Broadening Frontiers of Infrared Nonlinear Optical Materials with Ï

Subscriber access provided by Vanderbilt Libraries

Article

Broadening Frontiers of Infrared Nonlinear Optical Materials with #–Conjugated Trigonal Planar Groups Zhuang Li, Yi Yang, Yangwu Guo, Wenhao Xing, Xiaoyu Luo, Zheshuai Lin, Jiyong Yao, and Yicheng Wu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04981 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Broadening

Frontiers

of

Infrared

Nonlinear

Optical

Materials

with

π–Conjugated Trigonal Planar Groups

Zhuang Li,a,b,c Yi Yang,a,c Yangwu Guo,a,b,c Wenhao Xing,a,b,c Xiaoyu Luo,a,b,c Zheshuai Lin,a Jiyong Yao,a,b,* Yicheng Wua,d

a

Center for Crystal Research and Development, Key Lab Functional Crystals and Laser Technology of Chinese

Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China b

Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing

100049, P. R. China c University d Institute

of Chinese Academy of Sciences, Beijing 100190, PR China

of Functional Crystal Materials, Tianjin University of Technology, Tianjin 300384, P.R. China

*Corresponding author: Jiyong Yao; [email protected].

1

ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

ABSTRACT Trigonal planar units with high physicochemical stability and large polarizability anisotropy are one kind of promising fundamental building blocks (FBBs) for constructing novel nonlinear optical (NLO) materials. Though great achievements have been made in ultraviolet/deep ultraviolet (UV/DUV) region with trigonal planar units, little attention has been paid to them in infrared band owing to the lack of enough representatives. In this work, [AgSe3] and [HgSe3] are rationally proposed as NLO active FBBs. Besides, Ag6HgMSe6 (M = Si, Ge) are screened out by combined density functional theory calculations and experiments as new type IR NLO materials. Experiments demonstrate both Ag6HgSiSe6 and Ag6HgGeSe6 show strong second harmonic generation (SHG) responses, valuable phase–matchable features and congruent–melting thermal behaviors. Moreover, the great contributions of the trigonal planar units are also been discussed in detail.

2


Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Nonlinear optical materials can modulate laser frequency directly by means of sum frequency, difference frequency, frequency doubling, optical parametric oscillation (OPO) and optical parametric amplification (OPA), which are widely used in laser communication, laser guidance, minimally invasive surgery and atmospheric environment monitoring.1–10 Owing to multiple atomic connections and wide infrared (IR) transparent range, metal chalcogenides are the most promising materials for IR NLO applications. Among them, traditional chalcopyrite–type compounds AgGaQ2 (Q = S, Se) and ZnGeP2 have been practically used since 1970s.11–13 Though these traditional materials have good second order harmonic generation (SHG) responses, their applications are severely hindered by their intrinsic drawbacks, such as low laser damage thresholds (LDTs) for AgGaQ2, harmful two–photon absorption (TPA) at 1 μm for ZnGeP2, and non–phasematching behavior (small birefringence) for AgGaSe2.14 To explore IR NLO materials with good combined figure of merit, researchers screen out many microscopic NLO building motifs to construct novel molecular structures. These NLO building units usually includes the MX4 tetrahedra (M = Mn, Al, Ga, In, Si, Ge, Sn, P, etc.; X = S, Se, Te), tetrahedra centered by d10 metal cations (e.g., Cu+, Ag+, Zn2+, Cd2+, Hg2+), the polyhedra centered by the second–order Jahn–Teller (SOJT) distorted d0 metal cations (e.g., Ta5+, Zr4+) and some polar structural units centered by cations with stereochemically active lone pair electrons (e.g., As3+, Sb3+, Pb2+).15–23 Under the guidance of this idea, many high-performance NLO compounds have been discovered, such as BaGa4Q7 (Q = S, Se),24,25 A2Hg3M2S8 (A = Na, K; M = Si, Ge, Sn),26 3



Page 4 of 30

AgGa2PS6,27 Zn3P2S8,28 BaGa2MSe6 (M = Si, Ge),29 AAsSe2 (A = Li, Na),30 Ba4CuGa5Q12 (Q = S, Se),31 ACd4Ga5S12 (A = K, Rb, Cs)32 and Ba5CdGa6Se15.33 Nevertheless, π–conjugated planar groups, which should be among the most desirable FBBs for NLO materials, have been long ignored in IR NLO research field. In fact, π–conjugated planar groups have already made great achievements in constructing favorable UV/DUV NLO materials owing to their large microscopic second–order susceptibility and anisotropy. For instance, planar [BO3] and [B3O6] units are the main origin of the exceptional NLO properties of the world–renowned crystals KBe2BO3F2 (KBBF) and β–BaB2O4 (β–BBO) respectively.34,35 In addition, several other π–conjugated planar groups, such as [CO3],36 [NO3]37 and [C3N3O3],38 have also been proved to simultaneously possess large susceptibility and wide UV transmission range and regarded as potential UV NLO–active building blocks. By contrast, researches on chalcogenide containing π–conjugated planar groups, which may play an important role in IR NLO field, are very rare. In 2005, Halasyamani et al. first reported trigonal planar group [BS3],39 which is the sulfide analogue of the borate group [BO3]. However, subsequent experiments confirmed that the second–order nonlinear polarizability was relatively small compared with traditional IR NLO materials and most of the [BS3]-containing materials were often highly hydroscopic, which cast a shadow on its application prospect. In the following years, searching new type π–conjugated trigonal planar groups had stagnated until our research group found a stable trigonal planar unit [HgSe3] in Hg–base material BaHgSe2.40 [HgSe3] group not only has good physical and chemical stability, but also has high second–order polarizability. More commendably, BaHgSe2 features a congruent melting behavior which makes it a new type of IR NLO material with great application prospect. The discovery of the [HgSe3] group also rekindles our enthusiasm to broaden frontiers of IR NLO 4


Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


materials with π–conjugated trigonal planar groups. According to the diagonal rule on the periodic table of the elements and our previous exploration results, we focus our research on Hg–based and Ag–based materials. In this work, trigonal planar groups [AgSe3] and [HgSe3] with large polarizability anisotropy and high physicochemical stability are identified as superior FBBs to construct ideal candidates for IR NLO applications. Besides, Ag6HgMSe6 (M = Si, Ge) containing trigonal planar group [Ag/HgSe3] and [AgSe3] have been screened out by combined density functional theory calculations and experiments as promising IR NLO materials (Ag/Hg represents the crystallographic position shared by both Ag atoms and Hg atoms). Moreover, the strengthened SHG response induced by trigonal planar units has been discussed.

EXPERIMENTAL SECTION Computational details and methods: The first–principles calculations for Ag6HgSiSe6 and Ag6HgGeSe6 were performed by CASTEP,41 a plane–wave pseudopotential total energy package based density functional theory (DFT).42,43 The functional developed by Perdew–Burke–Emzerhoff (PBE) functional within the generalized gradient approximation (GGA) form were adopted to describe the exchange–correlation energy.44,45 The optimized norm–conserving pseudopotentials in the Kleinman–Bylander form for all the elements are used to model the effective interaction between atom cores and valence electrons. And Ag 4p105s1, Hg 5s25p65d106s2 Ge 4s24p2, Si 3s23p2, Se 4s24p4, electrons were treated as valence electrons, allowing the adoption of a relatively small basis set without compromising the computational accuracy. The high kinetic energy cutoff 1000 eV and dense 3×3×2 Monkhorst–Pack k–point meshes in the Brillouin zones were chosen for Ag6HgSiSe6

5



Page 6 of 30

and Ag6HgGeSe6. Our tests showed that the above computational set ups are sufficiently accurate for present purposes. It is well known that the energy band gaps calculated by standard DFT method are smaller than the measured values, due to the discontinuity of exchange–correlation energy. The scissor operators were adopted to shift all the conduction bands to match the calculated band gaps with the measured values. Based on the scissor–corrected electron band structure, the imaginary part of the dielectric function was calculated according to the electron transition from the valence bands (VB) to conduction band (CB). Consequently, the real part of the dielectric function is obtained by the Kramers–Kronig transform and the refractive index is determined. The SHG coefficients dij were obtained by the formula developed by Lin’s group.46,47 Synthesis: Ag (3N), Si (5N), Ge (5N), Se (5N) and HgSe (4N) were directly purchased from Sinopharm Chemical Reagent Co., Ltd. The binary starting materials Ag2Se, GeSe2 and SiSe2 were synthesized by heating the stoichiometric mixture of the elements in sealed silica tubes evacuated to 10−3 Pa. All manipulations were performed in an Ar–filled glovebox with H2O and O2 contents less than 0.1 ppm. Polycrystalline samples of Ag6HgSiSe6 and Ag6HgGeSe6 were obtained by traditional solid state reaction in a stoichiometric mixture of Ag2Se (0.442 g, 1.5 mmol), HgSe (0.14 g, 0.5 mmol), and SiSe2/GeSe2 (0.093 g, 0.5 mmol/0.115 g, 0.5 mmol). The mixture was loaded into a fused–silica tube. The tube was then evacuated to a vacuum of 10–3 Pa atmosphere and sealed. Afterwards, the sample was heated in a resistance furnace to 750 C in 15 h and stayed at that temperature for 120 h. And then the furnace was shut off.

6


Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Powder XRD measurement: The sample was carried out at room temperature with a Bruker D2 PHASER diffractometer (Cu Kα radiation with λ = 1.5418 Å). The 2θ range was 10–70° with a scan step width of 0.02° and a fixed counting time of 0.1 s per step. Diffuse reflectance spectra: A Cary 5000 UV–vis–NIR spectrophotometer with a diffuse reflectance accessory was used to record the spectra of the title compounds and BaSO4 as a reference in the range from 800 nm (1.55 eV) to 2500 nm (0.496 eV). Differential Scanning Calorimetry (DSC): A LabsysTM TG–DTA16 (SETARAM) thermal analyzer was used to study the thermal behavior by DSC analysis (the calorimeter was calibrated with Al2O3). The polycrystalline samples of Ag6HgSiSe6 and Ag6HgGeSe6 (ca. 30 mg) were placed in a silica tube (5mm o.d.×3mm i.d.), respectively, and subsequently sealed under a high vacuum. The heating and cooling rates were both 15 Kmin–1. Second–Harmonic Generation (SHG) Measurement: The measurement of optical SHG response of Ag6HgSiSe6 and Ag6HgGeSe6 was conducted by means of the Kurtz–Perry method with polycrystalline AgGaS2 as a reference.48 The fundamental light is the 2.09μm coherent light generated by a Q–switched Ho/Tm/Cr/YAG laser. The powder samples of Ag6HgSiSe6 and Ag6HgGeSe6 were sieved into a series of distinct particle size ranges of 20−41, 41−74, 74−105, 105−150, and 150−200 μm, respectively. Every sample was then pressed into a disk with diameter of 8 mm that was put between glass microscope slides and secured with tape in a 1 mm thick aluminum holder. The AgGaS2 sample was ground and sieved into the same particle size ranges, and then was pressed into disks with the same size and thickness.

