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Cite This: J. Phys. Chem. Lett. 2019, 10, 4303−4309

Midwavelength Infrared Photoluminescence and Lasing of Tellurium Elemental Solid and Microcrystals Dongsun Choi and Kwang Seob Jeong* Department of Chemistry, Korea University, Seoul 02841, Republic of Korea

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S Supporting Information *

ABSTRACT: Tellurium has been of great interest in physics, chemistry, material science, and more recently in nanoscience. However, information on the photoluminescence of Te crystals, crucial in understanding the material, has never been disclosed. Here, we present photoluminescence and lasing for the Te bulk crystal and microcrystals. Photoluminescence of Te bulk solid crystal was observed at 3.75 μm in the midwavelength infrared (MWIR) region, matching the theoretically predicted value well. With increasing the photoexcitation intensity or decreasing temperature, we successfully observed MWIR random lasing of the bulk Te crystals at 3.62 μm. Furthermore, the rod-shaped Te microcrystals efficiently exhibit second harmonic and third harmonic lasing at MWIR and short-wavelength infrared regions, respectively. Nonlinear coherent MWIR lasing from the Te microcrystals will serve as an excellent mid-IR light source, opening up new applications in infrared photonics, extremely longdepth penetration bioimaging, and optoelectronics.

T

medium for second harmonic generation, such as frequency doubling of a 10.6 μm CO2 laser to 5.3 μm.29 Considering the many efforts toward finding a midwavelength IR (MWIR) luminophore as a reference in the MWIR region, similar to the role of rhodamine in the visible region, it is imperative to measure the MWIR bandgap PL of the Te elemental crystal. In addition, such information can be used for gaining further understanding of optical properties of species such as artificially synthesized Te crystals: nanoparticles,23,30−33 nanorods,21,34,35 nanowires,24,26,32,34−37 nanotubes,22,25,35−38 nanobelts,22,39 two-dimensional tellurene,40−42 three-dimensional structures,11,43,44 and chiral nanostructures.12,13,22,27,45 Here, we present the experimentally acquired mid-infrared photoluminescence and lasing of Te elemental crystal; obtaining these results has been a long-standing challenge for several decades. Surprisingly, the strong PL emission intensity of the bulk Te crystal efficiently achieves random lasing by increasing the photoexcitation intensity or lowering the temperature. Furthermore, in the case of rod-shaped microcrystals, not only the fundamental but also unprecedented second harmonic generation (SHG) and third harmonic generation (THG) operating in whispering gallery mode were observed in the MWIR and short-wavelength infrared (SWIR) regions, respectively.

ellurium, a group 16 element, is a semiconductor in nature with a narrow bandgap.1 It is the fourth most abundant trace element in the human2 body and frequently found from xenolith.3 The Te element, discovered by FranzJoseph Mueller von Reichenstein in 1782, has been rigorously studied with and without other elements. Its crystal form has been intensively studied through theoretical approaches4−10 and absorption-based applications as well.11−17 Reitz et al. and Shinno et al. reported the theoretical bandgap value of the Te crystal in 1957 and 1973, respectively.4,5 Tutihasi et al. showed correlation between the absorption coefficient of the Te crystal and the computationally obtained value. These findings have demonstrated that a small energy bandgap can be observed in ambient conditions, which allows for Te to be applicable in thermoelectric materials and superconductors at room temperature.4−10,18,19 Further Te-based research has been expanded to the nanoscale dimension in recent years.20−27 Unfortunately, the bandgap photoluminescence (PL) of the Te crystal has never been experimentally determined at room temperature, although other physical properties have been thoroughly investigated both theoretically and experimentally, implying that potential optical properties of Te still remain hidden.19,20 To date, without the PL measurement of bulk Te, the laser emission of single-crystal Te under high pressure such as 2.0 kbars was observed once by Dresselhaus and co-workers in 1979.28 Unfortunately, further study of the lasing, such as threshold temperature, threshold photoexcitation fluence, or new emissive properties arising from different morphologies of the material, has not been reported. Instead of being used as an emitter, the Te crystal has been used as a nonlinear optical © XXXX American Chemical Society

