Next Article in Journal
Application of Electro-Technologies in Processing of Flax Fiber
Next Article in Special Issue
Towards Water-Free Tellurite Glass Fiber for 2–5 μm Nonlinear Applications
Previous Article in Journal
Preliminary Investigations into the Development of Textile Based Temperature Sensor for Healthcare Applications

Fibers 2013, 1(2), 11-20; doi:10.3390/fib1020011

Er3+/Ho3+-Codoped Fluorotellurite Glasses for 2.7 µm Fiber Laser Materials
Yaoyao Ma 1,2, Feifei Huang 1,2,, Lili Hu 1, and Junjie Zhang 1,*
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
Graduate School of Chinese Academy of Sciences, Beijing 100039, China
These authors contributed equally to this work.
Author to whom correspondence should be addressed; Tel.: +86-215-991-4297; Fax: +86-215-991-4516.
Received: 10 July 2013; in revised form: 6 August 2013 / Accepted: 12 August 2013 / Published: 16 August 2013


: This work reports the enhanced emission at 2.7 µm in Er3+/Ho3+-codoped fluorotellurite glass upon a conventional 980 nm laser diode. The significantly reduced green upconversion and 1.5 µm emission intensity in Er3+/Ho3+-codoped samples are observed. The results suggest that the Er3+: 4I13/2 state can be efficiently depopulated via energy transfer from Er3+ to Ho3+ and the detailed energy transfer mechanisms are discussed qualitatively. The energy transfer efficiency from Er3+: 4I13/2 to Ho3+: 5I7 is calculated to be as high as 67.33%. The calculated emission cross-section in Er3+/Ho3+-codoped fluorotellurite glass is 1.82 × 10−20 cm2. This suggests that Er3+/Ho3+-codoped fluorotellurite glass is a potential material for 2.7 µm fiber laser.
2.7 µm; Er3+/Ho3+-codoped; fluorotellurite glass; energy transfer mechanism

1. Introduction

Owing to the increased interest in mid-infrared laser fiber (2–5 µm) used in laser surgery and remote chemical sensing fields, considerable researches have been performed to searching for new materials to use as hosts for mid-infrared laser hosts especially for Er3+ 2.7 µm [1,2]. Among many alternatives, fluoride fibers have emerged as natural candidates for mid-infrared laser materials because of their low phonon energy which decreases the rate of phonon-assisted nonradiative transitions [3,4]. However, fluoride fibers suffer from poor thermal stability and require complex fabrication route. Usually, the mid-infrared emission of Er3+ can hardly be observed in oxide glasses owing to the large phonon energy. However, it is well known that oxide glasses are more chemically and thermally stable. Among all the oxide glasses, tellurite glasses emerge as good candidates for mid-infrared fiber laser materials because of their lowest phonon energy (760 cm−1) among all the oxide glasses with large refractive index and a broad transmission window (0.4–6 μm) [5,6,7].

Er3+ is an ideal luminescent center for 2.7 µm mid-infrared emission corresponding to the 4I11/24I13/2 transition. However, Er3+ suffers from self-terminating of the 4I11/2 level because of the much shorter lifetime of the emitting level (4I11/2) as compared to the terminal laser level (4I13/2). Fortunately, codoping with other ions such as Pr3+, Nd3+, Tm3+ and Ho3+ have been proved to be feasible alternatives to enhance the 2.7 µm emission [8,9,10,11]. The strong OH absorption around 3 µm is another important fact to obtain efficient Er3+ 2.7 µm emission. As is reported before [12], the addition of fluoride in the tellurite glasses was proved to be an effective way to reduce OH−1 groups and increase the radiative transition probabilities of 2.7 µm emission. Therefore, we prepare the Er3+/Ho3+-codoped fluorotellurite glass and evaluate the spectroscopic parameters based on the absorption spectra using the Judd-Ofelt theory. The detailed energy transfer processes based on the measured upconversion, near-infrared and mid-infrared fluorescence spectra are also discussed.

2. Experimental Section

The investigated fluorotellurite glasses in this study have the following molar compositions: 85TeO2-10PbF2-5LaF3-1ErF3-xHoF3(x = 0, 2), hereafter named TF glass. The samples were prepared using high-purity of powder. Well-mixed, 25 g batches of the samples were placed in an aluminum crucible and melted at 900 °C for 30 min. Then the melts were cast on a preheated steel plate and annealed for 3 h. at a temperature 10 °C below the Tg before they were naturally cooled to room temperature. The annealed samples were polished with a thickness of 1 mm for the optical property measurements.

