Mid-Infrared Intraband Transition of Metal Excess Colloidal Ag2Se

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Mid-Infrared Intraband Transition of Metal Excess Colloidal AgSe Nanocrystals Mihyeon Park, Dongsun Choi, Yoonchang Choi, Hang-beum Shin, and Kwang Seob Jeong ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00291 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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Mid-Infrared Intraband Transition of Metal Excess Colloidal Ag2Se Nanocrystals Mihyeon Park1†, Dongsun Choi1†, Yoonchang Choi1, Hang-beum Shin2 and Kwang Seob Jeong1*

Department of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul, 02841, Republic of Korea1 Corporate R&D, LG Chemical, Ltd. 10, Magokjungang 10-ro, Gangseo-gu, Seoul, 07796, Korea2 KEYWORDS: Silver selenide nanocrystals, Mid-IR intraband photoluminescence, Infraredspectroelectrochemistry, Thin-film transistor

ABSTRACT: Steady state intraband transition, which is a promising electronic transition of a colloidal quantum dot along with the bandgap transition, had been a long-standing challenge. The steady state intraband transition occurring between discrete electronic states in the conduction band of a colloidal nanocrystal has been reported only from the mercury chalcogenide nanocrystals for the last few years. Concerns about the toxicity of the mercury compound necessitate a new non-toxic system exhibiting a steady-state intraband transition. Here we present the steady state intraband absorption and

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photoluminescence of Ag2Se colloidal nanocrystals under ambient conditions. The mid-IR intraband transition is carefully investigated by means of FT-IR emission spectroscopy, spectroelectrochemistry, compositional analysis and transfer characteristics. Especially, the mid-IR intraband photoluminescence of the Ag2Se colloidal nanocrystal will open new avenues in the use of quantum confined colloidal systems for mid- and longwavelength infrared light source along with the bandgap transition that has been investigated for the last three decades.

Semiconductor colloidal nanocrystals have been of great interest due to their size-tunable bandgap transition afforded by the quantum confinement effect.[1-5] The confinement effect also leads to the size-tunable intraband transition occurring between electronic states in either the conduction band (CB) or the valence band (VB).[6-11] The steady state intraband transitions of HgS and HgSe colloidal quantum dots were reported by Jeong et al. and the mid-IR intraband transition-based optoelectronic applications have been rigorously investigated for the last few years because of their various potential applications such as telecommunications, thermography, IR photodetectors, and biosensors.[10, 12-15] The steady state intraband transition, 1Se-1Pe transition, is allowed when excess electrons occupy the lowest electronic state of the CB, 1Se.[10-12, 16] Due to the immediate oxidation of the electron in the CB created by photoexcitation via oxidative reaction induced by surrounding species such as water and oxygen molecules, it had been challenging to let electrons remain at the 1Se state. To date, the mercury chalcogenide colloidal quantum dot has been the only material that demonstrates this intraband transition under ambient conditions. The sufficiently long duration of


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the electron occupation at 1Se is attributed to the lower energy level of the 1Se than the H2/ H2O reduction potential ( -4.45 eV Vs. vacuum), leading to the air- stability of the nanocrystal. Although mercury chalcogenide nanocrystals exhibit this steady state intraband transition that has led to interesting studies over the last few years,[11, 16-20] there have been concerns in handling the toxic mercury ion compounds during synthesis. Especially, the organic mercury which is the intermediate product is extremely toxic to us that may prevent researchers from investigating such interesting intraband transitions. Therefore, there is a strong desire to replace the mercury elements with non-toxic elements to generate this mid-IR optical feature. Here we present steady

Figure 1. (A) Transmission electron microscope image (scale bar=10 nm) (B) Absorption spectra of Ag2Se nanocrystals with different sizes (C) Schematic of spectroelectrochemistry cell (D) Differential absorption spectra of Ag2Se nanocrystals with different electrochemical potentials. Positive potential (green) corresponds to the reduction of the nanocrystal films.