RESULTS AND DISCUSSION 7



Page 8 of 30

Conducting investigation of structure features and the spatial arrangement of NLO active units in chalcogenides before further characterization is far more effective than trial and error method. Thus, we comprehensively searched the Inorganic Crystal Structures Database (ICSD) to screen out crystals that contain the π–conjugated trigonal planar groups. We assume that if the sum of three angles around the central atom falls in the range of [350° 360°], it will be determined as the planar trigonal group, otherwise be identified as NH3–shaped pyramidal unit. After our preliminary screening and filtering out some compounds with severe disorder, the target compounds that meet the requirements are listed as follows: (1) Hg3AsQ4X (Q = S, Se; X = Cl, Br, I);49 (2) CuHg2S2I;50 (3) La3AgM’Q7 (M’ = Si, Ge, Sn; Q = S, Se);51 (4) KAg5S3;52 (5) Ag7TaS6;53 (6) K2Ag3Sb3S7;54 (7) Cu10Hg2Sb4S13;55 (8) Ag5SbS4;56 (9) Ag6HgMSe6 (M = Si, Ge).57 (Their crystal structure and the coordination of cations can be found in supporting information Figure S1–S16. Some calculation results of Hg3AsQ4X (Q = S ， Se ； X = Cl ， Br ， I) are given in Figure S17 and Table S1). According to the structural analysis, many of these compounds suffer from misalignment of the NLO active trigonal planar units, which greatly weaken their NLO effect. Among them, Ag6HgMSe6 (M = Si, Ge) show favorable spatial arrangement and strong NLO performance (The crystal structures of Ag6HgMSe6 were first reported by Parasyuk’ group with no physical properties reported57). Thus, they are selected for further discussion. Subsequently, we performed computational screening of Ag6HgMSe6 (M = Si, Ge), and conducted detailed prediction of their band gaps, SHG coefficients and birefringence (results are listed in Table 1). Besides, the contributions of different groups to their macroscopic nonlinear effects are calculated by means of the so–called real space atomic cutting method to illustrate the performance of the trigonal planar units. In that method, the real space of the crystal cell is divided 8


Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


into several individual sphere zones, whose sizes are primarily decided by the radius of the ions. But Ag6HgSiSe6 and Ag6HgGeSe6 have positional disorder (some Ag atoms and Hg atoms share the same crystallographic position, and the site occupancy was refined to be 0.5 Ag + 0.5 Hg), thus atom–cutting analysis cannot be used directly. To solve this problem, the disorder position is considered as with 100% Ag occupancy. As expected, both Ag6HgSiSe6 and Ag6HgGeSe6 demonstrate large SHG coefficients with the trigonal planar units making great contribution. What catches our attention is that Ag6HgSiSe6 shows unusual larger SHG coefficient than its Ge analogue (43.22 pm/V for Ag6HgSiSe6, 21.36 pm/V for Ag6HgGeSe6). Generally, Ge analogue should exhibit stronger NLO response because Ge has larger atomic radius and more dispersed electron cloud which gives rise to the larger microscopic second–order susceptibility than Si analogue when bonding with chalcogens. Through careful structural analysis, we give a reasonable explanation for this abnormal phenomenon. In Ag6HgSiSe6, trigonal planar [Ag/HgSe3] units and SiSe4 tetrahedra are connected alternately by sharing corner Se atoms to form a one–dimensional (1D) 1 ∞[HgSiSe5]4− anionic chain along the b–axis (Figure 1c). Further, these chains are linked with each other via trigonal planar [AgSe3] units to form a three dimensional framework. [AgSe4] tetrahedra and [Ag/HgSe2] units can be considered as filling in the voids ([AgSe4] units were removed for clarity in Figure 1b). It is obvious that the 1 ∞[HgSiSe5]4− anionic chain is the main origin of Ag6HgSiSe6’s large SHG response because the planar [Ag/HgSe3] triangles are aligned completely parallel to each other within an individual chain, and these chains extend in the same direction, which greatly strengthens the second–order nonlinear polarizability. Interestingly, Ag6HgGeSe6 adopts a similar structure as Ag6HgSiSe6 (Figure 1e), but its [Ag/HgSe3] triangles degenerate into [Ag/HgSe2] units. As is shown in Figure 1a, Ag/Hg1 cations in Ag6HgSiSe6 are coordinated to three 9



