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Abstract
Single-crystalline erbium chloride silicates have attracted extensive attention due to their high gain compatibility and silicon compatible properties. Long-lived near-infrared fluorescence is critical for reducing a pump density threshold when erbium containing materials are used as active devices. Here we developed a single-source chemical vapor deposition (CVD) method to grow high-quality single-crystalline erbium chloride silicate nanostructures. The growth mechanism is found composing of two steps, where silicon source comes from the minor evaporation of silicon substrate. The prepared single-crystalline erbium chloride silicate nanowires own diameter of about 200 nm with few lattice defects, and the fluorescence lifetime reaches up to 7.4 ms. A nanoscale thermometer based on their visible band fluorescence is realized.
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1. Introduction
Single-crystalline erbium chloride silicate nanostructures have attracted extensive attention due to their silicon compatible nature and high gain characteristics in near-infrared (NIR) optical communication band (1.5 µm). [1–4]. When single-crystalline erbium chloride silicates are used as active devices, such as optical amplifiers or light sources, the contained erbium concentration determines their gain coefficient. Single-crystalline erbium chloride silicates own erbium concentration of ∼1022 cm-3, which is about 2 or 3 orders of magnitude higher than that in traditional erbium-doped semiconductor materials [5,6]. The gain coefficient of single-crystalline erbium chloride silicate nanostructures has exceeded 10 dB/mm, providing sufficient gain to small-sized nanostructures or devices, such as an NIR optical amplifier and a vertical cavity surface emitting laser (VCSEL) [7–9].
Under a light excitation at 980 nm, the erbium ions, Er3+, in the erbium chloride silicates exhibit fluorescence emission both in near-infrared and visible band [10–12]. The fluorescence lifetime is a very important parameter that determines the pump density threshold of achieving a population inversion during achieving an amplifier or a laser [13]. However, materials containing such high erbium concentration typically exhibit a fluorescence quenching effect, which usually hinders realizing a low-threshold amplifier or laser [13–15]. It is challenging to obtain nanostructures with long fluorescence lifetime while maintaining high erbium concentration. In addition, besides the excellent application in the active devices in the NIR band, single-crystalline erbium chloride silicates have not yet been properly applied in visible band devices [11,16].
Here in this work, we developed a single-source chemical vapor deposition (CVD) growth method without silicon powder to grow single-crystalline erbium chloride silicate nanostructures. During the growth, silicon vapor is provided from the silicon substrate used to grow the nanostructures. The prepared single-crystalline erbium chloride silicate nanowires own few lattice defects and their fluorescence lifetime in NIR band reaches up to 7.4 ms. Furthermore, the silicate nanostructures can also be used to realizing a nanoscale thermometer based on their up-conversion luminescence in the visible band.
2. Experiments and methods
2.1. Material preparation
Erbium chloride silicate nanostructures are grown by a single-source chemical vapor deposition (CVD) growth method without silicon powder. As shown in Fig. 1(a), briefly, a ceramic boat with ErCl3 powder and another boat with Si substrate is placed into a quartz tube with tube diameter of 1 inch. And then the quartz tube is places into a horizontal furnace, where the ErCl3 powder locates at the central heating zone and the Si substrate locates at the downstream of the furnace. Before heating the furnace, argon gas is introduced into the tube to purge air with a flow rate of 500 sccm for 2 minutes and then acts as buffer gas at a constant flow rate of 96 sccm. The furnace temperature is raised gradually from room-temperature to 1150°C within 110 minutes, with a constant rising rate of ∼10 °C min-1, while containing the pressure around 1013 mbar in the quartz tube. At this moment, the temperature at ErCl3 boat and Si substrate is about 1145 °C and 1125 °C, respectively. Keep this condition for 60 minutes for material growth and then cool down the furnace to room temperature using another 110 minutes.
Fig. 1. Growth method and optical morphologies of erbium chloride silicate (ESC). (a) Schematic diagram for CVD method. (b)-(d) Optical photographs with three different morphologies appeared on the Si substrate, include silicon triangles (b), ESC nanowires (c) and ESC nanosheets(d).
2.2. Structure characterization
The phase and crystal structure of the as grown samples are characterized by an X-ray diffractometer (Lab XRD-6100), which works using Cu Kα radiation at 40 kV and 30 mA in a 2θ range from 10° to 80° and a high-resolution TEM (Tecnai G2 F20) with an accelerating voltage of 200 kV. The morphology and element distribution of the samples are observed by scanning electron microscope (SEM, TESCAN MIRA3 LMH) with electron beam energy of 20 kV and energy dispersive spectrometer (EDS, Ultim Max 20).