Received: May 28, 2019 Accepted: July 15, 2019 Published: July 15, 2019 4303

DOI: 10.1021/acs.jpclett.9b01523 J. Phys. Chem. Lett. 2019, 10, 4303−4309

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Figure 1. (A) SEM image of Te bulk crystals (inset), XRD spectrum of bulk Te crystal sample (top), reference of hexagonal Te (middle), and tellurium oxide (bottom). (B) Mid-IR emission spectra of Te bulk crystal and annealed Te crystals at room temperature. (C) Calculated Te bulk band structure. (Reprinted by permission from ref 6. Copyright 1969 Taylor & Francis Ltd.) (D) Emission spectra of the annealed Te sample at 78 K and room temperature. (E) MWIR emission spectra for the Te bulk crystal at various temperatures and MWIR emission intensities at maximum with various temperatures (inset). (F) MWIR emission spectra for the Te bulk crystal obtained using various pump fluence values (mJ/cm2) at 78 K; maximum emissive intensity obtained using various pump fluence (inset).

Photoluminescence of Te Elemental Solid in MWIR. Figure 1A shows the X-ray diffraction (XRD) spectrum (top) and the scanning electron microscopy (SEM) image (inset) obtained for the Te bulk crystals. The other two spectra are for the reference hexagonal Te and tellurium oxide (TeO2).11

The hexagonal structure of the bulk Te crystal showed peaks at 23.0°, 27.5°, 38.3°, 40.5°, 49.6°, and 56.9°, matching those obtained for the reference Te facets of (100), (101), (102), (110), (201), and (202), respectively. As shown in the XRD spectrum, TeO2 was also found in the commercially available 4304

DOI: 10.1021/acs.jpclett.9b01523 J. Phys. Chem. Lett. 2019, 10, 4303−4309

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temperature reduces the band tail density, based on the 1 Fermi−Dirac distribution, f (E) = , and the dis(E − EF)/ kT

Te sample because of natural oxidation of the surface under ambient conditions. To directly demonstrate the narrow bandgap of the Te crystal, we built a nanosecond laser-based infrared emission spectrometer. Briefly, linearly polarized 532 nm radiation from a frequency-doubled Nd:YAG pulsed laser operating at 1064 nm was used for photoexciting a sample. The pulsed laser with 5 ns duration was incident at an angle of 67° with respect to the sample surface. The emissive signal from the sample was detected by the MCT detector and integrated in the gate width of 10 ns. The gate integrated signal was converted to the spectrum by Fourier transform. The mid-IR bandgap PL of the Te bulk crystal was successfully measured (Figure 1B, left). The peak for the PL spectrum was centered at 2670 cm−1 (= 3.75 μm = 0.33 eV) with a full width at half-maximum (fwhm) value of 640 cm−1 consistent with the theoretical value.4−11,13 Figure 1C illustrates the calculated band structure of the Te crystal reprinted from ref 7. This band structure indicates a small and direct bandgap energy of 0.33 eV at the Hpoint,4−10,12,18,19 originating from the p-orbitals of Te.7,8,18,19 Considering the phonon energy of the Te crystal smaller than 148 cm−1(= 0.018 eV), the bandgap PL of 2670 cm−1 is significantly larger than the phonon energies by a factor of 18, virtually allowing the mid-IR PL measurement at room temperature.46 The sharp dip shown at 2350 cm−1 is attributed to the PL quenching by the asymmetric vibrational modes of the CO2 gas molecules under ambient conditions through the fast nonradiative electronic-to-vibrational energy transfer (EVET) process (Figure 1 B).47 Interestingly, it appears that annealing the Te bulk crystal makes the fwhm of the PL spectrum narrower and leads to generation of another peak at higher frequency. However, considering the blackbody radiation spectrum of the globar without the sample shows the same feature at the corresponding frequency (Figure S1), it is reasonable to regard the two features as simply the blueshifted emission spectrum partially quenched by the EVET process resulting from the vibrational modes of the coating materials on optical components or water molecules on the optical path. The blue-shift is attributed to the Burstein−Moss effect, the improved crystallinity, and the reduction of the tellurium oxide with a smaller refractive index (n = 2.27). Midwavelength IR Lasing of Te Elemental Solid. In order to suppress energy dissipative loss by the phonon scattering observed at room temperature, we performed the mid-IR emission experiments at a low temperature. Figure 1D and the inset represent the normalized and raw emission spectra of the annealed Te sample at 78 K(purple) and 298 K(red), respectively. The emission spectrum at 298 K corresponds to the PL of the bulk Te crystal, and the dip shown at 3300 cm−1 can be attributed to the EVET, as shown in Figure 1B. With a decrease of temperature, surprisingly, we observed lasing with narrow bandwidth at 78 K. It was very striking to observe the narrow emission from the Te crystal without a designed cavity. The narrow emission peak appeared at 2760 cm−1 (= 3.62 μm = 0.34 eV) with a fwhm of 36 cm−1. The temperaturedependent emission study for the bulk Te crystal was carefully carried out from 278 to 78 K (Figure 1E), and we successfully found the lasing threshold temperature of 150 K, as displayed in the inset of Figure 1E. It is worth noting that the Te bulk crystal did not comprise an ordered structure, and thus, such random lasing coherent feedback from the Te bulk crystal was unexpected.48−52 Generation of the random lasing of the Te bulk crystal at the low temperature suggested that lowering the

