3. It is also noted that higher carrier density of Cu3SnS4 NCs plays a decisive role in presenting a relatively stronger NIR absorption, while the number of free carrier per NC is probably insufficient to support LSPR in Cu2ZnSnS4 (CZTS).
2. Experimental details Chemicals. CuCl2 (98%), Zn(CH3COO)2 (99.98%), SnCl2 (98%), and thiourea (99.0%) were purchased from Alfa Aesar. Oleylamine (70%) was purchased from Aldrich.
Synthesis of Cu3SnS4nanocrystals.The preparations of Cu3SnS4 NCs were conducted in a glove box filled with nitrogen. In a typical synthesis of Cu/Sn< 3 Cu 3SnS4, metal precursor was made by vigorously stirring 0.405 mmol CuCl2, 0.15 mmol SnCl2, and 3 ml oleylamine at 60 ºC for 2 h. In a separate pot, 0.75 mmolthiourea, and 2 ml oleylamine were sufficiently mixed at 90ºC for at least 1 h and then raised to 180 ºC. Precursors containing cation and anion sources were mixed and heated to 225ºC for reaction. The NCs were allowed to grow for 20 min before naturally cooled down to about 60 ºC. 10 ml of ethanol was added to precipitate the NCs followed by centrifugation at 8000 rpm for 3min. The supernatant was discarded and the process was repeated for 2 more times to remove any poorly coordinated ligand. The obtained NCs were then re-dispersed in organic solvent. Synthesis of Cu2SnS3 and Cu2ZnSnS4 nanocrystals. The cation precursor for Cu2SnS3 (Cu/Sn < 2) was prepared by mixing 0.285 mmol CuCl2, 0.15 mmol SnCl2, and 3 ml oleylamine. The cation precursor for Cu-poor Cu2ZnSnS4 (Cu/(Zn+Sn) < 2) included 0.27 mmol CuCl2, 0.225 mmol Zn(CH3COO)2, 0.15 mmol SnCl2, and 3 ml oleylamine, and the reaction was performed at 250 ºC for 30 min. Other conditions for preparation and post treatment were kept the same as the preparation of Cu3SnS4 NCs. See Table S1 for the mole ratios of the ingredients for all the samples. Measurement and Characterization. D8 Focus X-ray diffraction (XRD) and Raman spectra (RenishawinVia Raman spectrometer) with an incident laser of 514.5 nm were used to determine the crystalline structures of nanocrystals. After drop-casting from toluene dispersions onto Cu grids, high-resolution transmission electron microscopy (HRTEM) images were obtained on JEM-2100 microscope for the prepared samples. Chemical compositions of the NCs were acquired by energy dispersive spectrometers (EDS) equipped on field emission scanning electron
microscopy (Hitachi S-4000). NIR absorbance of the NCs dispersions were detected by Shimadzu UV-3600. Surface analysis was studied with the help of Perkin-Elmer PHI 5000C Xray photoelectron spectroscopy (XPS).
3. Results and discussion Characterization of Cu3SnS4nanocrystals.In Figure 1 (a), the major diffraction peaks can be well indexed to orthorhombic Cu3SnS4 (JCPDS no. 36-0217) as reported in otherwork [25]. The obtained NCs are re-dispersed in tetrachloroethylene (TCE) due to its extremely low absorption in infrared region, and an absorbance band is observed with peak maximum located at 1815 nm in Figure 1 (b). Before attributing the NIR absorption to LSPR, two questions must be clarified. Firstly, Cu2-xS phases which are widely acknowledged having LSPR might exist as an impurity with the major Cu3SnS4 phase. Additionally, some other ternary phases, e.g. Cu2SnS3, should also be distinguished as these compounds have similar XRD pattern with Cu3SnS4. Therefore, composition of the sample is required to be ascertained especially when the calculated potential phase space of Cu3SnS4 is adjacent to those of CuS, Cu2S and Cu2SnS3 [26]. Secondly, NIR extinction can also arise from other factors, e.g. scattering effect [13]. So, further measurements should be taken to assign the NIR absorbance bands to LSPR. As shown in Figure 1 (c), EDS result indicates that the composition is close to stoichiometric Cu3SnS4 except for sulfur deficiency, which is also reported for Cu 3SnS4 NCs elsewhere [25,27]. Cu content is deliberately controlled by the mole ratio of cation in precursor to result in Cu/Sn < 3 composition. Further referring to Raman spectrum which is a powerful tool to determine the phase of the product, a single intensified peak at 318 cm-1 (Figure 1(d)) convinces the formation
