Near-Infrared Emission and Photon Energy Upconversion of Two

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Near-infrared Emission and Photon Energy Upconversion of Two-Dimensional Copper Silicates Weibo Chen, Yeqi Shi, Zhi Chen, Xiangwen Sang, Shuhong Zheng, Xiaofeng Liu, and Jianrong Qiu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04819 • Publication Date (Web): 17 Aug 2015 Downloaded from http://pubs.acs.org on August 19, 2015

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The Journal of Physical Chemistry

Near-infrared Emission and Photon Energy Upconversion of Two-Dimensional Copper Silicates Weibo Chen,a Yeqi Shi,a Zhi Chen,b Xiangwen Sang,a Shuhong Zheng,a Xiaofeng Liu,a*and Jianrong Qiua* a. School of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang, 310027. China. b. State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, Guangdong, 510640, China. * To whom correspondence should be addressed. E-mail: [email protected]; [email protected]; Fax: +86 057188925079; Tel: +86 057188925079

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Abstract

BaCuSi4O10 (Han blue), CaCuSi4O10 (Egyptian blue), and SrCuSi4O10 are pigments found in many ancient artifacts all over the world. Behind their brilliant color, we demonstrate here that these ancient pigments are strong candidates for photonic materials due to their bright Stokes and anti-Stokes emissions. These pigments give near-infrared emissions (NIR) from Cu2+ centered at around 930 nm under excitation of 440-800 nm light. This NIR emission can also be produced by pumping using a NIR laser diode. With the rise of pumping density, the emission bandwidth increases notably and stretches to the visible region, giving rise to bright, and broadband photon upconversion (UC). This photon UC process is interpreted in terms of laser-driven blackbody radiation from the ancient pigments.

1. Introduction The blue color is absent from ancient wall paintings made in the earliest cultural activities of humankind, because earth’s surface soil normally does not provide blue pigments. Egyptian blue (CaCuSi4O10) is the first-traces of man-made blue pigments date back to early Egyptian culture about 3600 BC1-2. In ancient China, Han blue (BaCuSi4O10) and Han Purple (BaCuSi2O6) were developed in Han dynasty (202 BC-220 AD) and they were widely used as blue pigments in artworks. Due to their brilliant colors, simple preparation method, and extremely high chemical and physical stability, they were widely used in artifacts and many of them were well preserved until today. The near-infrared photoluminescence of these pigments was not discovered until the last decade. Ajὸ et al.3 found the broad near-infrared emission centered at 920 nm from these pigments when


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excited by 532 nm or 632 nm light, which is originated from d-d transition of divalent copper ions. This spectra feature was further used for identification of these pigments in artifacts4. These early investigation stimulated further effort in the search for mechanistic understanding of the NIR luminescence and its potential applications5-6. Most work mainly focus on the exceptional near-infrared luminescence properties of freshly prepared samples or that from artifacts. Salguero et al.5 have studied the near-infrared emission properties of monolayer nanosheets of CaCuSi4O10. Zhuang et al.7 have found the energy transfer between Cu2+ and Yb3+ in Ca1xCuSi4O10:Ybx,

and acquired luminescence simultaneously from both Cu2+ and Yb3+ (1007 nm)

under visible light excitation. Borisov et al.8 have applied these pigments as optical chemosensors. Compared with normal fluorescence by Stokes emission, upconversion is an important nonlinear optical processes, which mainly originate from excited state absorption (ESA), energy transfer upconversion (ETU), and photon avalanche (PA)9. Upconversion from infrared to visible region has attracted considerable interest due to its numerous applications after its discovery in the mid1960s10. Recently, photon energy upconversion (PEUC) by thermal radiation based on lanthanide ions appears as a new UC pathway and has received growing attention11-14. Unlike normal upconversion mechanisms, photon energy upconversion by thermal radiation can reach much higher upconversion efficiency. For instance, Strek et al.15 reported that LiYbP4O12 yielded a 10 % efficiency for white light emission, and Wang et al.16 estimated the power efficiency of ZrO2:Yb3+ can reach up to 16 % under 980 nm laser excitation. In this research, we fabricated CaCuSi4O10, SrCuSi4O10 and BaCuSi4O10 pigments by solid state reaction. The influence of alkaline earth metals in luminescence property of Cu2+ was investigated. Bright photon energy upconversion light was first observed in these copper-silicon