1+e

sipative energy loss, which may facilitate the population inversion. Anderson localization is a possible model to explain the random lasing of the bulk Te crystal.48−53 Considering the refractive index (n = 4.80) of Te solid and the empty space of the bulk Te sample (n = 1) at vacuum (∼4.0 × 10−3 Torr), excellent scattering center behavior could be observed.54 It is worth noting that neither PL nor lasing was observed in the absence of the Te samples. As for the emission intensities, the lasing spectrum intensity was more than 2 orders of magnitude larger than the PL intensity. The length of the cavity created by the scattering sites (Lc) is generally estimated by calculating the micrometer wavelength of the interferogram in the case of the visible random lasing.55 Because there is no overlap in wavelength, the method reported could provide the random cavity size in the case of visible random lasing, such as the ZnO nanoneedles with a cavity size from 14 to 2 μm. However, in our case, the interferogram data showed only a wavelength of ∼3.7 μm (Figure S2), which is exactly the emission frequency we obtained. Also, the spectral range in our system is over the cutoff of the CaF2 window of 11 μm. Accordingly, the Lc can be either longer than 11 μm or identical to the emission wavelength of 3.7 μm, which is still in the reasonable range coincident with the previously reported value for different random lasing systems. In addition, we increased the photoexcitation pump fluence to determine the power dependence of the lasing at 78 K (Figure 1 E). Increasing the pump fluence generated a very narrow bandwidth emission spectrum, and the threshold pulse fluence was ∼2.7 mJ/cm2, which is a quite small threshold energy as compared to that of other systems such as TiO2 nanoparticles (∼1.02 × 102 mJ/cm2).51 Because of the randomly formed cavity, there was no preferred polarization direction from the bulk Te crystal sample (Figure S3). The structural and compositional properties of the Te bulk crystal were analyzed by SEM, energy-dispersive X-ray spectroscopy (EDS), as well as XRD, as shown in Figure 2. The SEM images provide the shape and size of the Te bulk crystals. As with other bulk crystals, they were anisotropic with irregular interparticle distance. Furthermore, the diameter of the Te crystal widely varied from ∼1 μm up to ∼25 μm, implying that the scattering distance was in the same range. The EDS mapping analysis and the XPS spectra helped determine the composition of the Te bulk crystal (Figure 2A). When comparing the EDS result for the Te bulk crystal to that obtained for the tellurium oxide, the tellurium elemental portion was found to be still significantly higher for the bulk Te crystal (Figure 2B). The Te 3d5/2 and 3d3/2 peaks were identified in the XPS spectra, supporting the presence of elemental Te.56 However, oxygen was identified on the bulk crystal sample in both EDS and XPS spectra, suggesting that, at least, the surface of the Te bulk crystal was readily oxidized to tellurium oxide. Tellurium Microcrystals Epitaxially Grown. The structural changes that Te underwent during annealing were thoroughly analyzed. Surprisingly, annealing at 773 K causes the formation of either solid rod-shaped Te microcrystals or hollow needleshaped microcrystals depending on the annealing conditions. The SEM images of the solid rod-shaped Te crystal in Figure 3A,C−E reveal a cross section of ∼9.5 ± 0.8 μm2 with various length scales up to ∼40 μm. The annealing lasted for 60−90 4305

