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[Boqing Liu](https://orcid.org/0000-0001-7987-3979), Kun Liang, [Qingyi Zhou](https://orcid.org/0000-0001-7261-1060), [Ahmed Raza Khan](https://orcid.org/0000-0002-0962-1861), [Zhuoyuan Lu](https://orcid.org/0009-0000-0345-8269), [Tanju Yildirim](https://orcid.org/0000-0002-0269-4718), [Xueqian Sun](https://orcid.org/0000-0002-0165-0481), [Sharidya Rahman](https://orcid.org/0000-0002-2195-0406), [Yun Liu](https://orcid.org/0000-0002-5404-3909), [Zongfu Yu](https://orcid.org/0000-0002-4536-1526), [Yuerui Lu](https://orcid.org/0000-0001-6131-3906)

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Giant second harmonic generation in two-dimensional tellurene with synthesis andthickness engineeringViewOnlineExportCitationRESEARCH ARTICLE |  FEBRUARY 18 2025Giant second harmonic generation in two-dimensionaltellurene with synthesis and thickness engineeringBoqing Liu  ; Kun Liang; Qingyi Zhou  ; Ahmed Raza Khan  ; Zhuoyuan Lu  ; Tanju Yildirim  ;Xueqian Sun  ; Sharidya Rahman  ; Yun Liu  ; Zongfu Yu  ; Yuerui Lu  Appl. Phys. Rev. 12, 011414 (2025)https://doi.org/10.1063/5.0218276Articles You May Be Interested InExciton states and oscillator strength in few-layer α-tellureneAppl. Phys. Lett. (March 2019)Microscopic origin of inhomogeneous transport in four-terminal tellurene devicesAppl. Phys. Lett. (December 2020)Memristors with tunable characteristics based on the tellurene/Nb-doped MoS2 heterojunction toward bio-realistic synaptic emulationAppl. Phys. Lett. (October 2023) 27 August 2025 00:09:57https://pubs.aip.org/aip/apr/article/12/1/011414/3336263/Giant-second-harmonic-generation-in-twohttps://pubs.aip.org/aip/apr/article/12/1/011414/3336263/Giant-second-harmonic-generation-in-two?pdfCoverIconEvent=citejavascript:;https://orcid.org/0000-0001-7987-3979javascript:;javascript:;https://orcid.org/0000-0001-7261-1060javascript:;https://orcid.org/0000-0002-0962-1861javascript:;https://orcid.org/0009-0000-0345-8269javascript:;https://orcid.org/0000-0002-0269-4718javascript:;https://orcid.org/0000-0002-0165-0481javascript:;https://orcid.org/0000-0002-2195-0406javascript:;https://orcid.org/0000-0002-5404-3909javascript:;https://orcid.org/0000-0002-4536-1526javascript:;https://orcid.org/0000-0001-6131-3906https://crossmark.crossref.org/dialog/?doi=10.1063/5.0218276&domain=pdf&date_stamp=2025-02-18https://doi.org/10.1063/5.0218276https://pubs.aip.org/aip/apl/article/114/9/092101/236661/Exciton-states-and-oscillator-strength-in-fewhttps://pubs.aip.org/aip/apl/article/117/25/253102/1061378/Microscopic-origin-of-inhomogeneous-transport-inhttps://pubs.aip.org/aip/apl/article/123/17/173503/2918941/Memristors-with-tunable-characteristics-based-onhttps://e-11492.adzerk.net/r?e=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&s=LjhIZAHv7X88sMghfBEvxti5tTYGiant second harmonic generation intwo-dimensional tellurene with synthesisand thickness engineeringCite as: Appl. Phys. Rev. 12, 011414 (2025); doi: 10.1063/5.0218276Submitted: 10 May 2024 . Accepted: 13 January 2025 .Published Online: 18 February 2025Boqing Liu,1 Kun Liang,1 Qingyi Zhou,2 Ahmed Raza Khan,1,3 Zhuoyuan Lu,1 Tanju Yildirim,4Xueqian Sun,1 Sharidya Rahman,1,5 Yun Liu,6 Zongfu Yu,2 and Yuerui Lu1,7,a)AFFILIATIONS1School of Engineering, College of Engineering, Computing & Cybernetics, The Australian National University,Canberra, ACT 2601, Australia2Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA3Department of Industrial and Manufacturing Engineering, University of Engineering and Technology Lahore (Rachna CollegeCampus), Gujranwala 54700, Pakistan4International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba,Ibaraki 305-0044, Japan5Department of Materials Science and Engineering, Monash University, Clayton, VIC 3800, Australia6Research School of Chemistry, College of Science, The Australian National University, Canberra, ACT 2601, Australia7ARC Centre of Excellence in Quantum Computation and Communication Technology ANU Node, Canberra, ACT 2601, Australiaa)Author to whom correspondence should be