2.3. Optical characterization
A self-built micro-area spectroscopy is used to executive the optical characterizations of the prepared nanostructures (see details in Fig. S1 in Supporting Information). The as grown samples are firstly transferred to a silica substrate before the optical experiment. A continuous-wave (CW) laser and a pulsed laser at the wavelength of 976 nm are used as excitation sources for steady-state and transient fluorescence detection, respectively. The excitation laser is firstly focused by an objective and the emitted fluorescence from the samples is collected by the objective above or underneath the sample stage, and then is collimated into a spectrometer. Steady-state fluorescence detection is performed by a spectrometer (iHR 550, Horiba) equipped with two liquid nitrogen-cooled CCDs, which responds fluorescence signal from ultra-violet (UV) to NIR bands (200-1700nm). In the collimated fluorescence beam path, a flip mirror is used to introduce the fluorescence into two photomultiplier tubes (PMT) mounted on a monochromator (Omni-λ3007i, Zolix). Two PMTs respond to visible and NIR light, respectively.
3. Results and discussion
Different from the previously reported growth method that used dual sources (silicon and ErCl3 powder) [17], our method employs only one source, ErCl3 powder. This approach can reduce silicon powder deposition on the growth substrate and keeps them clean. As shown in Fig. 1(b)-(d), there exist samples with various morphologies on the growth substrate, including triangle sheets, nanowires and nanosheets. In order to explore the growth mechanism, we carried out morphology and structural characterizations for the triangle sheets and nanowires. Figure 2(a) shows a Raman spectrum and its corresponding mapping (inset) of a typical triangle sample. The Raman shift peak of the triangle sample is at 516 cm-1, which is close to the Raman shift of silicon at 521 cm-1. In order to verify the composition of the triangle sheets, SEM image (Fig. 2(b1)) and two-dimensional element mapping are carried out. As shows in Fig. 2(b2) -(b3), the triangle sheet only contains two elements, Si and O, verifying that the triangle samples are silicon sheets with a small amount of oxidation on the surface. This also proves that under natural conditions, an oxide layer formed on the surface of the silicon wafer, providing an oxygen source for the growth of sample.
Fig. 2. Structure characteristics of triangle sheets. (a) Raman spectrum of a typical triangle sheet, with single Raman shifts at 516 cm-1. The inset is the Raman mapping of a triangle sheet. (b1) SEM image of a typical silicon triangle. (b2) -(b3) Element distribution of Si and O in the silicon triangle sheet in (b1).
Figure 3(a) shows a SEM image of a typical nanowire with length of ∼ 60 µm and diameter of ∼ 200 nm. The overall distribution of nanowires on the silicon wafer is shown in Fig.S2. The surface of the nanowire is quite smooth and the diameter is uniform through the entire wire. As shown in Fig. 3(b1)-(b4), the nanowire contains Si, O, Er, and Cl elements distributing all across the wire. From the transmission electron microscopic (TEM) characterization (Fig. 3(c)), it can be seen the nanowire is single-crystalline and owns high crystal quality with lattice spacing of 6.4 Å. Selected area electron diffraction (SAED) image (inset in Fig. 3(c)) demonstrates the crystal structure of the nanowires are tetragonal. The X-ray diffraction (XRD) pattern (Fig. 3(d)) of the nanowire is basically consistent with the standard card No.42-0365, from which the lattice constants are a = 0.6822 nm, b = 1.7652 nm, c = 0.6160 nm and the chemical formula of Er3(SiO4)2Cl can be obtained. The above characterizations demonstrate the grown nanowires are single-crystalline erbium chloride silicate (ESC). The nanowires have length reaching hundreds of micrometers, and own smaller wire diameters and length/diameter ratio than the nanowires prepared by the dual source growth method [17]. Based on the above determined structural parameters, the erbium concentration in the ESC single crystal can be estimated as about 4.1 × 1021 cm-3, which is two orders of magnitude higher than those erbium dopped materials, such as Er3+-doped silica with erbium concentration of 2.4 × 1015 cm-3 and Er3+-Yb3+ co-doped phosphate glasses with erbium concentration of 1.73 × 1020 cm-3 [18,19].
Fig. 3. Structure characterization of ESC nanowires. (a) SEM image of a single ESC nanowire, with typical nanowire diameter of ∼200 nm and length of ∼60 µm. (b1-b4) Two-dimensional element mapping of O, Er, Cl and Si in the ESC nanowire shown in panel (a). (c) High-resolution TEM image of the ESC nanowire with the lattice spacing of 6.4 Å. The inset is the SAED pattern, revealing its single crystal nature. (d) XRD pattern of ESC nanowires (below) and the data from JCPDS Card No.42-0365 of Er3(SiO4)2Cl (above).