The absorption spectra were recorded by a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrophotometer in the wavelength range of 400–1700 nm. The fluorescence spectra were measured with an Edinburg FLSP920 type spectrometer upon excitation at 980 nm. The fluorescence lifetime was measured with the instrument FLSP920 (Edinburgh instruments Ltd., UK). All the measurements were carried out at room temperature.

3. Results and Discussion

3.1. Absorption Spectra and Judd-Ofelt Analyses

Figure 1 shows the absorption spectra of Er3+ singly and Er3+/Ho3+-codoped TF glasses. All the intrinsic absorption transitions of Er3+ and Ho3+ in the region from 300 to 1700 nm are retained and labeled in Figure 1. The strong absorption around 980 nm of the Er3+/Ho3+-codoped sample indicates that this glass can be excited efficiently by a 980 nm laser diode (LD). It can be seen that Er3+: 4F9/2, Ho3+: 5F5 and Er3+: 4S3/2, Ho3+: (5S2 + 5F4) are very close, which show that the energy transfer processes in Er3+/Ho3+-codoped TF glasses are expected to be efficient.

Figure 1. Absorption spectra of Er3+ and Er3+/Ho3+-codoped samples.
Figure 1. Absorption spectra of Er3+ and Er3+/Ho3+-codoped samples.
Fibers 01 00011 g001 1024

The Judd-Ofelt theory [13,14] has been commonly applied to determine the important spectroscopic and laser parameters of rare earth doped glasses. Judd-Ofelt theory has been described in other literature in detail [15]. Basically, the Judd-Ofelt analyses were applied using the experimental oscillator strengths of the absorption bands obtained from absorption spectra. Judd-Ofelt intensity parameters and oscillator strengths provide indirect information of the symmetry and bonding of rare-earth polyhedra within the matrix. Experimental (fmea) and theoretical (fcal) oscillator strength for representative transitions of Er in TF glass are tabulated in Table 1. Then the Judd-Ofelt intensity parameters, Ωλ (λ = 2, 4, 6), can be derived using a least-square fitting approach and are shown in Table 2. Generally, Ωλ is closely related to the structure change of the sites of rare earth ligand and the basicity of the glass network. It is hypersensitive to the change of composition of host materials. As is shown in Table 2, the calculated Ω2 of Er3+ and Ho3+ in the present glass is lower than that other oxide glasses since the addition of fluoride in the tellurite glass can reduce the covalency and ligand of the glass matrix. The Ωλ parameters follow the trend Ω2 > Ω4 > Ω6 in present TF glass. It should be mentioned that the root-mean-square is 2.98 × 10−6 for Er3+/Ho3+-codoped TF glass. The larger value of the fitting is due to the overlap of energy levels of Er3+ and Ho3+ we select to calculate the intensity parameters Ωλ.

Table 1. Measured and calculated oscillator strength of Er3+ in TF glass.
Table 1. Measured and calculated oscillator strength of Er3+ in TF glass.
AbsorptionWavelength (nm)Oscillator strength (10−6)
Table 2. .Judd–Ofelt intensity parameters of Er3+ and Ho3+ in various glasses.
Table 2. .Judd–Ofelt intensity parameters of Er3+ and Ho3+ in various glasses.
Ωt(10−20 cm2)TFFluoridePhosphateGermanateSilicate
Ref.This work[16][17][17][17]
Ref.This work[18][19][20][21]

Table 3 represents the radiative transition probabilities (Ar), branching ratios (β) and radiative lifetime (τr) of certain levels of Er3+ ions which are calculated using the above obtained Judd-Ofelt intensity parameters. It is shown that the radiative probabilities Ar of Er3+: 4I11/24I13/2 transition is 53.26 s−1 for Er3+/Ho3+-codoped samples.