addressed: yuerui.lu@anu.edu.auABSTRACTSecond harmonic generation (SHG) is a prominent branch of non-linear optics (NLO) heavily reliant on conventional bulk NLO crystals. However,the difficulty in downsizing these crystals imposes technical limitations on the future of miniaturized NLO devices. Tellurene emerges as a promisingcandidate to overcome these restrictions, excelling in electrical applications and believed to possess a giant second-order optical susceptibility compa-rable to conventional NLO crystals. In this study, a face-to-face substrate configuration is employed for the synthesis of ultrathin tellurene via PVD.Our findings reveal that tellurene’s SHG performance surpasses that of monolayer transition metal dichalcogenides by two orders of magnitude,with maximum efficiency when the flake thickness is between 16 and 20nm under various wavelengths. High sensitivity to thickness variationencourages post-growth thinning through hydrogen plasma etching, enabling precise engineering of the flake thickness for optimal SHG. This estab-lishes a foundation for controlled tellurene thickness, further broadening its potential in diverse applications.VC 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International (CC BY-NC-ND) license (https://creativecommons.org/licenses/by-nc-nd/4.0/). https://doi.org/10.1063/5.0218276Since the discovery of graphene, intensive investigations intotwo-dimensional (2D) group III and V single-element transition met-als, including borophene,1 silicene,2 germanene,3 phosphorene,4 andantimonene,5 have gained significant momentum, due to their promis-ing semi-metallic and semiconducting properties.6 In 2017, 2D tellu-rium known as tellurene with a crystal structure consisting of atomichelical chains that spiral along one of the axes of the crystal lattice wassuccessfully fabricated.7,8 Bulk phase tellurium (Te) possesses intrigu-ing properties including semiconducting capabilities,9,10 photoconduc-tivity,11 thermoelectricity,12 and piezoelectricity,13 which are widelyused in electronics, sensors, optoelectronics, and energy devices.Tellurene not only well preserves these merits in its two-dimensionalform,14 but also exhibits extraordinary carrier mobility,15 environmen-tal stability,16 and low thermal conductivity,17 when compared to other2D materials, such as transition metal dichalcogenides (TMDCs) andblack phosphorus (BP).6,18,19 Tellurene’s intrinsic nanoscale propertiesdemonstrate potential for high-performance nano- and micro-devices,including field-effect transistors,20 Peltier coolers,21 and quantum Halleffect devices.22 Moreover, tellurene’s versatile optical properties sig-nify its potential as an outstanding candidate in future optical devices.For example, the exceptional light absorption of tellurene in the mid-infrared and infrared range offers a platform for the development ofAppl. Phys. Rev. 12, 011414 (2025); doi: 10.1063/5.0218276 12, 011414-1VC Author(s) 2025Applied Physics Reviews ARTICLE pubs.aip.org/aip/are 27 August 2025 00:09:57https://doi.org/10.1063/5.0218276https://doi.org/10.1063/5.0218276https://www.pubs.aip.org/action/showCitFormats?type=show&doi=10.1063/5.0218276http://crossmark.crossref.org/dialog/?doi=10.1063/5.0218276&domain=pdf&date_stamp=2025-02-18https://orcid.org/0000-0001-7987-3979https://orcid.org/0000-0001-7261-1060https://orcid.org/0000-0002-0962-1861https://orcid.org/0009-0000-0345-8269https://orcid.org/0000-0002-0269-4718https://orcid.org/0000-0002-0165-0481https://orcid.org/0000-0002-2195-0406https://orcid.org/0000-0002-5404-3909https://orcid.org/0000-0002-4536-1526https://orcid.org/0000-0001-6131-3906mailto:yuerui.lu@anu.edu.auhttps://creativecommons.org/licenses/by-nc-nd/4.0/https://doi.org/10.1063/5.0218276pubs.aip.org/aip/arefuture photonics, such as photodetectors.23–25 Meanwhile, given thewideband nonlinear saturation absorption and strong light–matterinteraction from the visible to infrared range, nonlinear photonic devi-ces based on tellurene can function as a saturable absorber,26 photonicdiode, and all-optical switcher.27Second