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compounds under NIR laser excitation. For convenience, CCO, SCO, and BCO are used to represent CaCuSi4O10, SrCuSi4O10 and BaCuSi4O10 respectively. 2. Experimental Methods 2.1 Materials Cu2(CO3)(OH)2 (AR, 54-57% Cu basis) is purchased from Aladdin Chemistry Co., Ltd. SiO2 (AR), MCO3 (M=Ca, Sr and Ba) (AR) and Na2CO3 (AR) are purchased from Sinopharm Chemical Reagent Co., Ltd. Yb2O3 (99.99%) is purchased from Shanghai Chemical Agent Ltd. China. All chemicals were used as received. 2.2 Sample Preparation Pigment samples were synthesized by a solid-state reaction method. Cu2(CO3)(OH)2, SiO2 and MCO3 (M=Ca, Sr and Ba) in a molar ratio of 1:8:2 as starting materials and 3 mol% Na2CO3 as fluxing agent were mixed in an agate mortar for 15 min. Then the mixture was calcined at 1000 °C for 24 h in air, and the products obtained were thoroughly ground into powder for different measurements. ZrO2:Yb3+28% was synthesized by a procedure described by Yan et al.16 2.3 Characterization The powder products were directly used for the structure and spectroscopic characterizations. Powder XRD patterns of products were collected on a RIGAKU D/MAX 2550/PC diffractometer with Cu Kα (λ=1.5406 Å) as the radiation source. Scanning electron microscopy (SEM) was performed using a Hitachi S-4800 FE-SEM. A UV-Vis spectrometer (Hitachi U4100) was employed to record the diffuse reflectance and a FLS920 fluorescence


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spectrophotometer (Edinburgh Instrument Ltd., UK) was used to collect the emission and excitation spectra. Both Xeon lamp and laser diode (980 nm) were used as the excitation source. All the measurements were performed at room temperature. 3. Results and Discussion 3.1 Morphology and Structure By simple solid state sintering, pure phase of CCO, SCO, and BCO phosphors were obtained. As can be seen from Fig. 1a, the X- ray diffraction (XRD) patterns of the compounds match well to the standard Joint Committee on Powder Diffraction Standards (JCPDS) card No.12-0512,


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No.49-1813, and No.82-1868 respectively. All the three compounds has the same tetragonal space group P4/ncc (No. 130) with a layered architecture, in which corner-sharing [SiO4] tetrahedrons form an extended 2D network, and Cu2+ is situated in the center of a square planar complex coordinated to four oxide ions. Crystal structure of BCO is illustrated in Fig. 1b. The increase in the size of alkali earth cation results in the expansion of the lattice cell along all the three axis (Table 1), as reflected by the small shift of diffraction peaks in Fig 1a. Table 1. Lattice parameters of the samples. Samples Space group (No.) a (Å) b (Å) CCO P4/ncc (130) 7.300 7.300 SCO P4/ncc (130) 7.366 7.366 BCO P4/ncc (130) 7.442 7.442

c (Å) 15.120 15.574 16.133

Fig. 2a and 2b are the SEM images of CCO, which demonstrate the platelet-like nature of this compound with clear layer structure and irregular shape. The SEM images of SCO and BCO show similar morphology as that of CCO.

Figure 2. SEM image of (a) and (b) CaCuSi4O10. (c) SrCuSi4O10. (d) BaCuSi4O10. 3.2 NIR Emission under Visible Light Excitation Absorption spectra of three phosphors is illustrated in Fig. 3a. Each absorption spectrum consists of three peaks (center at around 520 nm, 625 nm and 800 nm) corresponding to electronic


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transitions of square-planar Cu2+ (2B1g2A1g, 2Eg, and 2B2g). Due to the scattering effect, the absorption spectra are smooth compared to the excitation spectra, where the presence of fine structures is clear due to further spectral splitting. From CCO to SCO and BCO, absorption peaks of 2B1g2A1g and 2B1g2Eg have a slight redshift, while the peak of 2B1g2B2g exhibits a slight blue shift. This trend is due to both lattice expansion and the Jahn-Teller effect3.

Figure 3. (a) Absorption spectra of CaCuSi4O10, SrCuSi4O10 and BaCuSi4O10 respectively. The inset is the photograph of the phosphor powders filled in alumina sample holders. (b) Normalized excitation spectra (solid lines) and emission spectra (dotted lines). Fig. 3b depicts the normalized excitation and emission spectra of the samples. The excitation spectra of these pigment consists of a broadband covering 450 nm-700 nm, which match the transitions of 2B1g2A1g and 2Eg. A gradual redshift of emission peak of CCO, SCO and BCO