Table 3. The radiative transition probability of electric dipolar transitions (Aed), radiative transition probability of magnetic dipolar transitions (Amd), branching ratio (β) and radiative lifetime (τrad) of Er3+/Ho3+-codoped glasses.
Table 3. The radiative transition probability of electric dipolar transitions (Aed), radiative transition probability of magnetic dipolar transitions (Amd), branching ratio (β) and radiative lifetime (τrad) of Er3+/Ho3+-codoped glasses.
TransitionsAed (s−1)Amd (s−1)Er3+/Ho3+τ (ms)
β (%)

3.2. Fluorescence Spectra Analyses and Energy Transfer Mechanisms

Figure 2 presents the mid-infrared and near-infrared emission spectra of Er3+ and Er3+/Ho3+-codoped TF glasses under excitation at 980 nm. The emission at 2.7 µm corresponds to the transition of the Er3+: 4I11/24I13/2. The emissions at 1.53 and 2.05 µm come from the transition of Er3+: 4I13/24I15/2 and Ho3+: 5I75I8, respectively. 2.7 µm emission can be observed in both kinds of samples. However, the intensity of the 2.7 µm increases with the addition of the HoF3, which demonstrates the effective sensitization of Ho3+ ions.

Figure 2. Near-infrared and Mid-infrared fluorescence spectra of Er3+ and Er3+/Ho3+-codoped fluorotellurite glasses.
Figure 2. Near-infrared and Mid-infrared fluorescence spectra of Er3+ and Er3+/Ho3+-codoped fluorotellurite glasses.
Fibers 01 00011 g002 1024

From the experimental phenomenon and theoretical data, possible mechanisms [22] for the emission bands are discussed based on the simplified energy levels of Er3+ and Ho3+ presented in Figure 3. Ions on the Er3+: 4I15/2 state are excited to the 4I11/2 state by ground state absorption (GSA) when the sample is pumped by 980 nm LD. The involved energy transfer mechanisms processes based on the 4I11/2 level are as follows: excited state absorption (ESA1), Er3+: 4I11/2 + a photon→Er3+: 4F7/2; ETU1, Er3+: 4I11/2 + 4I11/24I15/2 + 4F7/2; ETU3, 4I11/2(Er3+) + 5I6(Ho3+)→4I15/2(Er3+) + 5F4(Ho3+); 4I11/24I13/2 transition with 2.7 µm emission; ET1(a nonresonant process), from the Er3+: 4I11/2 level to the Ho3+: 5I6 level, energy excess (1470 cm1) is given to the matrix; non-radiatively relaxation to the Er3+: 4I13/2 level.

The Er3+: 4I13/2 level is populated owing to the nonradiative relaxation from the upper 4I11/2 level. There exist four main energy transfer processes for the Er3+: 4I13/2 level in present glass as follows: excited state absorption (ESA2), Er3+: 4I13/2 + a photon→Er3+: 4I9/2; ETU2 [23], Er3+: 4I13/2 + 4I13/24I15/2 + 4F9/2; ETU4, 4I13/2 (Er3+) + 5I6(Ho3+)→4I15/2(Er3+) + 5F5(Ho3+) ; 4I13/24I15/2 transition with 1.5 µm emission; ET2(a nonresonant process), from the Er3+: 4I13/2 level to the Ho3+: 5I7 level, energy excess (1398 cm−1) is given to the matrix. After ET2 process, 2.05 µm emission can be observed due to the Ho3+: 5I75I8. The ESA2 process populates the Er3+: 4F9/2 level which relaxes radiatively to the ground state with red emission around 660 nm and non-radiatively to the next lower Er3+: 4I9/2 level. The energy stored in the Er3+: 4I9/2 can partly be non-radiatively decayed to the Er3+: 4I11/2 level, which is beneficial to the 2.7 µm emission.

Figure 3. Energy level schemes of Er3+ and Ho3+ and energy transfer sketch map between Er3+ and Ho3+.
Figure 3. Energy level schemes of Er3+ and Ho3+ and energy transfer sketch map between Er3+ and Ho3+.
Fibers 01 00011 g003 1024