harmonic generation (SHG) is a crucial nonlinear optical(NLO) process observed in crystals, where intense optical pumpingconverts energy into light with double its frequency, playing a key rolein frequency conversion for laser and imaging technology applica-tions.28 SHG studies in 2D materials are expected to make significantcontribution to the ongoing trend of miniaturization in optical devices,offering potential solutions to the limitations associated with conven-tional bulk materials like lithium niobate (LiNbO3).29 However, amongknown 2D materials, some stable candidates exhibit relatively weakSHG and require NLO modulation strategies to enhance perform-ace,29–32 while others demonstrate strong SHG potential but are proneto degradation in ambient conditions, such as GaSe33 and NbOI2.34These factors significantly influence the reliability and endurability oftheir sustained and robust SHG performance in practical applications.In previous studies, tellurene has been experimentally proven to haverobust piezoelectricity in ambient conditions,35 and it is theoreticallybelieved to possess a giant second-order coefficient v2 owing to its bro-ken inversion symmetry in the unique helical chain structure.36,37Despite the anticipated potential of 2D telluride (tellurene) to serve aspromising candidates for novel SHG emitters, there remains a lack ofcomprehensive studies experimentally elucidating its SHGperformance.In this work, extraordinary SHG performance under various exci-tation wavelengths has been experimentally observed from tellureneflakes, which are fabricated using the confined zone substrate configu-ration via the physical vapor deposition (PVD) method. This methodeffectively promotes a higher yield of high-quality tellurene flakes thin-ner than 30nm, compared to other vapor deposition methods.Comprehensive SHG measurements reveal that the intensity of tellur-ene flakes easily surpasses TMD monolayer films, by up to two ordersof magnitude, and moreover, the emission efficiency is highly sensitiveto the flake thickness and excitation wavelength. The hydrogen plasmaetching was thus adopted as the post-growth thinning process to real-ize modulation of the sample thickness, where the processed tellurenesamples with the well-preserved crystallinity consistently exhibit robustSHG performance across a range of excitation wavelengths. Theseresults emphasize the exceptional performance and potential of tellur-ene in nonlinear optics, while also highlighting the significant implica-tions of our methods for the production and processing of tellurene infuture applications.PVD is widely unutilized to synthesize high-quality single crystalfilms. Compared to the traditional way to place the substrate in PVDmethods for tellurene production, this work uses a face-to-face contactstacked substrate configuration and ultrahigh vacuum condition tosuccessfully produce tellurene flakes [Fig. 1(a)]. The high purity Teprecursor and stacked substrates are placed in the heating zone andgrowth zone near the insulation region prior to the growth procedure.An appropriately set distance between the heating and growth zones,coupled with proper pumping down procedures, synergistically con-tributes to the efficient deposition temperature for producing thin 2Dflakes (Supplementary material). With an increase in temperature, andthe application of a vacuum, the solid Te precursor vaporizes andconverts into Te molecules, which then can transfer into the narrowspace between substrates, finally depositing onto the bottom substrate[Fig. 1(b)]. During this process, appropriate temperature and timeempower Te molecules to overcome the formation energy barrier,enabling lateral movement for the van der Waals epitaxy.38Consequently, Te forms covalent bonds in a helical chain configura-tion, primarily aligned along the [000�1] direction, where this initialbonding arrangement triggers the growth of Te nanowires (1D), fol-lowed by the epitaxial growth in lateral and vertical directions, mani-festing in the formation of tellurene flakes [Figs. 1(b) and 