of orthorhombic Cu3SnS4 [28]. Cu2-xS can be excluded as little trace is detectable at ~475 cm-1. So it is certain to confirm that the phase presenting NIR absorption is Cu 3SnS4 without secondary phases. To the second question, one typical attribute of LSPR is that its resonance frequency depends on dielectric constant of the surrounding medium [13,29]. So three Cu3SnS4 NCs dispersions are made with different solvents: carbon tetrachloride (CCl4), tetrachloroethylene (C2Cl4), and carbon disulfide (CS2), and their refractive indices are 1.46, 1.51, and 1.63, respectively. Figure 1(e) illustrates that the absorbance band position red-shifts as the refractive index of the solvent increases, implying LSPR is the source of NIR absorption. Furthermore, the plasmonic sensitivity is estimated to be ~300 nm per refractive index unit (nm/RIU) for Cu3SnS4 NCs, which is comparable to those of other copper chalcogenides, e.g. Cu 2-xS QDs (~350 nm/RIU) [13], and CIS QDs (~250 nm/RIU) [22]. The non-linear change of the spectral shift is speculated to be caused by refractive effects of ligand shell [22]. TEM images display nearly monodispersed NCs with an average diameter of 14.7 ± 1.5 nm. In Figure 2 (c), the measured lattice spacing of 0.326 nm is corresponding to the (200) lattice planes of orthorhombic Cu3SnS4. The fast Fourier transform of an individual NC (Figure 2 (d)) is quite similar to the electron diffraction pattern for Cu3SnS4 nanosheet [25], manifesting an orthorhombic phase. One supposes the orthorhombic structure (Pmn21) of Cu2ZnSnS4 is based on a double wurtzite cell [30], and Chen et al. also indicate the wurtzite-stannite structure of I2II-IV-VI4 chalcogenides belongs to space group Pmn21 [31]. So, as for the crystal structure of orthorhombic Cu3SnS4, we believe it can be regarded as wurtzite-derived. Stoichiometry-dependence of LSPR for Cu3SnS4 NCs. In last section, we have attributed NIR absorption to LSPR for Cu/Sn < 3 Cu3SnS4 NCs. Niezgoda and coworkers claim a rapid crystal
growth of sulfur planes hinders complete filling of all cationic sites, creating cation vacancies [22]. On the assumption that charge neutralization sets a line between plasmonic and nonplasmonic, the amount of cation vacancies, which causes non-neutralization in charge, is deemed to lead to p-type conductivity and LSPR in CIS QDs [23], similar to binary copper compounds. However, why LSPR is still obvious for Cu3SnS4 NCs having extra cation stoichiometry? Even though all the samples are sulfur deficient, it is still difficult to ascribe the LSPR to S vacancies (VS) because they have been reported as deep donor levels for copper chalcogenides [32,33]. Since Cu3SnS4 could be achieved by only supplanting zinc atoms with copper ones in CZTS, its upper valence band is very likely to be mainly composed of Cu 3d and S 3p, and the feature of VS might be inherited despite of no calculation details are available yet. Deep donor level could act as carrier recombination center, making V S definitely not the reason to produce free carriers and LSPR. An investigation on the cations in Cu3SnS4 is further conducted, and the element valence states are analyzed by XPS. If it was charge neutral compound, Cu3SnS4 should have both monovalent and bivalent copper. From Figure 3 (a), two peaks representing Cu 2p3/2 and Cu 2p1/2 are located at 932.2 eV and 952.2 eV, respectively. The peak splitting of 20 eV is corresponded to Cu +, while signal of Cu2+ at ~942 eV is absent [34], indicating only monovalent coppers exist in Cu3SnS4 NCs. The binding energies of Sn 3d5/2 and Sn 3d3/2 are 486.6 eV and 495.0 eV, in accordance with the value of Sn4+, and the S 2p peak which is in the range of 160~164 eV informs the existence of S2(Figure 3 (b),(c)). The results are generally identical with Yi et al. in observing only Cu + in Cu3SnS4 NCs [25].