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state intraband transitions of Ag2Se nanocrystals occurring in the conduction band under ambient conditions. Experimental results show that the mid-IR intraband transition energy relies on the size of the Ag2Se nanocrystal synthesized, suggesting that the quantum confinement is still effective in these nanocrystals. Interestingly, the Ag2Se nanocrystal shows air-stability to some degree, making it beneficial in performing optical measurements and device fabrication as well. One-pot and single-step hot-injection methods were used for the nanocrystal synthesis.21-23 Briefly, an AgNO3 oleylamine solution (3 mL, 1.1 g/L) was heated to 140 °C under an argon atmosphere. The selenium precursor, prepared by dissolving 7.9 mg of Se powder in 0.1 mL of trioctylphosphine (1 M TOP-Se), was quickly injected into the AgNO3 solution, and the solution immediately turned black. The nanocrystal growth proceeded for 1-6 min., and chloroform and ethanol solutions were added to the product solution to halt the growth and to induce precipitation, respectively. The product solution was centrifuged to separate the Ag2Se nanocrystals, and the nanocrystals were redispersed in tetrachloroethylene (TCE). Spherical Ag2Se colloidal nanocrystals were obtained, as shown in Figure 1A and Figure S1, which is consistent with other studies.[21] As-synthesized Ag2Se nanocrystals exhibit a wavelength-tunable mid-IR transition by varying the nanocrystal size, as in Figure 1B and S2. The mid-IR absorption spectra are observed along with the NIR absorption feature. Interestingly, the mid-IR absorption feature is air-stable for a couple of week, as shown in Figure S3. Sahu et al. reported an excellent synthetic method for Ag2Se nanocrystals exhibiting similar wavelength-tunable bandgap spectra longer than 2.5 µm in the mid-IR regime.[22] The reported


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compositional information, the Ag/Se ratio, was close to ~2.0 or less. However, our own samples show an averaged value of ~2.3 as measured by energy dispersive X-ray spectroscopy (EDS) analysis in Figure S4. Interestingly, the excess metal portion of the nanocrystal is consistent with the previously reported HgS or HgSe nanocrystals.[10-12, 16] While the optical data obtained by Sahu et al. were interpreted as a bandgap transition for Ag2Se nanocrystals, our results clearly indicate that the mid-IR absorption peak of our sample is isolated from other transitions in the vicinity. These results imply that the observed mid-IR transition is not likely to be the bandgap transition.[11, 16, 17] Generally, the bandgap transition is well-overlapped with other interband transitions with a small energy gap in the range of a few hundred meV, leading to incompleteness in the full Gaussian line-shape (Figure S5). The absorption peak observed in Fig 1B, however, shows a full Gaussian line-shape with fwhm of 703 cm-1 without any overlapping with other interband transitions. To identify the origin of the mid-IR transition, we carefully performed spectroelectrochemistry (SEC) measurements for the Ag2Se nanocrystals with 2.4 nm and 3.2 nm radius in infrared regime in Figure 1D and Figure S6, respectively. The SEC measurement can directly monitor the spectral variance by reducing/oxidizing the nanocrystal with an electrochemical potential. Specifically, the SEC results can identify the origin of the electronic transition by finding the correlation between the bandgap transition and the intraband transition.[12, 24, 25] Because both the bandgap and the intraband transitions share the 1Se state, reducing/oxidizing the nanocrystal results in an immediate response. Under a negative electrochemical potential corresponding to the reduction of the nanocrystals, the mid-IR intraband transition at 2201 cm-1 gains strength while the bandgap transition at 5485 cm-1 is bleached. Conversely, the oxidation of the nanocrystal results in bleaching of the mid-IR transition and enhancement of the bandgap