1(c)].Stacking substrates appear to provide a confined-growth space config-uration, remarkably stabilizing the flow rate between substrates, whichestablishes a controllable growth environment for yielding thinflakes.39–41 While pumping down, the gas flow velocity outside of theconfined space is further increased to effectively lower deposition andcrystallization temperature, which is crucial for tellurene’s morphologi-cal evolution.24,42,43 Hence, the vacuum conditions and the arrange-ment of stacked substrates play vital roles in modulating the relevantgrowth energies and kinetics, contributing favorably to the formation,yield, and uniformity of thin tellurene flakes [Fig. 1(d)]. Taking advan-tage of this method, we find that more than half of the tellurene flakesare thinner than 35nm, and around 31% of the could be as thin as25nm [Fig. 1(e)], which is significantly larger yield compared to singlecrystal thin flakes (<50 nm) fabricated by other vapor depositionmethods,24,38,42,43 and is comparable to the products of solution syn-thesis methods.20 Raman tests, encompassing investigations into thick-ness, angle, and temperature dependencies, were conducted(supplementary material Note 2 and Figs. S1–S3), which strongly reaf-firm the high purity and crystalline quality of the synthesized tellurene,consistent with earlier findings in the literature.17,20,24,44Tellurene is believed to possess great potential for SHG, doublingfrequency of excitation light, owing to its helical chain structure andnon-centrosymmetric crystal structure [Fig. 2(a)].36,45 To confirmSHG properties, a tellurene flake was examined with a laser source at awavelength of 900 nm in response to varying excitation powers. Theresults demonstrate that the emission intensity became stronger as theexcitation power increased, which is described by a linear relationshipwith a fitting slope value approximately equal to 2.01 [Fig. 2(b)]. Thisindicated that the emission intensity is proportional to the square ofthe power of the laser beam, complying with the SHG principle,31,45–48further elucidating the SH emission of tellurene. In Fig. 2(c), a quickcomparison has been conducted between the tellurene flake and transi-tion metal dichalcogenide (TMD) materials, known for their robustSHG potential. Under identical excitation conditions, tellurene exhibitsa substantial SHG signal, surpassing the signal strength of variousmonolayer TMD flakes by nearly 100-fold. Moreover, it even outper-forms the SHG signal of similarly thick MoS2 with rhombohedralstacking by an order of magnitude. The extraordinary performanceobserved in tellurene is attributed to its giant second-order nonlinearsusceptibility, which is two orders of magnitude larger than that of theconventional nonlinear optical (NLO) crystal LiNbO3, and nearlytwice as large as that of another 2D NLO crystal, 3R-MoS2 (Fig. S4).Such high second-order nonlinear susceptibility is arisen from theunique lattice structure, where 2D flake is formed by stacking of triplehelix chain via van der Waals force. The quasi-one-dimensional struc-tures exhibit directional covalent bonding resulting in large opticalmatrix elements and large joint DOS for large v(2) values.49,50Applied Physics Reviews ARTICLE pubs.aip.org/aip/areAppl. Phys. Rev. 12, 011414 (2025); doi: 10.1063/5.0218276 12, 011414-2VC Author(s) 2025 27 August 2025 00:09:57https://doi.org/10.60893/figshare.apr.c.7623950https://doi.org/10.60893/figshare.apr.c.7623950pubs.aip.org/aip/areFIG. 