DOI: 10.1021/acs.jpclett.9b01523 J. Phys. Chem. Lett. 2019, 10, 4303−4309

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The inner diameter of the hollow needle-shaped nanocrystal was 10.7 ± 1.5 μm, and the outer diameter was 21.8 ± 0.9 μm (Figure 3B,F−H). The microcrystals were randomly oriented, and the separation among them was irregular. Interestingly, the SEM images of certain physically straight microcrystals in Figure 3B,F−H show a wave feature. The wavelength of the wave feature in the SEM image in Figure 3H is 10.2 ± 0.6 μm. The origin of this interesting feature is yet to be clearly understood, but it might be attributed to mechanical vibration caused by the resonance between the inner vacant cavity with a diameter of 10.7 ± 1.5 μm and the emission from the Te microcrystal. It was frequently observed that, during SEM measurement, the hollow needle-shaped microcrystals grown on the substrate were broken after the mechanical vibration during the measurement under an electron beam energy of 15 keV. This phenomenon suggests that the strain created by the mechanical vibration was sufficiently large to break the stem of the hollow needle-shaped microcrystal grown on the substrate. Amplified Spontaneous Emission and Lasing of Te Microcrystals. Panels A and B of Figure 3 show the EDS analysis results for the solid rod-shaped Te microcrystals and hollow needle-shaped microcrystals, respectively. This result and the fact that oxygen was found only in the silicon dioxide substrate at the bottom rather than the epitaxially grown microcrystals in Figure 3A imply that the Te microcrystal was mostly composed of Te and the portion of oxygen was negligible. The XRD spectrum (Figure S4) illustrates the high crystallinity and the hexagonal structure of the Te microcrystals. The hollow needle-shaped microcrystal explicitly showed a transition from amplified spontaneous emission to the amplified stimulated emission, the lasing. Figure 4A represents the raw emission spectra as a function of the temperature. The temperature range covered in Figure 4B was 118−298 K, and it showed both PL and lasing, simultaneously. At the threshold temperature of 114 K (Figure 4C), the emission intensity dramatically increased and reached 4.5 at 78 K, which is 196 times larger than the original PL intensity. The quality factor ( νc , Q-factor) of the lasing spectrum was 40.7 for the rodfwhm shaped microcrystal. The lasing feature appeared at 2760 cm−1. It is worth noting that the lasing found in the microcrystal operates in the whispering gallery mode of the microcrystal with the hexagonal micrometer-sized cross section (Figure 3E), and the fact that the lasing was still observed when only the microcrystals exist without the bulk Te with random morphology at the bottom infers that the lasing of the microcrystal does not result from the random lasing. The lasing was also generated with an increase in the photoexcitation pump fluence. We performed a power-dependent emission study at 78 K by carefully elevating the photoexcitation pump fluence (mJ/cm2). An increase in the photoexcitation pump fluence indeed generated lasing at 78 K for the hollow needleshaped Te microcrystal. The threshold pump fluence of the annealed sample was 2.1 mJ/cm2 (Figure 4D). Second Harmonic Generation and Third Harmonic Generation of Lasing of Tellurium Microcrystals. The Te crystal is known to be a nonlinear optical medium for second harmonic generation, such as frequency doubling of a CO2 laser at a wavelength of 10.6 μm to 5.3 μm.29 This nonlinear SHG is generated through phase matching in the Te crystal with the spiral nonsymmorphic (D3) point group symmetry. Interestingly, we observed unprecedented second harmonic (2ω) and

Figure 2. (A) SEM image, EDS mapping images, and compositional data of Te bulk crystal. (B) SEM image, EDS mapping images, and compositional data of tellurium oxide (TeO2). (C) XPS survey spectra of the Te bulk crystal and the TeO2. (D) Zoomed-in XPS spectra from 560 to 600 eV.