The growth of the ESC nanowires can be understood as a vapor-solid (VS) mechanism. As shown in Fig. 4, the growth contains two steps, the formation of silicon sources in step 1 and the growth of nanowires in step 2, respectively. In step 1, the silicon wafer is kept at a constant temperature of 1150°C for one hour, the silicon wafer volatilized, and triangular silicon sheets are formed in the original position. Then in the step 2, the silicon source is driven by the airflow to the silicon wafer to form nucleation sites, the ErCl3 powders are vaporized and carried to the silicon wafer. At this moment, the ErCl3 and silicon form a gas-solid interface on the nucleation sites, and the ErCl3 vapor will continuously deposit into the sites until it reaches to a supersaturated state. Hereafter, the crystals precipitate out of the interface and actuate the wires growing. The nanowires obtained through this single-source method own much thinner wire diameters than those grown using the dual-source method (see details in Figure S3 in Support Information). This is mainly because this grow method without silicon source comes from the volatilization of silicon on the substrate. In this case, the amount of silicon volatilization is small, and the reaction speed is slow, so the diameter of the nanowire is small. In fact, the evaporation of the silicon source on the silicon substrate is slight and occurs only in the high temperature region. Furthermore, the nanowires need to be transferred to other substrates for subsequent device fabrication such as an optical amplifier or a laser.
Fig. 4. Schematic diagram of the growth mechanism of ESC nanowires. The growth process can be divided into two stages that was the evaporation of silicon source in step 1 and the growth of ESC nanowire in step 2, respectively.
Erbium containing nanomaterials can be synthesized in many ways: sol-gel process, hydrothermal method, solvothermal method, combustion method, co-precipitation method, magnetron sputtering method and CVD method [1,20,21]. Among them, the liquid phase method has the advantages of simple equipment, easy access to raw materials and low reaction temperature, but the obtained samples contains many internal defects [22,23]. For the materials prepared by a combustion method, both erbium concentration and the fluorescence efficiency are quite low. For the materials prepared by a CVD method, the crystal structure usually contains few defects and hence the crystal quality is usually remarkable [24]. However, for the CVD method with dual evaporation sources, the evaporation speed of the silicon powder is hard to control, which usually leading to an over-deposition of the source on the growth substrate. This over-deposition process will make the surface of the material dirty, and even some of the grown materials are buried (see Fig. S4 in Supporting Information). For the single-source growth method in this work, the silicon source comes from the minor evaporation of the silicon substrate. The slow evaporation and reaction speed facilitates an ultra-uniform diameter and a highly smooth surface of the wires. This mechanism can be understood similar as an in-situ growth on silicon wafer.
The ESC nanowires are optically characterized by a self-built micro-area spectroscopy system (see details in Optical Experiments). The total fluorescence emission includes both visible and NIR bands. As shown in Fig. 5(a), the spectra peaks appeared at the visible band (525, 550 and 661 nm) and NIR band (1516 nm) are derived from the up-conversion and down-conversion fluorescence of Er3+, respectively. Figure 5(b) is a schematic diagram of luminescence energy level of Er3+ under a laser excitation at 980 nm. The Er3+ ions locate on ground state 4I15/2 absorb photons and transit to the excited state 4I11/2 through the ground state absorption (GSA) process. The ions on 4I11/2 state can absorb another photon and transit to the 4H7/2 state through an excited state absorption (ESA) process [10]. Next, the Er3+ will relax non-radiatively to the 2H11/2, 4S3/2 and 4F9/2 states and then transit back to the ground state accompanied by radiating green fluorescence at 525 nm, 545 nm and red emission at 651 nm, respectively. Simultaneously, because the 4I11/2 state is unstable, the Er3+ will transit to the metastable state 4I13/2 with non-radiative transition process and lead to a particle population in the 4I13/2 state, and then particles relaxation to the ground state produces the NIR fluorescence at 1.5 µm band. Log-log plot of the fluorescence intensity as a function of the excitation laser power is shown in Fig. 5(c). The slope for the visible fluorescence is about 1.29 and that for the NIR fluorescence is about 0.65, confirming the two-photon process for the up-conversion fluorescence and single-photon process for the down-conversion fluorescence, respectively. The diameter of the collimated pump beam focused on the sample is 50µm. Silicate materials have good thermal stability. We also perform a verification test as shown in Fig. S5, to ensure that the samples are not damaged during the test. It can be observed that the slope value does not change significantly after the forward and reverse variable power tests. So, we believe that the sample is not damaged during the test.
Fig. 5. Fluorescence detection of ESC nanowires. (a) Schematic diagram of luminescence energy level of Er3+ under 980 nm laser excitation. (b) Emission spectra in visible (red line) and infrared (black line) region. (c) Log-log plots of fluorescence intensity (visible in red line and infrared in black) dependent on excitation power. (d) Transient fluorescence decay curves of ESC nanowires, with lifetimes of ∼340 µs and ∼7.4 ms in visible (red line) and infrared (black line) region, respectively.