The visible emission spectra for Er3+ doped and Er3+/Ho3+-codoped glasses upon 980 nm excitation are shown in Figure 4. Three visible emission peaks centered at 525, 550 and 660 nm are observed. As discussed above, the stored energy in the 4F7/2 level after ESA1 process decays non-radiatively to the lower levels 2H11/2 and 4S3/2. Then the Er3+: (2H11/2 + 4S3/2)→4I15/2 transitions bring green emissions (525 and 548 nm). Meanwhile, the excited Ho3+ ions at (5S2 + 5F4) and 5F5 levels also generate 550 and 660 nm emissions, respectively. It is noted that the fluorescence intensity of red emissions becomes stronger when codoped with Ho3+ while the green emission becomes weaker. Because the energy gaps of the ET1 and ET2 process are relatively small, so both processes can happen efficiently. It is expected that the ESA1 and ESA2 processes will be reduced while the sample is codoped with Ho3+, so part of green and red emission can be attributed to the Ho3+ upconversion emissions. Since the energy gap of the ET2 process is smaller than that of ET1 process, the energy transfer efficiency of ET2 is expected to be higher than that of ET1, this is also proved from the extremely decreased 1.5 µm emission in the Er3+/Ho3+-codoped samples, consequently ions on the 5I7 level are much more than that on the 5I6 level. Then the 5F5 level that is populated through ETU4 generates stronger 660 nm emission while the 5S2 (5F4) levels that are populated through ETU3 generate weaker 550 nm emission.

3.3. Fluorescence Spectra Analyses and Energy Transfer Mechanisms

The energy transfer efficiency has been estimated from the measured lifetime of the 1.53 µm emission of Er3+ singly and Er3+/Ho3+-codoped samples by the following formula [24]:

Fibers 01 00011 i001
where τEH and τE are the Er3+ lifetime monitored at 1.53 µm with and without Ho3+ ions, respectively. The lifetime decay curves of the Er3+: 4I13/2 level with and without Ho3+ in TF glasses are measured and shown in Figure 5. The values for lifetime of Er3+: 4I13/2 level for Er3+ and Er3+/Ho3+-codoped samples are 5.25 ms and 1.71 ms, respectively. The decrease of lifetime of the Er3+: 4I13/2 state indicates the existence of ET2 process. In addition, the energy transfer efficiency from the Er3+: 4I13/2 to Ho3+: 5I7 level is calculated to be 67.33%.

Figure 4. Upconversion emission spectra of Er3+ and Er3+/Ho3+-codoped glass samples.
Figure 4. Upconversion emission spectra of Er3+ and Er3+/Ho3+-codoped glass samples.
Fibers 01 00011 g004 1024
Figure 5. Fluorescence decay curves of Er3+: 4I13/2 level in Er3+ singly and Er3+/Ho3+-codoped samples.
Figure 5. Fluorescence decay curves of Er3+: 4I13/2 level in Er3+ singly and Er3+/Ho3+-codoped samples.
Fibers 01 00011 g005 1024

3.4. Cross-Sections Analyses

The emission cross section is an important factor for evaluating the emissive ability of luminescent center. The absorption and emission cross sections could be calculated from Füchtbauer–Ladenburg equation [25] and McCumber theory [26]:

Fibers 01 00011 i002
Fibers 01 00011 i003
where λ is the wavelength, Arad is the spontaneous transition probability, I(λ) is the fluorescence spectra intensity, n and c represent the refractive index and the speed of light, ZL and ZU are partition functions of the lower and upper manifolds, respectively. The maximum absorption (σabs) and emission cross section (σem) (both peaking at 2708 nm) in Figure 6 are 1.54 × 10−20 cm2 and 1.82 × 10−20 cm2, respectively, which is larger than the values reported in Ref. [27,28,29]. Hence, Er3+/Ho3+-codoped TF glass with promising properties has potential applications for 2.7 µm laser material.

Figure 6. Absorption and emission cross sections of Er3+: 4I11/24I13/2 in TF glasses.
Figure 6. Absorption and emission cross sections of Er3+: 4I11/24I13/2 in TF glasses.
Fibers 01 00011 g006 1024

4. Conclusions

Enhanced 2.7 µm emission was obtained in Er3+/Ho3+-codoped fluorotellurite glass. This suggests that Ho3+ can be a feasible approach to obtain efficient Er3+ 2.7 µm emission pumped by common 980 nm LD in fluorotellurite glass for practical applications. It was also found that the green upconversion and 1.5 µm emissions extremely decreased in the Er3+/Ho3+-codoped glass. The energy transfer mechanisms between Er3+ and Ho3+ were discussed in detail and the energy transfer efficiency was calculated to be 67.33%. Larger absorption and emission cross sections of Er3+: 4I11/24I13/2 in TF glasses were obtained which were 1.54 × 10−20 cm2 and 1.82 × 10−20 cm2, respectively. These results suggest that Er3+/Ho3+-codoped fluorotellurite glass has potential applications for 2.7 µm fiber laser materials.