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transition strength. Notably, the oscillator strength of the mid-IR transition is comparable to that of the near-IR transition based on the SEC result in Fig 1D. The comparability of the oscillator strength of the mid-IR transition feature was also observed from the HgSe or HgS CQDs exhibiting the intraband transitions. Furthermore, the fwhm of the mid-IR transition (~ 750 cm-1, Table S1, Figure S7) is similar to that of the HgS CQD, and much broader than the narrow fwhm (~300 cm-1 for the HgS CQDs reported by Shen et al.) of the localized surface plasmon resonances, suggesting that the mid-IR transition is indeed the intraband transition of the conduction band (CB) of the Ag2Se nanocrystal, and the quantum confinement effect is still dominant over the carrier density effect, leading to the size dependent mid-IR transitions.[26] To note, the bandgap energy obtained by the SEC is much larger than the bandgap transition estimated by using the parameters for the tetragonal Ag2Se nanocrystal (me* = 0.32 and mh* = 0.54, EBulk BG = 565 cm-1, ENC BG = 3420 cm-1, r = 2.3 nm).[22] However, the measured bandgap energy is coincident with the bandgap energy estimated by the orthorhombic structure (me*=0.1 and mh*= 0.75, EBulk BG = 1450 cm-1, ENC BG = 6506 cm-1, r = 2.3 nm), although the crystal structure determined by the XRD results appears to be metastable cubic (or tetragonal) structure.[27, 28] There is controversy over the values of the effective masses of the electron and hole of the various crystal structures of the Ag2Se. The experimental value we obtained will provide useful information for understanding the physical properties of the effective mass of carriers of the cubic Ag2Se nanocrystals. Since it turns out that the mid-IR absorption spectrum is still affected by the quantum confinement in Fig1B and 2A, the number of electrons in the conduction band is in the range of one or two electrons per nanocrystal. In this case, the k·p approximation can be useful to understand the electronic structure of nanocrystals and it can estimate the intraband transition


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energy by finding the correlation between the intraband transition and the bandgap transition. In order to figure out the intraband transition energy based on the spectroelectrochemistry, we performed the k·p approximation to calculate the non-parabolic energy dispersion, = + by using a Hamiltonian, H = 0

+

of a 2-band model . Ak is the matrix −

element of the perturbing potential. In a sphere of radius R, the values for k of 1Se and 1Pe are and

.

, respectively. The intraband transition energies for 1Se-1Pe corresponds to the difference

between E1P and E1S . The valence band dispersion and Coulombic interactions are not considered e

e

in this model. The bulk bandgap energy used here is 565 cm-1 corresponding to the bulk bandgap energy of the tetragonal structure of Ag2Se. The model fits well the experimental data in some degree. For instance, 5482 cm-1 (0.680 eV) of the bandgap energy gives 2230 cm-1 (0.276 eV) that is not much different from the experimental value, 2201 cm-1(0.272 eV). Further theoretical study, however, will be needed to understand the fine electronic structure of the Ag2Se CQDs with advanced calculation methods.


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Figure 2. (A) Ag2Se CQD intraband absorption and emission spectra (B) Schematic of the Ag2Se CQD/ZnO FET device (C) Photo-response of the transfer characteristic of the Ag2Se CQD/ZnO TFT device under mid-IR irradiation. (D) Zoomed-in from -4 V to 2 V applied gate voltage.

The intraband transition necessarily creates an exciton in the 1Se-1Pe state under photoexcitation. The formation of the exciton, the e-h pair, should result in the radiative recombination , and also affects the charge transfer characteristics of the transistor. To confirm the formation of the exciton of the Ag2Se nanocrystal in the CB, we measured the mid-IR emission spectra and carried out a photocurrent measurement using the Ag2Se nanocrystal/ZnO thin film transistor.