1. Two-dimensional tellurene crystals synthesized with PVD method. (a) Schematic of tellurene flakes grown using the PVD method with a face-to-face contact substrateconfiguration under vacuum conditions. The face-to-face contact substrate is depicted in the inset. (b) Schematic diagram of the growth mechanism and morphologic transitionof tellurene flakes realized by delivering vaporized Te into the tight space between substrates, with time and temperature. The light gray spheres, rods, and flakes represent Teatoms, nano-wires, and thin tellurene flakes, respectively. (c) 3D illustration of the atomic structure of a tellurene nano-flake. (d) Atomic force microscopic (AFM) image of a rep-resentative tellurene flake with a thickness of 17.5 nm, where the height profile is extracted along the white dashed line. (e) Thickness statistics of tellurene flakes synthesizedby our illustrated method, where 15 to 25 nm thick flakes account for approximately 31% of the total tellurene yield.Applied Physics Reviews ARTICLE pubs.aip.org/aip/areAppl. Phys. Rev. 12, 011414 (2025); doi: 10.1063/5.0218276 12, 011414-3VC Author(s) 2025 27 August 2025 00:09:57pubs.aip.org/aip/areAdditionally, the existence of lone-pair electrons is conducive to gener-ating induced dipole oscillations in response to optical electric fields,thus leading to large SHG effects.36,51 The nonlinear anisotropicresponse was also investigated by employing a polarization-dependentexperimental setup to examine the angle-dependent SHG response ofthe tellurene flake under parallel configuration. As depicted in thepolar plot [Fig. 2(d)], the SHG intensity displayed a two-lobe symme-try, indicating the obvious dependence on the angle (h) between thepolarization of incident light and the orientation of the crystal lattice.Specifically, a maximum and minimum SHG response was observedFIG. 2. Characterization of second harmonic generation (SHG) in tellurene flakes. (a) Schematic illustration of SHG mechanism in a 2D tellurene flake, wherein a pump laserreflects the surface at double the frequency and half the wavelength. (b) Power dependence of the SHG signal with both axes presented in logarithmic scale. The spheres rep-resent the measured values, while the solid line indicates the linear fit with a slope of 2.01. (c) Comparison of SHG emission from a tellurene flake and several monolayer tran-sition metal dichalcogenide flakes under 900 nm illumination and the 3 R-MoS2 with similar thickness. (d) Polar plot of angle resolved SHG. (e) Analytical (blue solid line) andexperimental data (blue solid dots) for SHG emission vs tellurene thickness, ranging from 10 to 90 nm, under an excitation wavelength of 900 nm. (f) SHG measurement con-ducted on two flakes having thicknesses of 12.2 and 60 nm under varied excitation wavelengths, ranging from 830 to 1040 nm. The enhancement curve suggests two flakeswith different thickness show consistent response at different wavelengths but with different trends.Applied Physics Reviews ARTICLE pubs.aip.org/aip/areAppl. Phys. Rev. 12, 011414 (2025); doi: 10.1063/5.0218276 12, 011414-4VC Author(s) 2025 27 August 2025 00:09:57pubs.aip.org/aip/arealong the crystal direction [1210] and [0001], respectively, when thesample was rotated, and the orientation of the sample was perpendicu-lar and parallel to the polarized incident light. This distinctive aniso-tropic behavior could be attributed to the five non-zero and only twoindependent second-order NLO susceptibility elements induced bypoor crystalline symmetry (D43),36 which is corroborated by the well-fitted curve derived by the analytical solution (supplementary materialNote 3). Notably, 2D form of Te does not differentiate the anisotropicbehavior compared with the observation in 1D nanowires,45 owing tothe shared crystalline structure of triple helix chains present in bothforms.Flake thickness also substantially influences the SHG emission asdemonstrated in Fig. 2(e), which depicts the extracted SHG intensityfrom thicker tellurene flakes spanning a thickness range from 4 to90 nm, all measured at a wavelength of 900nm. When the samplethickness is between 17nm to approximately 30nm, there is a distinctdecrease in SHG, followed by an increase and decline in SHG intensi-ties associated with the thicker flakes [Fig. 2(e)]. This variation is alsoobserved in the tapered-face sample, where robust emission is appar-ent across regions with a thickness of less than 20nm, and the stron-gest local emission is centered around a thickness of 17 nm (Fig. S7).This behavior can be mainly attributed to two aspects, which are thevarying absorption for the pump light wavelength and the light