This research was financially supported by the Chinese National Natural Science Foundation (No. 51172252).

Conflicts of Interest

The authors declare no conflicts of interest.


  1. Zhu, X.; Jain, R. 10W level diode-pumped compact 2.78 µm ZBLAN fiber laser. Opt. Lett. 2007, 32, 26–28. [Google Scholar] [CrossRef]
  2. Sanghera, J.S.; Shaw, L.B.; Aggarwal, I.D. Chalcogenide Glass-Fiber-Based Mid-IR Sources and Applications. IEEE J. Sel. Top. Quant. 2009, 15, 114–119. [Google Scholar] [CrossRef]
  3. Jackson, S.D.; King, T.A.; Pollnau, A.M. Diode-pumped 1.7 W erbium 3 µm fiber laser. Opt. Lett. 1999, 24, 1133–1135. [Google Scholar] [CrossRef]
  4. Tokita, S.; Murakami, M.; Shimizu, S.; Hashida, M.; Sakabe, S. Liquid-cooled 24 W mid-infrared Er3+:ZBLAN fiber laser. Opt. Lett. 2009, 34, 3062–3064. [Google Scholar] [CrossRef]
  5. Wang, J.S.; Vogel, E.M.; Snitzer, E. Tellurite glass: a new candidate for fiber devices. Opt. Mater. 1994, 3, 187–203. [Google Scholar] [CrossRef]
  6. Richards, B.; Tsang, Y.; Binks, D.; Lousteau, J.; Jha, A. Efficient ~2um Tm3+-doped tellurite fiber laser. Opt. Lett. 2008, 33, 402–404. [Google Scholar] [CrossRef]
  7. Mori, A.; Ohishi, Y.; Sudo, S. Erbium-doped tellurite glass fibre laser and amplifier. Electron. Lett. 1997, 33, 863–864. [Google Scholar] [CrossRef]
  8. Golding, P.S.; Jackson, S.D.; King, T.A.; Pollnau, M. Energy transfer processes in Er3+-doped and Er3+,Pr3+-codoped ZBLAN glasses. Phys. Rev. B 2000, 62, 856–864. [Google Scholar] [CrossRef]
  9. Chou, Y.G.; Kim, K.H.; Lee, B.J.; Shin, Y.B.; Kim, Y.S.; Heo, J. Emission properties of the Er3+:4I11/2–4I13/2 transition in Er3+ and Er3+/Tm3+-codoped Ge-Ga-As-S glasses. J. Non-Cryst. Solids 2000, 278, 137–144. [Google Scholar] [CrossRef]
  10. Zhong, H.; Chen, B.; Ren, G.; Cheng, L.; Yao, L.; Sun, J. 2.7 μm emission of Nd3+, Er3+codoped tellurite glass. J. Appl. Phys. 2009, 106, 083114. [Google Scholar] [CrossRef]
  11. Zhang, L.; Yang, Z.; Tian, Y.; Zhang, J.; Hu, L. Comparative investigation on the 2.7 μm emission in Er3+/Ho3+ codoped fluorophosphate glass. J. Appl. Phys. 2011, 110, 093106. [Google Scholar] [CrossRef]
  12. O’Donnell, M.D.; Miller, C.A.; Furniss, D.; Tikhomirov, V.K.; Seddon, A.B. Fluorotellurite glasses with improved mid-infrared transmission. J. Non-Cryst. Solids 2003, 331, 48–57. [Google Scholar] [CrossRef]
  13. Ofelt, S.G. Intensities of Crystal Spectra of Rare-Earth Ions. J. Chem. Phys. 1962, 37, 511. [Google Scholar] [CrossRef]
  14. Judd, B.R. Optical Absorption Intensities of Rare-Earth Ions. Phys.l Rev. 1962, 127, 750–761. [Google Scholar]
  15. Xu, R.R.; Tian, Y.; Wang, M.; Hu, L.L.; Zhang, J.J. Spectroscopic properties of 1.8 μm emission of thulium ions ingermanate glass. Appl. Phys. B 2010, 102, 109–116. [Google Scholar]