Page 10 of 30

Se atoms with Ag/Hg−Se bond lengths varying from 2.425 Å to 2.709 Å which fall in a reasonable range and are in accordance with those for BaHgSe2. However, the coordination environment of Ag/Hg1 cations in Ag6HgGeSe6 is a little different. They are linked to two normal Se atoms with Ag/Hg−Se bond lengths of 2.431 Å and 2.525 Å and a third Se atom at a much longer distance of 3.243 Å (Figure 1d) which is too large for Ag/Hg−Se bond. Consequently, the appropriate coordination geometry of Ag/Hg1 cations in Ag6HgGeSe6 should be linear [Ag/HgSe2]. Apparently, the parallelly aligned [Ag/HgSe3] triangles show much larger second–order nonlinear polarizability than linear [Ag/HgSe2] units, as shown in Table 1. This can be the origin why Ag6HgSiSe6 demonstrates twice as much SHG coefficient as its Ge analogue does. On the other hand, birefringence is an important factor that determines whether the crystal can be phase–matchable. Many compounds with large SHG response are prevented from application due to their non–phase–matchable features resulted from small birefringence (for example, RbZn4In5Se12, CsZn4In5Se12 and KCd4Ga5Se12).58 From a structural point of view, the birefringence of the compound depends on the anisotropy of electron distribution, which is largely determined by the structure of fundamental building blocks (FBBs) and their arrangement mode and packing density.14,59 Generally, trigonal planar units exhibit much larger anisotropy than the common tetrahedra units owing to their π–conjugated effect. Thus, the introduction of trigonal planar units can usually result in not only larger SHG coefficient but also suitable birefringence for phase–matching (Table S2 shows the comparison between several benchmark crystals and materials that contain trigonal planar units). Certainly, the parallel alignment and high density of trigonal planar units can further strengthen the optical anisotropy and increase the crystal’s birefringence. Consequently, the large birefringence of Ag6HgSiSe6 and Ag6HgGeSe6, which makes them easy to be phase–matchable, 10


Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


can be easily understood (Δn = 0.207, 0.233 for Ag6HgGeSe6 and Ag6HgSiSe6, respectively, as shown in Table 1). And, owing to the coparallel arrangement of [Ag/HgSe3] triangles in 1 ∞[HgSiSe5]4− anionic chain, Ag6HgSiSe6 shows even larger birefringence than its Ge analogue. The electronic band structures along the highly symmetrical path in the Brillouin zone of Ag6HgSiSe6 and Ag6HgGeSe6 are depicted in Figure 2a and 2c, which illustrate they are direct transition semiconductor with both the maximum of valence bands (VBs) and minimum of conduction bands (CBs) situated at  point. The calculated bad gaps are 0.75eV and 0.86 eV for Ag6HgSiSe6 and Ag6HgGeSe6, respectively. As illustrated in the picture of total and partial density of states of Ag6HgSiSe6 and Ag6HgGeSe6 (Figure 2b and 2d), the upper region of the VBs is primarily composed from Ag 4d orbitals, Hg 5d orbitals and Se 4p orbitals, while the bottom of the CBs principally originates from Ag 4p orbitals, Hg 5p orbitals, Si 3p (Ge 4p) orbitals and Se 4s and 4p orbitals. As is well known, material’s optical properties mainly stem from the electronic transitions between the VBs and the CBs adjacent to the band gap. Thus, the [AgSe3] and [AgSe4] units are the main origin of the linear and nonlinear optical properties of Ag6HgGeSe6 while [AgSe3], [AgSe4] and [Ag/HgSe3] are responsible for the optical properties of Ag6HgSiSe6. To further investigate the NLO properties of Ag6HgSiSe6 with uniformly oriented trigonal planar units and Ag6HgGeSe6, various characterizations have been conducted. The powder samples of Ag6HgSiSe6 and Ag6HgGeSe6 were prepared by high temperature solid state reaction. According to our experimental observation, they are stable in the air for months and don’t dissolve in the water. As illustrated in Figure 3, the measured XRD powder patterns match well with the simulated patterns generated using the CIF data from Parasyuk’ group,57 which indicates the high purity of the samples Several weak calculated peaks don’t appear obviously in the experimental patterns. It should be 11



Page 12 of 30

owing to the relatively large background noise and the preferred orientation of the powder samples. The optical band gap of Ag6HgMSe6 (M = Si, Ge) are deduced from the plot of F(R) versus hν based on the Kubelka−Munk equation. As can be clearly seen in Figure 4, the band gap of 0.92 eV and 1.0 eV can be designated to Ag6HgGeSe6 and Ag6HgSiSe6, respectively. Moreover, there is almost no optical absorption below the band gap, which indirectly verifies the purity of the polycrystalline samples because the light transmission has not been disturbed. Both Ag6HgGeSe6 and Ag6HgSiSe6 exhibit small band gap, which may not make for increasing the LDT. Nevertheless, the band gap is not the only factor that determines LDT value. Crystal quality, chemical composition and thermal conductivity are also decisive factors. A good example is that ZnGeP2 has a small band gap of 1.75 eV but demonstrates much larger LDT than AgGaQ2 (Q = S, Se) when pumping with 2 μm light. Owing to the lack of large size crystals of Ag6HgMSe6 (M = Si, Ge), their accurate infrared transmission range cannot be obtained. However, they are selenides that contains heavy metal like Ag and Hg. Thus, their infrared transmission range can be easily up to 20 μm and cover the two atmosphere windows (i.e. 3–5 μm and 8–12 μm). Consequently, they may have important applications in the far IR or even the terahertz range, which have aroused increasing interests. Besides, the thermal behaviors of Ag6HgGeSe6 and Ag6HgSiSe6 are evaluated by the differential scanning calorimetric measurement. As demonstrated in Figure 5, during the entire heating/cooling cycle, only one endothermic peak and one exothermic peak exist. This indicates Ag6HgGeSe6 and Ag6HgSiSe6 melt congruently. The powder XRD patterns of the residues show almost no change compared to the original ones, which further verifies their congruent–melting property. The melting points of Ag6HgGeSe6 and Ag6HgSiSe6 are 915 and 939 °C, which are comparable to that of AgGaS2 and ZnGeP2 (998 °C and 1025 °C). It is worth noting that the credible 12


Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


thermal behavior of Ag6HgMSe6 (M = Si, Ge) would make it feasible to use the Bridgman–Stockbarger technique to grow bulk crystals for practical applications. SHG responses of Ag6HgGeSe6 and Ag6HgSiSe6 were assessed by means of the Kurtz−Perry method with a Q–switched Ho: Tm: Cr: YAG laser (2.09 μm, 3 Hz, 50 ns). As depicted in Figure 6a, the SHG intensities show a rising tendency with the increase of the particle size and then approach to saturation, which indicates Ag6HgGeSe6 and Ag6HgSiSe6 have good phase–matchability at the common 2 μm pumping light. The result is in accordance with their large calculated birefringence, which makes it easy to achieve phase–matching. Importantly, the good phase–matchable behavior is one of the prerequisites for applicable IR NLO materials. Consistent with the forgoing calculation prediction, Ag6HgSiSe6 exhibits strong SHG response, which is almost four times that of Ag6HgGeSe6 (Figure 6b). The experimental result commendably proves that the coparallel alignment of trigonal planar units can greatly strengthen materials’ SHG response. According to the first principles calculations, Ag6HgSiSe6 and Ag6HgGeSe6 have much larger SHG coefficients than AgGaS2 and thus should exhibit much stronger SHG intensity. However, the experimental result shows the SHG responses of Ag6HgSiSe6 and Ag6HgGeSe6 are only 2 and 0.5 times that of AgGaS2 at the particle size of 150−200 μm. This is because the fundamental light we used is at the wavelength of 2 μm and thus the output frequency–doubled light is at 1 μm. Owing to their small band gaps (1.0 eV and 0.92 eV), Ag6HgSiSe6 and Ag6HgGeSe6 have relatively strong absorption of the frequency–doubled light. Consequently, the relatively weaker experimental SHG intensity is reasonable.

CONCLUSION 13



Page 14 of 30

In summary, [AgSe3] and [HgSe3] with large microscopic second–order susceptibility and high physicochemical stability are rationally proposed as NLO active groups. The representative compounds Ag6HgSiSe6 and Ag6HgGeSe6 are screened out by first principles calculations combined with structural analysis. Ag6HgSiSe6’s strengthened SHG response originated from the co-parallel alignment of [Ag/HgSe3] triangles has also been revealed. Experiments demonstrate that Ag6HgSiSe6 and Ag6HgGeSe6 show strong second harmonic generation (SHG) responses (2 and 0.5 times that of AgGaS2), valuable phase–matchable features and congruent–melting thermal behaviors, which make them potential IR NLO materials. Moreover, this study may shed light on broadening frontiers of IR NLO materials with π–conjugated trigonal planar groups.

ASSOCIATED CONTENT Supporting Information Crystal structure and the coordination of cations of the screened compounds Electronic band structure of Hg3AsQ4X (Q = S, Se; X = Cl, Br, I) Band gap, the second−order polarization tensor and the contribution of different building blocks to NLO effect of Hg3AsQ4X (Q = S, Se; X = Cl, Br, I) Space group, SHG coefficient and birefringence of several benchmark crystals and compounds that contain trigonal planar units

ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (No. 91622123，No. 51472251, No. 61675212)

14


Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References (1) Petrov, V. Parametric down-Conversion Devices: The Coverage of the Mid-Infrared Spectral Range by Solid-State Laser Sources. Opt. Mater. 2012, 34, 536–554. (2) Gong, P.; Liang, F.; Kang, L.; Chen, X.; Qin, J.; Wu, Y.; Lin, Z. Recent Advances and Future Perspectives on Infrared Nonlinear Optical Metal Halides. Coord. Chem. Rev. 2019, 380, 83–102. (3) Hu, C. L.; Mao, J. G. Recent Advances on Second-Order NLO Materials Based on Metal Iodates. Coord. Chem. Rev. 2015, 288, 1–17. (4) Xia, Z.; Poeppelmeier, K. R. Chemistry-Inspired Adaptable Framework Structures. Acc. Chem. Res. 2017, 50, 1222–1230. (5) Luo, X.; Li, Z.; Guo, Y.; Yao, J.; Wu, Y. Recent Progress on New Infrared Nonlinear Optical Materials with Application Prospect. J. Solid State Chem. 2019, 270, 674–687. (6) Liang, F.; Kang, L.; Lin, Z.; Wu, Y. Mid-Infrared Nonlinear Optical Materials Based on Metal Chalcogenides: Structure–Property Relationship. Cryst. Growth Des. 2017, 17, 2254–2289. (7) Guo, S. P.; Chi, Y.; Guo, G. C. Recent Achievements on Middle and Far-Infrared Second-Order Nonlinear Optical Materials. Coord. Chem. Rev. 2017, 335, 44–57. (8) Liang, F.; Kang, L.; Lin, Z.; Wu, Y.; Chen, C. Analysis and Prediction of Mid-IR Nonlinear Optical Metal Sulfides with Diamond-like Structures. Coord. Chem. Rev. 2017, 333, 57–70. (9) Kanatzidis, M. G. Discovery-Synthesis, Design, and Prediction of Chalcogenide Phases. Inorg. Chem. 2017, 56, 3158–3173. (10) Chung, I.; Kanatzidis, M. G. Metal Chalcogenides: A Rich Source of Nonlinear Optical Materials. Chem. Mater. 2014, 26, 849–869. 15