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Figure 2A represents the mid-IR absorption and the photoluminescence spectra of the Ag2Se nanocrystals with different sizes (Figure S8). The mid-IR intraband PL spectrum of the sample (1) to (2) appears at from 2053 cm-1 to 2021 cm-1, and their fwhm are ~ 520 cm-1. The fwhm of (3)-(6) emission spectra are not considered for the calculation since the emission spectra are partially overalpped with the vibrational mode (asymmetric stretching) of water molecule showing at ~1600 cm-1. The vibrational mode (asymmetric stretching) of the CO2 gas molecule also efficiently quenches the mid-IR PL emission at 2349 cm-1 shown as a sharp dip in Figure 2A, and the quenched spectra recover with increasing the dry N2 purging time in Figure S9. The intraband PL spectra were obtained by photoexcitation of the Ag2Se nanocrystal films with a 532 nm Nd:YAG SHG pulsed laser. The photoexcitation power and the beam cross-section are 30.1 mW and 0.64 cm2, respectively. A gated-integrator collects the mid-IR emission signal that is overlapped with the laser pulse of 5 ns duration. The thermal emission component was ruled out by dividing the raw emission data by the thermal emission data. The details of the mid-IR emission spectrometer are described in the experimental method section. Interestingly, the Stokes shift of ~ 404-413 cm-1 is observed from the Ag2Se CQDs in Figure 2A, which is constrast to the negligible stokes shift of HgS CQDs. The meaningful data of the Stokes shift of Ag2Se CQDs will be only the two data from the bottom because the two PL spectra are not overlapped with the water absorption peaks. The Stokes shift, tentatively attributed to the exchange splitting of the electronic states involved in the intraband transition, will be beneficial to increase the intraband PL quantum yield by avoiding the self-absorption of emission. Further bandgap engineering would offer more options to improve the intraband PL intensity as well.


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Accordingly, the presence of the mid-IR PL spectrum is a direct evidence of the intraband exciton, confirming that the mid-IR absorption observed is not the LSPR showing no radiative recombination, but indeed the intraband transition. The Ag2Se CQDs with size-tunable mid-IR intraband transitions can be used for wavelengthselective photodetector if the intraband transition functions as an IR color filter. Figure 2B illustrates the Ag2Se/ZnO TFT device structure. Aluminum, aluminium oxide (Al2O3) and chromium were used as the source/drain electrodes, the dielectric and gate, respectively. For the photocurrent response measurement of the Ag2Se nanocrystals, a thin ZnO layer was used as an active layer instead of the Ag2Se nanocrystal films due to the small on/off ratio of the Ag2Se TFT device, as in the HgS CQD TFT device.[18] Figure 2C and 2D (zoomed-in) show the transfer characteristics of the Ag2Se/ZnO TFT device. The black curve indicates the IR light off state, while the red and blue curves correspond to the results under IR irradiation with and without the Ge filter blocking the IR light wavelengths shorter than ~2 µm (5000 cm-1), respectively. A globar was used as the IR light source. Apparently, the characteristic immediately responds to the IR irradiation, corroborating the generation of the intraband exciton under the photoexcitation. The negative threshold voltage (Vth) shift indicates that positive charges are created in the Ag2Se nanocrystal layer, enhancing the electron accumulation at the ZnO layer. In the presence and absence of the Ge filter, the Vth values are -0.61 V and -0.43 V, respectively. For reference, the ZnO TFT device without the Ag2Se nanocrystal layer does not respond to the mid-IR irradiation, as shown in Figure S10. In conclusion, the steady state intraband transition of the metal excess Ag2Se nanocrystals has been presented. The spectroelectrochemistry experiments identify the intraband absorption feature through finding the strong correlation between the mid-IR transition and the NIR


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bandgap transition. The photoexcitation to the Ag2Se nanocrystal solid induces the formation of the intraband exciton, leading to novel intraband PL spectra in the mid-IR regime. In addition, the intraband exciton gives rise to the negative shift of the Vth in the transfer characteristic of Ag2Se/ZnO TFT, which can be used for wavelength selective IR TFT photodetector by using the neat Gaussian line shape intraband transitions. The non-toxic Ag2Se nanocrystals exhibiting the steady state intraband transition will serve as promising materials for telecommunications, midIR bio-imaging, bio-optoelectronic, photovoltaics, and opto-magnetic applications.


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ASSOCIATED CONTENT Supporting Information Experimental details and characterization data, including experimental method, figures, and references. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions †These authors contributed equally. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work is supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, & Future Planning (NRF2016R1C1B2013416),

Ministry

of

Education

(NRF-20100020209).

Author

gratefully

acknowledge use of the facilities of the Korea Basic Science Institute (KBSI).


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Mid-Infrared Intraband Transition of Metal Excess Colloidal Ag2Se

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