Tunable near-Infrared Luminescence of PbSe Quantum Dots for

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Tunable near-Infrared Luminescence of PbSe Quantum Dots for Multigas Analysis Long Yan,†,‡ Yu Zhang,*,†,§ Tieqiang Zhang,§ Yi Feng,§ Kunbo Zhu,§ Dan Wang,§ Tian Cui,§ Jingzhi Yin,† Yiding Wang,† Jun Zhao,‡,∥ and William W. Yu*,†,‡,∥ †

State Key Laboratory on Integrated Optoelectronics and College of Electronic Science and Engineering, Jilin University, Changchun 130012, China ‡ College of Material Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China § State Key Laboratory of Superhard Materials and College of Physics, Jilin University, Changchun 130012, China ∥ Department of Chemistry and Physics, Louisiana State University, Shreveport, Louisiana 71115, United States ABSTRACT: Multigas sensing is highly demanded in the fields of environmental monitoring, industrial production, and coal mine security. Three near-infrared emission wavelengths from PbSe quantum dots (QDs) were used to analyze the concentration of three gases simultaneously through direct absorption spectroscopy, including acetylene (C2H2), methane (CH4), and ammonia (NH3). The corresponding lower detection limits for the three gases were 20, 100, and 20 ppm, respectively, with an accuracy of 2%. This study demonstrates that QDs with tunable emissions have great potential for simultaneous and uninterfered multiplex gas analysis and detection due to the advantages of the easy tunability of multiplex emitting wavelengths from QDs.

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due to their many advantages like dielectric and sparks free, remote sensing, no electromagnetic interference, no chemical contamination and reaction, geometric versatility to form arbitrary shapes, ease of sampling, noninvasive, and fast response.48,49 It is worth noting that multiple gases often coexist in complicated environments. For example, the gas in coal mines may be composed of methane (CH4), ammonia (NH3), carbon monoxide (CO), sulfur dioxide (SO2), hydrogen sulfide (H2S), etc. Therefore, multiplex gas detection is quite necessary, and usually several wavelengths are required for the simultaneous multiplex gas detection using spectroscopy methods. In this work, we employed size tunable emission spectra of QDs to monitor the concentrations of multiplex gases. The analysis and detection of three gases (NH3, CH4, and C2H2) at the same time were based on the tunable NIR emission of PbSe QDs and direct absorption spectroscopy technique of gas sensing. The good selectivity of multiplex gas simultaneous concentration measurement indicated that this kind of NIR emitting nanomaterials has great potential in plenty of fields on account of its low-cost, small size, high efficiency, and multiplex detection.

ecently, semiconductor quantum dots (QDs) have been investigated and shown very unique properties, such as the quantum confined optical property.1−8 They usually possess high photoluminescence (PL) quantum yield (QY) with sizedependent tunable wavelength emissions, which makes them promising for light conversion.9−18 Among them, the PbSe bulk material has a small band gap of 0.28 eV at room temperature and a very large exciton Bohr radius of 46 nm.19 As a result, PbSe QDs show very strong quantum confinement and high quantum yield in the near-infrared (NIR) region.20 Their band edge photoluminescence peaks span over a wide infrared wavelength region of 1−4 μm.21−24 The wavelength can be adjusted merely by changing the particle size to cover the particular absorption frequencies of many kinds of gases in the NIR region. As a matter of fact, the NIR emitting QDs with narrow bandgaps have already been used in electroluminescent devices,25,26 photodetection,12 photovoltaics,27,28 and biomedical imaging.29,30 In recent years, several methods based on chemical surface modification of QDs have been proposed for gas sensing.31−33 They detected some toxic gases according to the chemical interactions between a given chemical species and the surface of the nanoparticles. As a popular subject, the detection and quantification of flammable and poisonous gases in ambient air are considered extremely important in areas such as environmental monitoring, coal mine safety, industrial production, and automobile exhaust checking.34−37 A number of methods have been used to detect gas species including spectroscopy, electrochemistry, and photoacoustics.38−47 Optical techniques are adopted in online gas analysis and air quality monitoring © 2014 American Chemical Society

Received: August 15, 2014 Accepted: October 27, 2014 Published: November 4, 2014 11312

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Figure 1. Normalized absorption spectra and TEM images of PbSe QDs with diameter of (a) 4.6 nm, (b) 5.1 nm, (c) 6.1 nm, and the normalized PL spectra of (d) 4.6 nm, (e) 5.1 nm, and (f) 6.1 nm PbSe in chloroform corresponding to the absorption line intensities (blue) of C2H2, CH4, and NH3, respectively.