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(932, 936 and 964 nm respectively) is observed, which is consistent with the crystal field theory. From these spectra, the size of alkali earth ions exerts an obvious impact on the transition energies. The increasing of ironic radius from Ca2+ to Ba2+ causes lattice expansion, therefore produces weaker crystal field, resulting in the redshift of emission peak. The crystal field (CFS) energy (10Dq) of samples are estimated as listed in Table. S1. The CFS energies for CCO, SCO and BCO are 12710 cm-1, 12410 cm-1 and 12140 cm-1, respectively. In addition, we also calculated other CFS parameters according to the equations given by Ye et al.17. The results given in Table. S1 indicate that weaker crystal field (for larger alkali earth ions) leads to redshift of emission peak for Cu2+ with d9 configuration in the square-planar (D4h) coordination environment, similar as the result observed by Ye et al.17 in CCO. To understand the spectral characteristics of the pigment, we show in Fig. 7b the energy diagram of Cu2+. In the octahedral ligand field, the 2D ground term of free Cu2+ ions splits into 2T2g and 2

Eg energy levels18-19. In these pigments the coordination of Cu2+ can be better described as a

square-planar (D4h) structure, in which Cu2+ coordinates with four oxygen atoms (Cu2+ linked by [SiO4]). The planar coordination is less symmetry than Oh, which leads to further splitting of 2T2g (into 2Eg and 2B2g) and 2Eg (into 2A1g and 2B1g)3. Since the spin-orbit coupling constant of Cu2+ ions is large20-21, along with the spin-orbit coupling, energy level of Cu2+ finally splits into five energy levels. As the ion radius gets larger from Ca2+ to Ba2+, the symmetry of octahedral ligand field gets lower and splits more obviously, resulting in the spectral shift in Fig. 3a and Fig. 3b. 3.3 Photon Energy Upconversion under NIR Excitation Bright Upconversion luminescence (UCL) was observed for the first time in copper-silicon compounds. Fig. 4a shows the emission spectra of BCO under excitation of a 976 nm diode


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laser. Here the off-resonance pumping of the B2g level of Cu2+ is possibly helped with the lattice phonons via electron-phonon coupling. BCO starts to emit weak red light at the excitation density of 15 W/cm2. With further increase of excitation power, the emission light of BCO gradually turns white and finally emits very intense broadband white light at 580 W/cm2. The emission light intensity gradually increases with the rise of excitation power, and the spectrum edge gradually extend to shorter wavelength. Meanwhile the emission peak shows a gradual blue shift. These are the characteristics of blackbody radiation with the increase in temperature.

Figure 4. (a) Emission spectra of BaCuSi4O10 under irradiation of a 976 nm laser with increasing power. (b) Log-log plot of integrated emission intensity of BaCuSi4O10 (in the range from 300 to 850 nm) versus 976 nm laser power density. The solid lines are linear fitting, giving slopes of 1.58 and 4.56.


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It can be seen in Fig. 4b that the power density dependence can be divided into three periods. The integrated UCL intensity first increase (with slope=1.58) with the increase of laser power. Then, as the excitation power increases further, UCL intensity experiences a slight drop and then dramatically increase with slope=4.56. This trend is also observed in other literatures22-23, which is probably caused by photon avalanche or thermal avalanche24. Fig. 5a shows a typical irradiance spectrum (laser power: 505 W/cm2) together with the fitting using Planck’s law:

்ܲ (λ) = a

2ℎܿ ଶ λହ

1 ௛௖ ିଵ ݁ ஛௞ಳ ்

Where T is the absolute temperature of the blackbody, l is wavelength, a is a fitting parameter, h is the Planck constant, c is the speed of light and kB is the Boltzmann constant. The emission spectrum fits well with the Planck’s law at different temperatures. As can be seen in Fig. 5b, variation of blackbody temperatures shows a similar trend as that of the UCL intensity.


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Figure 5. (a) Irradiance spectrum (dotted) and the fitted curves (solid line) using the Plankian radiation at 1620 K for the high energy side of the emission spectrum. (b) Dependence of the blackbody temperature on the excitation power density. Each emission spectrum was fitted according to Plank’s law to obtain the blackbody temperature. Near-resonance pumping at a shorter wavelength (808 nm) results in a similar white light emission due to spectral broadening as the pumping powder increases (SI). The emission intensity exhibits a very similar dependence on the excitation power as the case of excitation using 980 nm laser diode, as shown in Fig. S1a in supporting information. In addition, the white broadband emission can be also produced by the excitation with the NIR part of the Xeon lamp (using an 800 or 900 nm long pass filter) as shown in Fig. 6, or a 1064 nm laser diode (see Fig. S1 and S2), suggesting that the emissions produced under different excitation conditions are of the same origin.