  16. Ivanova, S.; Pelle, F. Strong 1.53 μm to NIR-VIS-UV upconversion in Er-doped fluoride glass for high-efficiency solar cells. J. Opt. Soc. Am. B 2009, 26, 1930–1938. [Google Scholar] [CrossRef]
  17. Zou, X.; Izumitani, T. Spectroscopic properties and mechanisms of excited state absorption and energy transfer upconversion for Er3+-doped glasses. J. Non-Cryst. Solids 1993, 162, 68–80. [Google Scholar] [CrossRef]
  18. Peng, B.; Izumitani, T. Optical properties, fluorescence mechanisms and energy transfer in Tm3+, Ho3+ and Tm3+-Ho3+ doped near-infrared laser glasses, sensitized by Yb3+. Opt. Mater. 1995, 4, 797–810. [Google Scholar] [CrossRef]
  19. Reisfeld, R.; Hormadaly, J. Optical intensities of holmium in tellurite, calibo, and phosphate glasses. J. Chem. Phys. 1976, 64, 3207–3212. [Google Scholar] [CrossRef]
  20. Xu, R.; Pan, J.; Hu, L.; Zhang, J. 2.0 μm emission properties and energy transfer processes of Yb3+/Ho3+ codoped germanate glass. J. Appl. Phys. 2010, 108, 043522. [Google Scholar] [CrossRef]
  21. Feng, L.; Wang, J.; Tang, Q.; Liang, L.; Liang, H.; Su, Q. Optical properties of Ho3+-doped novel oxyfluoride glasses. J. Lumin. 2007, 124, 187–194. [Google Scholar] [CrossRef]
  22. Zhang, X.D.; Xu, T.F.; Dai, S.X.; Nie, Q.H.; Shen, X.; Lu, L.; Zhang, X.H. Investigation of energy transfer and frequency upconversionin Er3+/Ho3+ co-doped tellurite glasses. J. Alloy Compd. 2008, 450, 306–309. [Google Scholar] [CrossRef]
  23. Pollnan, M. The route toward a diode-pumped 1 W erbium 3 μm fiber laser. IEEE J. Sel. Top. Quant. 1997, 33, 1982–1990. [Google Scholar] [CrossRef]
  24. Carnall, W.T.; Fields, P.R.; Rajnak, K. Spectral Intensities of the Trivalent Lanthanides and Actinides in Solution. II. Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, and Ho3+. J. Chem. Phys. 1968, 49, 4412–4423. [Google Scholar] [CrossRef]
  25. Stephen, A.P.; Chase, L.L.; Smith, L.K.; Kway, W.L.; Krupke, W.F. Infrared Cross—Section Measurements for Crystals Doped with Er3+, Tm3+, and Ho3+. IEEE J. Quantum Elect. 1992, 28, 2619–2630. [Google Scholar]
  26. McCumber, D.E. Theory of Phonon Terminated Optical Masers. Phy. Rev. 1964, 134, A299–A306. [Google Scholar] [CrossRef]
  27. Guo, Y.; Tian, Y.; Zhang, L.; Hu, L.; Chen, N.K.; Zhang, J. Pr3+-sensitized Er3+-doped bismuthate glass for generating high inversion rates at 2.7 μm wavelength. Opt. Lett. 2012, 37, 3387–3389. [Google Scholar] [CrossRef]
  28. Tian, Y.; Xu, R.; Zhang, L.; Hu, L.; Zhang, J. Observation of 2.7 μm emission from diode-pumped Er3+/Pr3+-codoped fluorophosphate glass. Opt. Lett. 2011, 36, 109–111. [Google Scholar] [CrossRef]
  29. Tikhomirov, V.K.; Méndez-Ramos, J.; Rodríguez, V.D.; Furniss, D.; Seddon, A.B. Laser and gain parameters at 2.7 μm of Er3+-doped oxyfluoride transparent glass–ceramics. Opt. Mater. 2006, 28, 1143–1146. [Google Scholar] [CrossRef]
Fibers EISSN 2079-6439 Published by MDPI AG, Basel, Switzerland RSS E-Mail Table of Contents Alert