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precipitation once with acetone.16−18,20 The final products were dispersed in chloroform for LED fabrication and stored in an argon filled glovebox. Fabrication of the NIR LEDs with Three Peak Wavelengths. PbSe QDs were mixed with a UV glue (NOA60 from LIENHE Fiber Optics) in chloroform to form a homogeneous mixture by vortex and ultrasonic treatment. Then, the mixture was transferred into a vacuum chamber to remove chloroform and bubbles. One size PbSe QDs/UV glue composite was applied on a blue GaN chip to form a layer. Another sized PbSe QDs-UV glue composite was applied to form a second layer. Finally, three thin layers with three sizes were formed. Each layer was polished. Typically, 6.1 nm PbSe QDs were first deposited on the GaN chip followed by 5.1 and 4.6 nm PbSe QDs to fabricate the NIR QDs light source with strong multiple emitting wavelengths. The concentration of PbSe QDs with different particle sizes in UV glue was kept as 5.0 × 10−3 mmol·L−1. The QDs’ luminous intensity could be controlled by adjusting the PbSe QDs-UV glue composites

EXPERIMENTAL SECTION Synthesis of PbSe QDs. The PbSe QDs employed in the present study were synthesized according to the method previously reported by Yu et al.20 Briefly, a mixture of 0.892 g of PbO, 2.26 g of oleic acid (OA), and 12.848 g of 1-octadecene (ODE) was loaded into a 100 mL three-neck flask. After 10 min of nitrogen flow to remove the air, the three-neck flask was heated to 170 °C. After PbO powder completely disappeared and the solution became colorless, 6.9 mL of Se− trioctylphosphine solution (containing 0.637 g of Se powder) was quickly injected into the vigorously stirred solution. The temperature of the reaction mixture was then maintained at 143 °C for QD growth. At a certain reaction time, 30 mL of toluene was injected into the three-neck flask and then the flask was submerged in a room-temperature water bath to completely quench the reaction. It should be pointed out that a series of purification operation procedures were carried out to remove excess reaction precursors and ODE before utilization. QDs were purified by phase extraction twice with methanol and 11313

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Figure 2. (a) Fabrication of NIR QDs light source and (b) variation of NIR light source spectrum fabricated by different ways: (1) 5.1 nm PbSe QDs were deposited; (2) 6.1 nm PbSe QDs were deposited; (3) 5.1 nm PbSe QDs were deposited first followed by 6.1 nm; (4) 6.1 nm PbSe QDs were deposited first followed by 5.1 nm; (5) 5.1 and 6.1 nm PbSe QDs were mixed and deposited together. (c) Evolution of the QDs light source spectra at different working bias from 2.5 to 3.5 V.

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RESULTS AND DISCUSSION CH4 is unavoidable to be released during the exploitation of coal and it outbursts in both underground and surface mines. When it reaches a concentration of 5−15% in the air in a closed environment, CH4 becomes dangerously explosive. Thus, it is extremely important to detect the concentration of CH4 in coal mines.50 Similarly, C2H2 and NH3 are released during industrial production and they are extremely explosive gases when their concentrations reach 2.5%−80% and 16−25%, respectively. Therefore, we chose PbSe QDs with the particle sizes of 4.6, 5.1, and 6.1 nm, corresponding to their respective first absorption peaks of 1437, 1592, and 1862 nm, as shown in Figures 1a−c. Figure 1d shows the PL spectrum of 4.6 nm PbSe QDs and the absorption spectrum of C2H2. The PL peak of PbSe QDs was 1515 nm with a full width at half-maximum (fwhm) of 150 nm, which covered the entire absorption spectrum of C2H2 gas (from 1500 to 1550 nm). The PL spectrum of 5.1 nm PbSe QDs had a PL peak of 1665 nm and a fwhm of 143 nm which corresponded to the CH4 absorption spectrum as shown in Figure 1e. Figure 1f indicates the PL peak of 6.1 nm PbSe QDs located on 1943 nm with a fwhm of 185 nm; the main absorption spectrum of NH3 from 1900 to 2060 nm was completely covered by this emission light. The largest absorption coefficients of CH4, C2H2, and NH3 are 1.33 × 10−21, 1.34 × 10−20, and 1.22 × 10−20 cm−1, respectively. Therefore, the detectable lower limit of CH4 could be 1 order of magnitude higher than the other two at the same detection conditions. The NIR light source based on PbSe QDs with three particle sizes was fabricated, as shown in Figure 2a. The blue GaN LED was employed as the excitation light source, and PbSe QDs were used as photoluminescent materials. According to Figure