Not only should Cu+ vacancies (VCu) be one kind of defects in Cu3SnS4 due to their low formation energy when Cu incorporation is lesser than stoichiometry, as indicated by XPS results, both Cu2+ vacancies and Cu2+ related antisites could also be created in Cu3SnS4 as well. Cu2+ related antisites ( Cu Cu ) are most probably be formed when Cu+ occupies the positions 2
originally belong to Cu2+ in a similar way as Cu+ replaces Zn2+ to form CuZn antisites in CZTS. As for CZTS, a significant amount of hole carriers can be produced when a high population of CuZn antisites is ionized [33]. In our case, domination of such monovalent Cu in cation sublattice could also supply more free carriers to sustain LSPR, and somehow explain the high self-doping hole density in Cu3SnS4. To further elucidate the influences of defects on LSPR, Cu 3SnS4 NCs with Cu/Sn ≈ 3, and Cu/Sn > 3 compositions are then prepared under the same condition except for different starting ratios of cation to offer an experimental control. Not only are the crystallite sizes of these NCs very close to that of the Cu/Sn < 3 sample as estimated by Scherrer analysis, their Raman spectra overlap with each other as well (Figure 1 (a), (d), Figure S1 (a), (b) and Figure S2 (a), (b)). A comparison of absorbance spectra is given in Figure 4.Three LSPR bands are observed except that they are red-shifted and dampened in intensity with the increase of copper compositions, indicating a decrease in free carrier concentration. Once again, XPS detects no Cu2+ in Cu3SnS4 with Cu/Sn > 3 as shown in Figure S3. On the other hand, EDS results (Figure 1 (c), Figure S1 (c) and Figure S2 (c)) for the three samples show that Cu content increases at the expanse of decreasing Sn when the composition turns from Cu/Sn < 3 to Cu/Sn > 3. So it is reasonable to speculate that when the amount of Cu rises, additional Cu would probably take Sn sites to form Cu Sn defects and fill part of VCu. Further considering LSPR is always prominent and the valence state of Cu remains intact when Cu incorporation is enhanced, influences on the
amount of Cu Cu antisites, a major contributor of free carrier, can be neglected. As deep acceptor 2
level defect CuSn increases and shallow acceptor VCu decreases, a smooth decline in hole concentration, acting as a detrimental factor for LSPR, is observed. In addition, the random change in sulfur composition proves again that the regular shift in LSPR cannot be a consequence of VS. Interband absorbance might also be collaterally affected. Zhao found in UV-vis absorption that interband of Cu2-xS NCs showed a blue shift comparing to bulk material and contributed it to both quantum size effect and Moss-Burstein effects [35]. On condition that the sizes of the three kinds of NCs are kept similar, the blue-shifted bandgaps are mainly ascribed to the latter. Figure 5 illustrates Moss-Burstein effects in which optical bandgaps are associated with free carrier concentration [1]. When hole density is reduced, the optical bandgaps for Cu/Sn ≈ 3 and Cu/Sn > 3 Cu3SnS4 NCs are estimated to be 1.47 eV and 1.5 eV, respectively (Figure 4 (b)). Both values are close to the report on orthorhombic Cu 3SnS4 NCs [25], but < 0.1 eV narrower than their Cu/Sn < 3 counterparts of which higher doping level is presumed. It should be noted that the acceptor-related VCu can be enhanced over time as oxidation proceeds, resulting in an increase in LSPR frequency and intensity of Cu2-xS or Cu2-xSe [4-7]. The NIR absorptions of Cu3SnS4 NCs, on the other side, are more stable in the ambient. A second test was performed after the same sample was preserved in air for two weeks. Similar to CIS NCs [22], the intensity, shape, and position of the two absorption curves for Cu3SnS4 NCs remain almost the same. So oxidation is not responsible to produce additional free carrier in Cu3SnS4, and self-doping is very likely completed when the phase is formed. Other Cu-Sn-S NCs presenting LSPR. Except for Cu3SnS4, absorption in NIR has also been observed in other ternary Cu-Sn-S materials [24]. Cu2SnS3 is studied as it is p-type conductive
and has relatively larger chemical potential space [26]. Wurtzite Cu2SnS3 NCs and CZTS NCs have been reported by the authors before [36], and herein, by adjusting stoichiometry, Cu2SnS3 NCs with Cu/Sn < 2 and CZTS NCs with Cu/(Zn+Sn) < 2 (Cu-poor) are prepared. The information of their characteristics is given in Figure S4 to Figure S6. NIR absorption is also observed in Cu2SnS3 NCs, and we can also attribute it to LSPR as the peaks shift with the change of organic solvent (Figure 6). But LSPR of wurtzite Cu2SnS3 is obviously reduced comparing to wurtzite-derived orthorhombic Cu3SnS4 as indicated in Figure 7 (a), and little absorption is found for CZTS NCs. With the assistance of Drude model [13,22], a relationship between plasma frequency (ωp) of the free carriers in the NCs and LSPR frequency (ωsp) is given in the following equation (1):
sp =
p 2 1 + 2 m
2
(1)