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Figure 6. Emission spectra of CaCuSi4O10, SrCuSi4O10 and BaCuSi4O10 under excitation of concentrated simulated sunlight with (a) 800-nm long pass filter and (b) 900-nm long pass filter. 3.4 Discussion Bright white UCL has been reported by some research groups. Wang et al.16 have reported the photon energy upconversion of ZrO2:Yb3+ under 976 nm excitation with excitation power density higher than 364 W/cm2. The measured radiation efficiency increases to 16 % as the excitation power density reaches 800 W/cm2. The mechanism is ascribed to blackbody emission. Other materials, such as lanthanide oxides25-26, Si and SiC nanoparticles27, and carbon nanotubes28 were also found to emit white light under intense 976 nm excitation due to blackbody emission. However, the origin of a laser-induced white-light emission is still of much controversy. Beside blackbody emission, possible mechanism includes thermal avalanche process24, 29, defect emissions30, electron-hole combination31 and anti-Stokes emissions15.


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Figure 7. (a) Schematic showing the photon energy upconversion and NIR emission. (b) Simplified energy level diagram of Cu2+ ions in tetragonally distorted octahedral crystal field environment in BaCuSi4O10 lattice. In the copper silicate pigments examined here, divalent Cu2+ in a very high concentration ensures high absorption efficiency for the NIR laser radiation. When the excitation power density is higher than 15 W/cm-2 (@980 nm), the temperature of the laser spot on the sample surface will rise notably above room temperature. As phonon excitation is strongly temperature-dependent, the rise in sample temperature leads to much enhanced electron-phonon coupling Interactions. Due to the admixing of transitions from phonon sub-levels (Fig. 7b), the emission spectra are gradually broadened with the increase of pumping density; eventually reddish emission in the visible are observed. Increasing further the pumping density above 50 W/cm2 leads to a slight drop in UCL intensity. The exact mechanism is still unclear, but a similar process is observed in


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RE-doped systems, in which the anomaly has been ascribed photon avalanche or thermal avalanche process24. When the power density reach up to 220 W/cm2, the bright broadband emission is dominated by a mechanism similar to blackbody radiation14,

32

, featuring a

continuous rise of blackbody temperature with pumping density and a slope of 4.56 in the UCL intensity-pumping power relation. On the other hand, at high pumping densities, the 3d levels of Cu2+ would become saturated, therefore enabling the promotion of electrons further to the conduction band. This multiphoton process associate with a photon avalanche mechanism was supported by the photocurrent measurement for several RE3+ doped compounds and the white emission has been ascribed to a charge transfer process22, 33-35. Fig. 7a is schematic showing the photon energy upconversion and NIR emission. Concerning the visible light upconversion, this process might be dominated by the interplay of thermal radiation and broadened photoluminescence originated from the transition of B2gB1g. The emission peak by PL does not change with the increase in the excitation laser energy. In comparison, the peak of thermal radiation driven by laser excitation is expected to experience blue shift with the increase of laser power, according to the law of blackbody radiation. Practically both processes could occur under laser excitation, and thus the two mechanisms are not easily distinguishable from the emission spectra. Since the emission spectra can be successfully fitted by the law of black body radiation, it is reasonable to suggest that laser driven thermal radiation may be dominating, especially under high powder laser excitation. To evaluate the efficiency of white light generation, we have made a systematical comparison of the efficiency for the copper silicates and other compounds reported to show highly efficiency white light thermal radiation (such as ZrO2:Yb3+ and Yb2O3)16, 33. As shown in Fig. S3, under excitation by a 976 nm laser at 20 W/cm2, the luminescence intensity of BCO is the highest,


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followed by SCO and then BCO, while at the same condition ZrO2:Yb3+28% and Yb2O3 do not show observable emission. This result indicates that the threshold laser energy for visible light radiation of CCO, SCO and BCO are lower than that of ZrO2:Yb3+28% and Yb2O3. With the excitation power further increase to 400 W/cm2, luminescence intensity of CCO become the highest, followed by BCO and SCO. Luminescence of Yb2O3 is stronger than that of ZrO2:Yb3+28%, but negligible compared to CCO, SCO and BCO. When the excitation power density reach up to 1000 W/cm2, luminescence of Yb2O3 become the strongest, probably because it has the highest content of Yb3+. At this laser energy, luminescence of CCO, SCO and BCO becomes unstable, as can be speculated from the fluctuation of emission spectra. With further increase of excitation power density, photon energy upconversion of CCO, SCO and BCO is quenched, while the luminescence intensity of Yb2O3 and ZrO2:Yb3+28% keeps going up because they are more stable at high temperature and have relatively higher melting point. Indeed, laser damage occurs for the copper silicate pigments, despite that thermal analysis indicate the stability of these compounds under 1273 K (Fig. S4). As can be seen from Fig. S3d, black dots are formed on the surface of the sample, which are obtained by laser irradiation for 60 second with power densities of 50, 400 and 1000 W/cm2 respectively (from left to right). Large amount of heat is generated by laser and the temperature rise quickly on the laser spot. When the power density is relatively low (

Near-Infrared Emission and Photon Energy Upconversion of Two

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