Figure 3. (A) EDS mapping images and compositional data of rodshaped Te microcrystals. (B) EDS mapping images and compositional data for the hollow needle-shaped Te microcrystals. (C−E) SEM images for the solid truncated hexagonal rod-shaped Te microcrystals. (F−H) SEM images for the hollow needle-shaped Te microcrystals.

min in our experiments, and the overall height of the Te microcrystal was proportional to the duration of annealing. 4306

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Figure 4. (A) MWIR emission intensity of the hollow needle-shaped microcrystal as a function of wavenumber at various temperatures. (B) Zoomed-in MWIR emission spectra exhibiting both PL and lasing emissive features. (C) MWIR emission intensity of the hollow needle-shaped microcrystals as a function of temperature. (D) MWIR emission intensity of the hollow needle-shaped microcrystals as a function of pump fluence. (E) Fundamental (ω = 3.62 μm), second harmonic generation (SHG, 2ω = 1.81 μm), and third harmonic generation (THG 3ω = 1.21 μm) of the rod-shaped Te microcrystal.

the third-harmonic (3ω) lasing at 5519 cm−1(1.81 μm) and 8277 cm−1 (1.21 μm), respectively, in addition to the fundamental at 2760 cm−1 (ω = 3.62 μm) from the rodshaped microcrystals in the absence of the bulk Te at the bottom in Figure 4E. The phase matching condition, υph(2ω)

= υph(ω), was met because the tellurium is a birefringent crystal. The wave orthogonality between the fundamental and the second harmonic waves was yet to be identified in our experiment because of the irregular growth direction and the inhomogeneous size distribution of the microcrystal. However, 4307

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if possible, it will be worth figuring out the orthogonal direction between of the fundamental (ω) and the second harmonic (2ω) lasing of tellurium microcrystals that are sophisticatedly aligned, such as self-assembled microcrystals or photonic crystals, in the near future. In general, for the SHG conversion efficiency analysis of a nonlinear medium, the phase match angle is examined by measuring the intensity of the SH lasing with varying the angle and power of the incident beam. However, in our case, the visible nanosecond laser pulse was not used for the second harmonic generation but for photoexciting the semiconductor material, and therefore the incidence angle of the pump laser pulse is not as meaningful as in the general SHG efficiency measurement. The SHG and THG emission peaks were pronounced for the rod-shaped Te microcrystal, suggesting that the hexagonal rod-shaped microcrystal having an inner length close to the emission wavelength is likely to serve as a cavity for the lasing and that the optical gain is enhanced because of the increase in the oscillator strength in the 1-D microcrystal. This is also supported by the fact that a single-crystal Te ingot with centimeter dimensions shows neither SHG nor THG (Figure S5). The enhancement of the SHG can be understood by the second harmonic power equation as well, P2 ∝

L2P1P1deff (ω0)2 cn12n2λ12

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.S.J.). ORCID

Kwang Seob Jeong: 0000-0003-3246-7599 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT, & Future Planning (NRF2016R1C1B2013416) and the Ministry of Education (NRF2018R1D1A1A02085371).