Page 16 of 30

(11) Boyd, G. D.; Buehler, E.; Storz, F. G. Linear and Nonlinear Optical Properties of ZnGeP2 and CdSe. Appl. Phys. Lett. 1971, 18, 301–304. (12) Tell, B.; Kasper, H. M. Optical and Electrical Properties of AgGaS2 and AgGaSe2. Phys. Rev. B 1971, 4, 4455–4459. (13) Harasaki, A.; Kato, K. New Data on the Nonlinear Optical Constant, Phase-Matching, and Optical Damage of AgGaS2. Japanese J. Appl. Physics, Part 1 Regul. Pap. Short Notes Rev. Pap. 1997, 36, 700–703. (14) Zhou, M.; Yang, Y.; Guo, Y.; Lin, Z.; Yao, J.; Wu, Y.; Chen, C. Hg-Based Infrared Nonlinear Optical Material KHg4Ga5Se12 Exhibits Good Phase-Matchability and Exceptional Second Harmonic Generation Response. Chem. Mater. 2017, 29, 7993–8002. (15) Fan, H. T.; Xu, C. H.; Wang, Z. H.; Wang, G.; Liu, C. J.; Liang, J. K.; Chen, X. L.; Wei, Z. Y. Generation of Broadband 17-μJ Mid-Infrared Femtosecond Pulses at 375 Μm by Silicon Carbide Crystal. Opt. Lett. 2014, 39, 6249. (16) Wang, S.; Zhan, M.; Wang, G.; Xuan, H.; Zhang, W.; Liu, C.; Xu, C.; Liu, Y.; Wei, Z.; Chen, X. 4H-SiC: A New Nonlinear Material for Midinfrared Lasers. Laser Photonics Rev. 2013, 7, 831–838. (17) Huang, H.; Han, X.; Li, X.; Wang, S.; Chu, P. K.; Zhang, Y. Fabrication of Multiple Heterojunctions with Tunable Visible-Light-Active Photocatalytic Reactivity in BiOBr-BiOI Full-Range Composites Based on Microstructure Modulation and Band Structures. ACS Appl. Mater. Interfaces 2015, 7, 482–492. (18) Feng, J.; Hu, C.; Li, B.; Mao, J. LiGa2PS6 and LiCd3PS6 : Molecular Designs of Two New Mid-Infrared Nonlinear Optical Materials LiGa2PS6 and LiCd3PS6 : Molecular Designs of Two 16


Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


New Mid-Infrared Nonlinear Optical Materials. 2018, 6, 2–9. (19) Guo, Y.; Liang, F.; Yao, J.; Lin, Z.; Yin, W.; Wu, Y.; Chen, C. Nonbonding Electrons Driven Strong SHG Effect in Hg2GeSe4: Experimental and Theoretical Investigations. Inorg. Chem. 2018, 57, 6795–6798. (20) Wu, K.; Yang, Z.; Pan, S. Na4MgM2Se6 (M = Si, Ge): The First Noncentrosymmetric Compounds with Special Ethane-like [M2Se6]6- Units Exhibiting Large Laser-Damage Thresholds. Inorg. Chem. 2015, 54, 10108–10110. (21) Mei, D.; Yin, W.; Feng, K.; Lin, Z.; Bai, L.; Yao, J.; Wu, Y. LiGaGe2Se6: A New IR Nonlinear Optical Material with Low Melting Point. Inorg. Chem. 2012, 51, 1035–1040. (22) Luo, Z. Z.; Lin, C. S.; Cui, H. H.; Zhang, W. L.; Zhang, H.; He, Z. Z.; Cheng, W. D. SHG Materials SnGa4Q7 (Q = S, Se) Appearing with Large Conversion Efficiencies, High Damage Thresholds, and Wide Transparencies in the Mid-Infrared Region. Chem. Mater. 2014, 26, 2743–2749. (23) Chung, I.; Song, J.-H.; Jang, J. I.; Freeman, A. J.; Kanatzidis, M. G. Na2Ge2Se5: A Highly Nonlinear Optical Material. J. Solid State Chem. 2012, 195, 161–165. (24) Lin, X.; Zhang, G.; Ye, N. Growth and Characterization of BaGa4S7 : A New Crystal for Mid-IR Nonlinear Optics. Cryst. Growth Des. 2009, 9, 1186–1189. (25) Yao, J.; Mei, D.; Bai, L.; Lin, Z.; Yin, W.; Fu, P.; Wu, Y. BaGa4Se7: A New Congruent-Melting IR Nonlinear Optical Material. Inorg. Chem. 2010, 49, 9212–9216. (26) Wu, K.; Yang, Z.; Pan, S. Na2Hg3M2S8 (M = Si, Ge, and Sn): New Infrared Nonlinear Optical Materials with Strong Second Harmonic Generation Effects and High Laser-Damage Thresholds. Chem. Mater. 2016, 28, 2795–2801. 17