where γ is the full width at half maximum (FWHM) of the LSPR peak, and εm represents the dielectric constant of the surrounding medium (εm = 2.28 for TCE, omitting the effect from the ligands). As shown in Figure 7 (a), the LSPR peak for Cu 3SnS4 NCs locates at 0.68 eV and its FWHM is measured as 1.19 eV. Thus, ωp is estimated at 3.23 eV from equation (1). Dopant density further depends on ωp as:
ωp
N h e2 ε0mh
(2)
whereε0 is free space permittivity, and mh is hole effective masses. As mentioned above, Cu3SnS4 is quite analogical to CZTS of which mh is calculated to be 0.22m0 (m0 is electron mass) [37,38] except for Cu substitution on Zn, leading to a certain extent of dispersed and delocalized valance band maximum which facilitates a relatively lower effective
hole mass. So we speculate mh for Cu3SnS4 as approximately 0.2m0 and substitute it into equation (2), obtaining Nh = 4.0 × 1019 cm-3. Similarly, as the LSPR maximum and FWHM for Cu2SnS3 NCs are 0.56 eV and 0.70 eV, respectively, and mh is reported as 0.33m0[39], an estimation of Nh is given at 2.7 × 1019 cm-3, within an order of magnitude comparing to that of Cu2SnS3 films, the other study presenting NIR absorption [24]. Although higher than Cu2SnS3, Nh of Cu3SnS4 NCs is still lower than the reported data [24]. Based on the fact that Cu2SnS3 NCs are nearly stoichiometric like the sulfurized films, but Cu3SnS4 NCs are quite sulfur deficient, the existence of VS in Cu3SnS4 which act as carrier combination center can be regarded as a major factor to be blamed for reducing hole density. On the other hand, no LSPR is found for CZTS as it has a relatively lower hole concentration within the range of 10 15~1018 cm-3 [40-44]. According to the volume of the NCs which is acquired from the average dimensions shown in HRTEM images (Figure 2 and Figure S5), we further estimate the number of free carriers at 55 per Cu3SnS4 NC and 35 per Cu2SnS3 NC, respectively. The number of carriers for an individual CZTS NC might be too small to support a LSPR mode.
4. Conclusions LSPR in NIR is observed in orthorhombic Cu3SnS4 NCs for the first time as proved by the dependence of resonance frequency on medium’s dielectric constant. The formation of Cu Cu 2 antisites is used to elucidate the high carrier concentration in Cu 3SnS4. More CuSn defects
which are detrimental for LSPR will also be produced when cationic composition turns from Cu/Sn < 3 to Cu/Sn > 3, explaining the slightly red-shifted and weakened LSPR bands. A further research is performed on the absorption differences between Cu 3SnS4 and other analogical semiconductors, i.e. Cu2SnS3 and CZTS. As a result of calculation, Nh of the two ternary
compounds are on the same order, both over the magnitude to support LSPR. Little absorption in NIR is detectable in CZTS NCs due to its lowerhole density.
Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC) (No.61006010),
Innovation Program of Shanghai
Municipal
Education Commission
(No.14YZ079), Shanghai Municipal Natural Science Foundation(14ZR1430500), also partially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013-016467).
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Figure captions Figure 1 (a) XRD pattern for orthorhombic Cu3SnS4 NCs and the standard data (JCPDS no. 360217) is plotted below for reference. (b) Absorbance spectrum for Cu 3SnS4 dispersion in TCE. The optical bandgap estimated by plotting (αE)2 as a function of the photon energy with α being the absorption coefficient is shown in the inset. (c) EDS and (d) Raman spectra of the sample. (e) Absorbance spectra for NCs dispersed in different solvents, and the inset shows the dependence of LSPR maximum on the solvent refractive index. Figure 2 (a), (b) TEM, (c) HRTEM images for orthorhombic Cu3SnS4 NCs, and (d) their corresponding fast Fourier transform. Figure 3 XPS analyses of the three elements in Cu 3SnS4 with Cu/Sn< 3. Figure 4 (a) Absorbance spectra for Cu/Sn> 3, Cu/Sn ≈ 3, and Cu/Sn< 3 Cu3SnS4 NCs dispersions in TCE. (b) The optical band gap estimation. Figure 5 Band structures of plasmonic p-type Cu3SnS4nanocrystals. Intrinsic bandgap (W), and optical bandgap (Eg) which is defined as the difference between the bottom of the conduction band and the highest occupied state in the valence band become larger when doping level is increased. Figure 6 Absorbance spectra of wurtzite Cu2SnS3 NCs dispersed in different solvents, and the inset shows the dependence of LSPR maximum on the solvent refractive index. Figure 7 (a) Absorbance spectra for wurtzite Cu2SnS3, CZTS, as well as orthorhombic Cu3SnS4 NCs dispersions in TCE. (b) Estimations of band gap energy for different NCs. WurtziteCu 2SnS3
NCs has an optical bandgap of 1.45 eV, and Eg of wurtzite CZTS (1.54 eV) is equal to the previous reports on CZTS films or nanoparticles.
Highlights
Orthorhombic Cu3SnS4 nanocrystals were obtained via hot-injection method.
The ternary nanocrystals with localized surface plasmon resonances are reported.
Localized surface plasmon resonances are caused by high carrier concentration.
Cu+ occupy the positions originally belong to Cu2+ to form Cu Cu antisite defects.
CuSn defects are detrimental for localized surface plasmon resonances.
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