The fluorescence emission intensity and lifetime of Er3+ are usually affected by the erbium concentration of their host materials. When the erbium ion concentration increases, the fluorescence intensity usually decreases and the lifetime is shortened. This is generally come from an Er3+ clustering phenomenon caused by the distance shortening in high-erbium-concentration materials [25,26]. Whether the NIR fluorescence lifetime can be maintained longer in our ESC crystal is the key issue that need to clarify, because a long-lived NIR fluorescence is beneficial to reducing the pumping threshold during light amplification or lasing [27]. As shown in Fig. 5(d), the fluorescence decay curves are recorded by our micro-area transient fluorescence spectrometer, and the exponentially fitted lifetime at 651 nm and 1.5 µm is about 340 µs and 7.4 ms, respectively. Compared with the materials those owns erbium concentration around ∼1021 cm-3 [1,17], our ESC material owns longest NIR fluorescence lifetime (see supporting information in Table S1). There may be two reasons why we are able to obtain long-lived NIR fluorescence of Er3+. On the one hand, the prepared samples own higher crystal quality, which usually implies weaker non-radiative process during the optical transitions. On the other hand, our samples have weaker water absorption properties comparing to the previously reported erbium chloride silicate nanostructures, and the corresponding non-radiative center, OH−, will also decrease [10,26,28]. Put our sample and the sample prepared by dual-source chemical vapor deposition method under the same conditions for two hours to observe the water absorption on the surface of the sample. We found that our samples had only a few blisters on the surface, while the other samples had great quantity of blisters. This long-lived NIR fluorescence of Er3+ facilitates the realization of energy level population inversion at low pump density, and expands the application of erbium chloride silicate.
As an excellent up-conversion luminescent material, ESC nanowires can be used as a thermometer. The 525 nm and 550 nm luminescence are produced by the transition 2H11/2 →4I15/2 and 4S3/2→ 4I15/2, respectively. Due to the relatively narrow energy gap between the 2H11/2 and 4S3/2 state, 710 cm-1, the particles distributed on the 4S3/2 state can be easily excited to the 2H11/2 state by thermal energy [29]. Figure 6(a) shows the green up-conversion (UC) emission spectra at four temperatures of 180 K, 220 K, 260 K, and 300 K. As the temperature rises, the green emission at 550 nm gradually becomes weaker. Fluorescence intensity ratio (FIR) between the peaks at 525 nm and 550 nm dependent on temperature can be used to evaluate the temperature variation [30]. The expression of FIR is shown in Eq. (1) [31],
where I525 and I550 are the fluorescence intensity at 525 nm and 550 nm, respectively. B is a constant that usually related to the degeneracy of the energy level, the rate of spontaneous emission and the phonon energy of the matrix. ΔE is the energy gap between the 2H11/2 and 4S3/2 levels, kB is the Boltzmann's constant and T is the absolute temperature in the system. To evaluate the dependence of luminescence performance on temperature, the natural logarithm of FIR within the temperature range of 150 K - 300 K is plotted versus T-1 and illustrated in Fig. 6(b). By linear fitting using the Eq. (2) [32], the parameter ln(B) =1.58 and C =1087 are obtained. The energy difference between 2H11/2 and 4S3/2 is obtained to be about 720 cm-1 for the ESC nanowire, which is close to the experimental value (710 cm-1).Fig. 6. The temperature sensing ability of ESC nanowires. (a) Up-conversion emission intensity of ESC nanowires at 525 nm and 550 nm upon excitation at 980 nm for different temperatures; (b) Logarithmic peek intensity ratio at 525 nm and 550 nm dependent on the reciprocal of temperature.
4. Conclusions
We developed a single-source CVD method of preparing single-crystalline ESC nanostructures. This method not only saves source materials but also avoid the over-deposition of sources and keeping the surface of the material clean. Compared with the traditional CVD growth method with dual sources, the diameter of the grown nanowires grown with single-source is thinner and uniform. The nanowires have clean surface without obvious impurities, and they own excellent fluorescence performance both in visible and near-infrared bands. Especially, the NIR fluorescence lifetime reaches up to 7.4 ms, which is the longest lifetime values under such high erbium concentration of 4.1×1021 cm-3 so far. We also investigated the fluorescence intensity ratio of nanowires at 525 and 550 nm emission band as a function of temperature in order to explore performance as a temperature sensor.
Funding
Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation,Changsha University of Science and Technology (2019XK2001, 2020XK2001); National Natural Science Foundation of China (51972105, 52072117, 61635001).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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