thickness in this design. The thicknesses were determined to be 48.0, 165.5, and 671.5 μm for 6.1, 5.1, and 4.6 nm PbSe QDs, respectively. Fabrication of the LEDs for Mutual-Absorption Analysis. 5.1 and 6.1 nm QDs-UV glue composites were deposited on two LED chips respectively, which were named LED 1 and LED 2. LED 3 was obtained when 5.1 and 6.1 nm QDs-UV glue composites were deposited on one LED chip one after another. A reversed order deposition of 6.1 and 5.1 nm QDs-UV glue composites on one LED chip was also obtained as LED 4. The thicknesses of 5.1 and 6.1 nm QDs-UV glue composites were adjusted to be 165.5 and 48 μm and their concentration in UV glue was 5 × 10−3 mmol·L−1. After that, 5.1 and 6.1 nm QDs-UV glue composites were mixed at a 165.5:48 volume ratio, and then the mixture was deposited on a chip to form a film of 213.5 μm as LED 5. Finally, ultraviolet light curing was executed. Characterizations. Absorption spectra were recorded using a Shimadzu UV-3600 spectrophotometer. The photoluminescence (PL) properties of PbSe QDs in tetrachloroethylene solvent and the spectra properties of NIR light source were measured on an Omni-λ300 Monochromotor/Spectrograph. All the absorption and PL spectra were recorded at room temperature. Transmission electron microscopy (TEM, JEOL FasTEM-2010) was used for observing the particle size and shape. The TEM specimens were prepared in a glovebox, where purified PbSe QDs were dispersed in chloroform and dropped on carbon-coated copper grids, and then the solvent was evaporated off. 11314

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Figure 3. Schematic illustration of the experimental setup.

Figure 4. PL spectrum of the NIR light source after absorbed by a certain amount of single gas [(a) C2H2, (b) CH4, (c) NH3], and the variation tendency of optical power (area integral) of the other two PL peaks after absorbed by (d) C2H2, (e) CH4, and (f) NH3.

intensity of the emission spectrum of 5.1 nm QDs in LED 3 was stronger because much more blue light was absorbed by 5.1 nm QDs. Due to the less blue light irradiating and the reflection of 5.1 nm QDs, the intensity of emission spectrum of 6.1 nm QDs decreased seriously despite absorbing the light emitted by 5.1 nm QDs. The emission spectrum intensities of 5.1 and 6.1 nm QDs in LED 5 decreased simultaneously compared with LED 4 due to the strong mutual absorption of the well-mixed two sized QDs. Although the emission intensities of both 5.1

1, the emission from QDs with small particle size will be absorbed by QDs with large particle size. If all the PbSe QDs with three particle sizes were mixed together, the device performance would be limited because the reabsorption among different size QD nanoparticles, which would affect the output intensity of the device. Figure 2b shows the NIR light source output intensity when using two sized QD nanoparticles with different layer formation to investigate the reabsorption phenomenon as an example. Compared with LED 4, the 11315

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Figure 5. Area integrals of PL spectra of (a) C2H2, (b) CH4, and (c) NH3 at 15 standard concentrations and their working curves fitted by Matlab. The insets show their accuracy curves.

y = 1763 × e−x/318 + 7667

and 6.1 nm QDs were lower than their individual single layers (LEDs 1 and 2) due to the reflection of the second layers, LED 4 demonstrated the well balanced emission. Therefore, this no-mutual-absorption layered structure is proven to be an effective design to optimize the output intensity and it weakens the reabsorption between the QDs. The luminescence spectra of the as-fabricated NIR QDs light source under different forward bias are shown in Figure 2c, in which the emission peaks of PbSe QDs were located at 1526, 1676, and 1949 nm, respectively. When the forward bias increased from 2.8 to 3.2 V, the intensity of three emission peaks increased simultaneously with a good stability. Figure 3 shows the experimental setup consisting of the NIR QDs light source, the gas cell, and the optical spectrometer. Impulse voltage was employed to drive the light source. After converged by the convex lens, an NIR beam transmitted through the gas cell (the cell length L = 30 m) and was received by the optical spectrometer. The experimental data were collected and handled by a lock-in amplifier and a computer. Nitrogen and target gases were loaded into the gas cell in the same time but with different flow rates to obtain the different gas concentrations. According to Beer−Lambert law, when a light beam passes through the gas, the gas molecules absorb light energy:

I = I0e−KCL

(2)

where y is the area integral of the PL spectrum and x is the gas concentration. We detected different concentration of C2H2 for one more time and used the calibrated formula to calculate the measured concentration to check the sensitivity and accuracy of the system. Comparing with the standard concentration, the accuracy curve is shown in Figure 5b. The sensitivity was 2 × 10−5 (20 ppm) and the accuracy was better than 2%. The same calibrating experiment and accuracy analysis were performed to obtain the calibrated formulas of CH4 and NH3. Figure 5c,e shows the integral output signals of PL spectra for CH4 and NH3. Their calibrated formulas were y = 5670 × e−x/4683 + 20563