Page 18 of 30

(27) Feng, J. H.; Hu, C. L.; Xu, X.; Li, B. X.; Zhang, M. J.; Mao, J. G. AgGa2PS6: A New Mid-Infrared Nonlinear Optical Material with a High Laser Damage Threshold and a Large Second Harmonic Generation Response. Chem. - A Eur. J. 2017, 23, 10978–10982. (28) Li, Z.; Jiang, X.; Zhou, M.; Guo, Y.; Luo, X.; Wu, Y.; Lin, Z.; Yao, J. Zn3P2S8: A Promising Infrared Nonlinear-Optical Material with Excellent Overall Properties. Inorg. Chem. 2018, 57, 10503–10506. (29) Yin, W.; Feng, K.; He, R.; Mei, D.; Lin, Z.; Yao, J.; Wu, Y. BaGa2MQ6 (M = Si, Ge; Q = S, Se): A New Series of Promising IR Nonlinear Optical Materials. Dalt. Trans. 2012, 41, 5653. (30) Bera, T. K.; Jang, J. I.; Song, J.-H.; Malliakas, C. D.; Freeman, A. J.; Ketterson, J. B.; Kanatzidis, M. G. Soluble Semiconductors AAsSe2 (A = Li, Na) with a Direct-Band-Gap and Strong Second Harmonic Generation: A Combined Experimental and Theoretical Study. J. Am. Chem. Soc. 2010, 132, 3484–3495. (31) Kuo, S. M.; Chang, Y. M.; Chung, I.; Jang, J. I.; Her, B. H.; Yang, S. H.; Ketterson, J. B.; Kanatzidis, M. G.; Hsu, K. F. New Metal Chalcogenides Ba4CuGa5Q12 (Q = S, Se) Displaying Strong Infrared Nonlinear Optical Response. Chem. Mater. 2013, 25, 2427–2433. (32) Lin, H.; Zhou, L. J.; Chen, L. Sulfides with Strong Nonlinear Optical Activity and Thermochromism: ACd4Ga5S12 (A= K, Rb, Cs). Chem. Mater. 2012, 24, 3406–3414. (33) Yin, W.; Iyer, A. K.; Li, C.; Yao, J.; Mar, A. Ba5CdGa6Se15, a Congruently-Melting Infrared Nonlinear Optical Material with Strong SHG Response. J. Mater. Chem. C 2017, 5, 1057–1063. (34) Nikogosyan, D. N. Beta Barium Borate (BBO) - A Review of Its Properties and Applications. Appl. Phys. A Solids Surfaces 1991, 52, 359–368. (35) Chen, C.; Xu, Z.; Deng, D.; Zhang, J.; Wong, G. K. L.; Wu, B.; Ye, N.; Tang, D. The Vacuum 18


Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ultraviolet Phase-Matching Characteristics of Nonlinear Optical KBe2BO3F2 crystal. Appl. Phys. Lett. 1996, 68, 2930–2932. (36) Zou, G.; Jo, H.; Lim, S. J.; You, T. S.; Ok, K. M. Rb3VO(O2)2CO3: A Four-in-One Carbonatoperoxovanadate Exhibiting an Extremely Strong Second-Harmonic Generation Response. Angew. Chemie - Int. Ed. 2018, 57, 8619–8622. (37) Zhao, S.; Yang, Y.; Shen, Y.; Zhao, B.; Li, L.; Ji, C.; Wu, Z.; Yuan, D.; Lin, Z.; Hong, M.; Cooperation of Three Chromophores Generates the Water-Resistant Nitrate Nonlinear Optical Material Bi3TeO6OH(NO3)2. Angew. Chemie - Int. Ed. 2017, 56, 540–544. (38) Kalmutzki, M.; Stroebele, M.; Wackenhut, F.; Meixner, A. J.; Meyer, H. J. Synthesis, Structure, and Frequency-Doubling Effect of Calcium Cyanurate. Angew. Chemie - Int Ed. 2014, 53, 14260–14263. (39) Kim, Y.; Martin, S. W.; Ok, K. M.; Halasyamani, P. S. Synthesis of the Thioborate Crystal ZnxBa2B2S5+x (x ≈ 0.2) for Second Order Nonlinear Optical Applications. Chem. Mater. 2005, 17, 2046–2051. (40) Li, C.; Yin, W.; Gong, P.; Li, X.; Zhou, M.; Mar, A.; Lin, Z.; Yao, J.; Wu, Y.; Chen, C. Trigonal Planar [HgSe3]4- Unit: A New Kind of Basic Functional Group in IR Nonlinear Optical Materials with Large Susceptibility and Physicochemical Stability. J. Am. Chem. Soc. 2016, 138, 6135–6138. (41) Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. First-Principles Simulation: Ideas, Illustrations and the CASTEP Code. J. Phys. Condens. Matter 2002, 14, 2717–2744. (42) Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. 19



Page 20 of 30

Phys. Rev. 1965, 140, 1133–1138. (43) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C. First Principles Methods Using CASTEP. Zeitschrift Fur Krist. 2005, 220, 567–570. (44) Perdew, J. P.; Zunger, A. Self-Interaction Correction to Density-Functional Approximations for Many-Electron Systems. Phys. Rev. B 1981, 23, 5048–5079. (45) Ceperley, D. M.; Alder, B. J. Ground State of the Electron Gas by a Stochastic Method. Phys. Rev. Lett. 1980, 45, 566–569. (46) Lin, Z.; Jiang, X.; Kang, L.; Gong, P.; Luo, S.; Lee, M. H. First-Principles Materials Applications and Design of Nonlinear Optical Crystals. J. Phys. D. Appl. Phys. 2014, 47, 253001. (47) Lin, J.; Lee, M. H.; Liu, Z. P.; Chen, C. T.; Pickard, C. J. Mechanism for Linear and Nonlinear Optical Effects in β-BaB2O4 Crystals. Phys. Rev. B 1999, 60, 13380–13389. (48) Kurtz, S. K.; Perry, T. T. A Powder Technique for the Evaluation of Nonlinear Optical Materials. J. Appl. Phys. 1968, 39 (8), 3798–3813. (49) Beck, J.; Hedderich, S.; Köllisch, K. Hg3AsE4X (E = S, Se; X = Cl, Br, I), a Family of Isotypic Compounds with an Acentric, Layered Structure. Inorg. Chem. 2000, 39, 5847–5850. (50) Keller, H. L.; Wimbert, L. Über Münzmetall-Quecksilber-Chalkogenidhalogenide. IV Hydrothermalsynthese Und Kristallstruktur von CuHgSI Und CuHg2S2I. Zeitschrift fur Anorg. und Allg. Chemie 2004, 630, 331–336. (51) Hwu, S. J.; Bucher, C. K.; Carpenter, J. D.; Taylor, S. P. A Solid-State Diastereomer, AgLa3GeS7. Inorg. Chem. 1995, 34, 1979–1980. (52) Emirdag, M.; Schimek, G. L.; Kolis, J. W. KAg5S3. Acta Crystallogr. Sect. C Cryst. Struct. 20


Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Commun. 1998, 54, 1376–1378. (53) Wada, H.; Onoda, M. Crystal Structure and Conductivity of a New Compound, Ag7TaS6. J. Less-Common Met. 1991, 175, 209–217. (54) Yao, H. G.; Ji, M.; Ji, S. H.; Zhang, R. C.; An, Y. L.; Ning, G. L. Solvothermal Syntheses of Two Novel Layered Quaternary Silver-antimony (III) Sulfides with Different Strategies. Cryst. Growth Des. 2009, 9, 3821–3824. (55) Johnson, N. E. X-Ray Powder Diffraction Data for Synthetic Varieties of Tetrahedrite. Powder Diffr. 1991, 6, 43–47. (56) Leitl, M.; Pfitzner, A.; Bindi, L. Preferred Ion Diffusion Pathways and Activation Energies for Ag in the Crystal Structure of Stephanite, Ag5SbS4. Mineral. Mag. 2009, 73, 17–26. (57) Gulay, L. D.; Olekseyuk, I. D.; Parasyuk, O. V. Crystal Structures of the Ag6HgGeSe6 and Ag6HgSiSe6 Compounds. J. Alloys Compd. 2002, 343, 116–121. (58) Lin, H.; Chen, L.; Zhou, L. J.; Wu, L. M. Functionalization Based on the Substitutional Flexibility: Strong Middle IR Nonlinear Optical Selenides AXII4XIII5Se12. J. Am. Chem. Soc. 2013, 135, 12914–12921. (59) Chen, C.; Wu, Y.; Li, R. The Anionic Group Theory of the Non-Linear Optical Effect and Its Applications in the Development of New High-Quality NLO Crystals in the Borate Series. Int. Rev. Phys. Chem. 1988, 8, 65–91.

21



Page 22 of 30

Figure Captions Figure 1 (a) From left to right are coordination environments of Ag3, Ag1 (Ag2), Ag/Hg2 and Ag/Hg1 in Ag6HgSiSe6. (b) Crystal structure of Ag6HgSiSe6 ([AgSe4] units are not shown for clarity). (c) 1 ∞[HgSiSe5]4− anionic chain. (d) From left to right are coordination environments of Ag1 (Ag2), Ag/Hg1, Ag/Hg2 and Ag3 in Ag6HgGeSe6. (e) Crystal structure of Ag6HgGeSe6 ([AgSe4] units are not shown for clarity). (f) 1 ∞[HgGeSe4]2− anionic chain Figure 2 (a) The calculated band structure of Ag6HgSiSe6. (b) The total and partial density of states of Ag6HgSiSe6. (c) The calculated band structure of Ag6HgGeSe6. (d) The total and partial density of states of Ag6HgGeSe6. Figure 3 Experimental and calculated powder X–ray diffraction patterns of Ag6HgSiSe6 (a) and Ag6HgGeSe6 (b). Figure 4 UV/vis-NIR diffuse reflectance spectrum of Ag6HgSiSe6 (red) and Ag6HgGeSe6 (black). Figure 5 Differential scanning calorimetric curves of Ag6HgSiSe6 (green) and Ag6HgGeSe6 (purple). Figure 6 (a) Phase-matching curves for Ag6HgSiSe6 (red) and Ag6HgGeSe6 (green) with AgGaS2 (black) as a reference. (b) Oscilloscope traces of SHG signals for Ag6HgSiSe6 (red) and Ag6HgGeSe6 (green) with AgGaS2 (black) as a reference at a particle size of 150–200 μm.

22


Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1

23



Figure 2

24


Page 24 of 30

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3

25



Figure 4

26


Page 26 of 30

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5

27



Figure 6

28


Page 28 of 30

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Space groups, calculated band gaps at Level of GGA approximations, birefringence (Δn) at 2.09 nm, and SHG coefficients (dij) with a correction of the band gap by using scissors operator for Ag6HgMSe6 (M = Si, Ge). Space group

Eg (eV) (GGA)

d31 (pm/V)

d32 (pm/V)

d33 (pm/V)

Δn

Pmn21

0.86

1.919

21.36

14.04

0.207

GeSe4

–2.22

9.83

8.78

AgSe3

2.83

13.84

22.28

AgSe4

3.76

17.70

7.64

Ag/HgSe2

–1.18

–5.81

–8.88

–2.11

43.22

19.19

SiSe4

8.13

21.34

9.54

Ag/HgSe3

–6.19

38.66

–6.43

AgSe4

–12.32

24.83

8.02

Ag/HgSe2

2.097

0.79

4.47

Ag6HgGeSe6

Ag6HgSiSe6

Pmn21

0.75

29


0.233


Table of Contents

30


Page 30 of 30

Broadening Frontiers of Infrared Nonlinear Optical Materials with Ï

Recommend Documents

Broadening Frontiers of Infrared Nonlinear Optical Materials with Ï