inter-ference arisen within the flake. First, a linear optical reflection andtransmission measurement reveals that the thicker Te flake exhibitshigh absorption ability for pump light in the NIR range (Fig. S5),which indicates that the less portion of pump light can be convertedinto double frequency output light. This is further supported bythe results obtained from the transmission SHG measurements, as thestrong forward SHG intensity observed in the thinner regions ofthe flake diminishes as the flake’s thickness increases to larger values,as shown in Fig. S8. Meanwhile, the backward SHG behavior of the Teflake on the transparent substrate also exhibits a similar variation withthickness. This fact can be explained by the correlation between flakethickness and pump light wavelength. Specifically, when the Te flake isexposed to the pump light, the thinner Te flake causes the interferenceof the fundamental and SHG waves within substrate that serves as theFabry–P�erot cavity, reaching its maximum SHG when the thickness isaround 17nm. If the thickness keeps increasing, bulk nonlinear coeffi-cient and surface interface will dominate the emission process. Thenormal incident light reflects, bounces back, and travels forth betweentwo interfaces (Te–air and Te–substrate), resulting in a strong interfer-ence effect among the upwardly reflected lights. The round trip phaseof the fundamental wave inside the Te flake can thus lead to eitherweaker light–matter interaction and reduced SHG intensity or con-structive interference that enhances the overall SHG signal (Fig. S9).The finite-difference time domain (FDTD) simulation model (supple-mentary material Note 4) was employed to provide a numerical solu-tion and reaffirm the trend of experimental data regarding the varyingflake thickness under the excitation of a 900 nm source and otherwavelengths (Fig. S10). To investigate the influence of wavelength onSHG behavior for different thickness flakes, two different flakes havinga thickness of 12.2 and 60nm were measured under increasing excita-tion wavelengths, ranging from 830nm to 1040nm [Fig. 2(f)]. The12.2 nm thick flake exhibited the strongest emission at an excitationwavelength of 1040 nm and experienced an overall increasing trendwith increased wavelength. On the other hand, for another flake with60nm thickness, the largest emission was observed at a wavelength of870 nm, and the overall emission would decrease with a longer wave-length. A comparison between the SHG response of the 12.2 nm flakedivided by the SHG response of the 60nm thick flake responserevealed an increasing trend (denoted as the enhancement factor),where the shared second-order nonlinear optical susceptibility of tel-lurene results in consistent responses at given excitation wavelengths,but the thickness effect results in differing SHG magnitudes. Due tothe fact that tellurium becomes very absorptive around this wavelengthrange,36 a thicker flake does not possess a stronger light–matter inter-action for an enhanced SHG signal. As can be shown in FDTD simula-tion, the pump light gets reflected or absorbed before it reaches thedeeper part of the flake; thus, the SHG only happens strongly near thesurface (Fig. S9, supplementary material Note 4). This also emphasizesthe importance of selecting an appropriate thickness for Te flakes innonlinear optical device applications.Given that the proper thickness of a tellurene flake is an impor-tant factor in optimizing the nonlinear optical performance, an effec-tive method for modulating the flake thickness can further influencethe application prospects and value of 2D tellurene. Herein, hydrogen(Hþ) plasma was employed in this work as a post-growth treatmentprocess to systematically etch and reduce tellurene flake thickness. Asimilar approach was previously demonstrated to fabricate few layeredBP.52 When the Hþ plasma treatment process starts, Hþ collide anduniformly distribute on the top surface of the thick Te flakes, accord-ingly, continuously accelerated Hþ react with the surface Te atomsand form H2Te gas. As H2Te gas has thermal instability and can ulti-mately decompose into H2 and Te at room temperature,53 this etchingmethod effectively realizes the controllable removal of undesired mole-cules, thus reducing the thickness of the flakes [Fig. 3(a)]. Due to thedynamic equilibrium state of the reaction between Hþ and Te duringthe treatment process, thickness modulation provides a reliable etchingrate and homogeneity, as confirmed by characterization tests con-ducted prior to and after the treatment (Fig. S11). As illustrated inFig. 