(3)

y = 6474 × e−x/370 + 17450

(4)

The detection sensitivities of CH4 and NH3 were 1.0 × 10−4 (100 ppm) and 2.0 × 10−5 (20 ppm), respectively. Because of a smaller gas absorption coefficient, the sensitivity of CH4 was lower than those of C2H2 and NH3. The same accuracy of 2% was obtained according to Figure 5d,e. When C2H2 with different concentrations was loaded into the gas cell, the area integral ranging from 1610 to 1840 nm and 1890 to 2070 nm corresponding to CH4 and NH3 absorption were analyzed according to Figure 4d. It is obvious that the corresponding PL intensity was stable, which meant the absorption of C2H2 almost did not interfere with the PL peaks relevant to CH4 and NH3. The same results were obtained when CH4 or NH3 was loaded into the gas cell (Figures 4e-f). The conclusion was thus obtained that there was little interference to measure one gas while the other two were present. To further analyze the selectivity of this system, the mixture of C2H2, CH4, and NH3 was loaded into the gas cell with different ratios (Table 1). Figure 6 shows the corresponding PL spectra with the mixtures. The variation trend of the PL spectra was the same as the PL spectra with single gas. It means the interference among the three gases is little. On the basis of the above three fitting formulas, we adopted the method of cross calibration to figure out the measured concentration. Figure

(1)

where I0 and I denote the input and output light intensities, respectively; K is the absorption coefficient of that gas; C is the gas concentration; and L is the cell length. It is worth noting that there was no filter needed in this system, which is simpler than normal structures. Figure 4a shows the evolution of NIR QDs light source spectra with the increasing concentration of C2H2 in N2. Compared with the absorption line of C2H2 in Figure 1d, it is obvious that the intensity ranging from 1500 to 1560 nm decreased because of the absorption of C2H2 and the intensity at 1525 nm produces a maximal decline. The variations of NIR QDs light source spectra with the increasing concentration of CH4 in N2 are shown in Figure 4b. With the increase of concentration, the intensity of the PL peak decreases and this phenomenon conforms to Beer−Lambert law. The same evolution was observed for NH3 as shown in Figure 4c. The PL intensity at wavelength of 1900 to 2060 nm dramatically decreases with the increase of NH3 concentration. By using the designed system, the concentrations of a series of prepared C2H2 samples within 0−800 ppm were measured (20 °C, 101.325 kPa). Figure 5a shows the output signals at 15 standard concentrations of C2H2. The curve fitting was made using Matlab, and the fitting formula was obtained as

Table 1. Concentrations of Mixed Gas Samples for the Selectivity Analysis (Figure 6a)

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gas sample

C1

C2

C3

C4

C5

C2H2 (ppm) CH4 (ppm) NH3 (ppm)

100 1000 100

250 3000 250

400 5000 400

550 7000 550

700 9000 700

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Figure 6. (a) Evolution of NIR light source spectra with the change of concentration of mixed gases (detailed compositions are listed in Table 1). The comparisons between standard and measured concentrations of (b) C2H2, (c) CH4, and (d) NH3.

61225018), the National Postdoctoral Foundation (2011049015), the Jilin Province Key Fund (20140204079GX), the Shandong Natural Science Foundation (ZR2012FZ007), the Hong Kong Scholar Program (XJ2012022), the State Key Laboratory on Integrated Optoelectronics (IOSKL2012ZZ12), NSF (CHE-1338346), and 3M Faculty Award.

6b−d shows the comparison between standard and measured concentrations for C2H2, CH4, and NH3 in mixtures, and their accuracies were all better than 2%. This as-fabricated NIR QDs light source can detect C2H2, CH4, and NH3 in the same time and with the same high accuracy.

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CONCLUSIONS In summary, we reported a design of NIR emitting QDs-based multiplex gas sensing with high selectivity and accuracy. Such emitting nanomaterials and light sources emit multiple wavelengths, making them ideal in the field of environmental monitoring and coal mine security. If each light with a different wavelength was modulated by different frequencies, the optical spectrometer could be replaced by a tiny optical detector. Therefore, the detecting system would be further simplified. Compared to other infrared light sources for gas detection including infrared thermal emitter and semiconductor lasers, this NIR LED has relative high modulate rate without the large thermal inertia, narrow emitting band without obvious interference, low cost, and small volume. We postulate this type of QD-based gas sensing method will be of interest to researchers working in the fields of gas analysis and detection.

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AUTHOR INFORMATION

Corresponding Authors

*Y. Zhang. E-mail: [email protected]. *W. W. Yu. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National 863 Program (2011AA050509), the National Natural Science Foundation of China (61106039, 51272084, 61306078, 11317

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