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REFERENCES

(1) Cohen, B. L. Anomalous behavior of tellurium abundances. Geochim. Cosmochim. Acta 1984, 48, 203−205. (2) Taylor, A. Biochemistry of tellurium. Biol. Trace Elem. Res. 1996, 55, 231−239. (3) Goldfarb, R. J. Tellurium-the bright future of solar energy. U.S. geological survey fact sheet 2015, 2, 2014−3077. (4) Reitz, J. R. Electronic band structure of selenium and tellurium. Phys. Rev. 1957, 105, 1233. (5) Shinno, H.; Yoshizaki, R.; Tanaka, S.; Doi, T.; Kamimura, H. Conduction band structure of tellurium. J. Phys. Soc. Jpn. 1973, 35, 525−533. (6) Betbeder-Matibet, O.; Hulin, M. Semi-empirical model for the valence band structure of tellurium. Phys. Status Solidi B 1969, 36, 573−586. (7) Li, J.; Ciani, A.; Gayles, J.; Papaconstantopoulos, D. A.; Kioussis, N.; Grein, C.; Aqariden, F. Non-orthogonal tight-binding model for tellurium and selenium. Philos. Mag. 2013, 93, 3216. (8) Peng, H.; Kioussis, N.; Snyder, G. J. Elemental tellurium as a chiral p-type thermoelectric material. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 195206. (9) Caldwell, R. S.; Fan, H. Y. Optical properties of tellurium and selenium. Phys. Rev. 1959, 114, 664. (10) Callen, H. B. Electronic structure, infrared absorption, and hall effect in tellurium. J. Chem. Phys. 1954, 22, 518−522. (11) Wang, Y.; Qiu, G.; Wang, R.; Huang, S.; Wang, Q.; Liu, Y.; Du, Y.; Goddard, W. A.; Kim, M. J.; Xu, X.; Ye, P. D.; Wu, W. Field-effect transistors made from solution-grown two-dimensional tellurene. Nat. Electron. 2018, 1, 228−236. (12) Peng, H.; Kioussis, N.; Snyder, G. J. Elemental tellurium as a chiral p -type thermoelectric material. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 195206. (13) Agapito, L. A.; Kioussis, N.; Goddard, W. A.; Ong, N. P. Novel family of chiral-based topological insulators: elemental tellurium under strain. Phys. Rev. Lett. 2013, 110, 176401. (14) Mayers, B.; Xia, Y. Formation of tellurium nanotubes through concentration depletion at the surfaces of seeds. Adv. Mater. 2002, 14, 279−282. (15) Lin, Z. H.; Yang, Z.; Chang, H. T. Preparation of fluorescent tellurium nanowires at room temperature. Cryst. Growth Des. 2008, 8, 351−357. (16) Mohanty, P.; Kang, T.; Kim, B.; Park, J. Synthesis of single crystalline tellurium nanotubes with triangular and hexagonal cross sections. J. Phys. Chem. B 2006, 110, 791−795. (17) Hawley, C. J.; Beatty, B. R.; Chen, G. N.; Spanier, J. E. Shapecontrolled vapor-transport growth of tellurium nanowires. Cryst. Growth Des. 2012, 12, 2789−2793. (18) Glushkov, M. V.; Itskevich, E. S.; Kosichkin, Y. V.; Tolmachev, A. N.; Shirokov, A. M. Two types of carriers in tellurium. In The Physics of Selenium and Tellurium; Gerlach, E., Grosse, P., Eds.; Springer: Berlin, 1979; pp 164−167.

,

where L, P1, deff, ω0, c, n1, n2, and λ1 are the phase match length inside the crystal, fundamental beam power, effective nonlinear coefficient, beam radius, velocity of light in vacuum, refractive index of the fundamental, refractive index of the second harmonic, and wavelength of the fundamental wave, respectively. As the cross section of the tellurium becomes smaller from the bulk to a few hundred nanometers, the effective fundamental beam radius becomes smaller as well, leading to larger second harmonic power. The quadratic relation would be revealed if better control in the synthesis of the tellurium microcrystal is realized. In conclusion, we present the MWIR PL and lasing of the Te bulk crystal and microcrystals. The maximum peak for the PL appeared at 2670 cm−1 (= 3.7 μm = 0.33 eV), and its fwhm was 640 cm−1. Annealing the Te bulk crystal formed rodshaped or hollow needle-shaped microcrystals. Surprisingly, increasing the photoexcitation power or decreasing the temperature generated random lasing of the Te bulk crystal. Furthermore, the SHG and THG were observed in the range of MWIR−SWIR from the rod-shaped Te microcrystal. Considering the endeavors that have been done to achieve the mid-infrared luminescence from materials such as graphene, nanomaterials, photonic crystals, etc., the discovery of the remarkably strong PL and lasing of the Te elemental solid and microcrystals is indeed meaningful and will provide new opportunities to realize new mid-IR photonics, extremely long-depth penetration bioimaging, and optoelectronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01523. Method and materials, Te sample preparation, X-ray diffraction results, midwavelength IR emission spectra, scanning electron microscopy images, X-ray photoelectron spectroscopy spectra (PDF) 4308

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DOI: 10.1021/acs.jpclett.9b01523 J. Phys. Chem. Lett. 2019, 10, 4303−4309