3(b), through monitoring every etching treatment, the same treat-ment duration produces a comparable etched thickness, leading to anoverall removal rate with a linear relationship between the etcheddepth and treatment duration. At the same time, to ensure the nonlin-ear optical properties of treated flakes, the crystallization and puritywere examined by Raman tests after each treatment. The spectraclearly showed three separate Raman active modes, and the sharppeaks suggest the high crystal clarity of the Te flakes following the thin-ning down process [Fig. 3(c)]. A significant blue shift for the E2 and A1modes can be observed when the flake thickness was reduced, becauseof hardened vibration modes induced by the modulation of interchainand intrachain interactions.20,54,55 This corresponds well with theresults of untreated flakes (Fig. S1) in previous reports.20,56Considering the safety concerns associated with the use of Te, mono-elemental Te and hydrogen gas can be efficiently separated and col-lected following the decomposition of H2Te using techniques such asmembrane gas separation and cryogenic distillation based on the dis-tinct physical properties of solid Te and gaseous H2. To further mini-mize the risks, encapsulation engineering provides an effective strategyto isolate Te flakes during its use, although it is thermodynamically sta-ble.16,57,58 For instance, Te flakes can be securely sealed within van derWaals (vdW) layered materials, which offer excellent gas impermeabil-ity, mechanical properties, and stability.59,60 Additionally, the recyclingApplied Physics Reviews ARTICLE pubs.aip.org/aip/areAppl. Phys. Rev. 12, 011414 (2025); doi: 10.1063/5.0218276 12, 011414-5VC Author(s) 2025 27 August 2025 00:09:57https://doi.org/10.60893/figshare.apr.c.7623950https://doi.org/10.60893/figshare.apr.c.7623950https://doi.org/10.60893/figshare.apr.c.7623950https://doi.org/10.60893/figshare.apr.c.7623950pubs.aip.org/aip/areand reuse of Te play a critical role in reducing potential harm to boththe environment and human health while promoting sustainableresource utilization.To provide comprehensive insights into the SHG variationinduced by the various thickness conditions of treated samples, atreated tapered-face flake serves as a great platform for investigatingvarying thickness and related SHG behavior [Figs. 4(a) and 4(b)].After the Hþ treatment, the tapered-face tellurene flake still has highcrystallinity and purity, regardless of the thickness change along thelength of the flake (Fig. S12). The SHG intensity distribution over theflake surface was imaged under different excitation wavelengths, aspresented in Fig. 4(b). As the wavelength increased from 830 to1040 nm, two strong emission regions with a distinct physical thick-ness difference had obvious variations in the SHG intensity. When theexcitation wavelength was larger than 900nm, the local emissionmaximum named ‘peak A’ had less emission intensity than that ofanother local maximum “peak B,” whose corresponding thickness isaround 67 and 17nm, respectively [Fig. 4(c)]. It is worth noting thateven though the treated sample surface is no longer smooth as com-pared to prior to Hþ etching, the treated flake remains responsive tochanges in the excitation wavelength. The positions of the two peaksboth exhibited a slight shift toward the thicker region of the flake asthe excitation wavelength became longer. For comparison, the FDTDmethod was utilized to simulate the SHG profiles using the smoothsurface obtained from the effective thickness of the treated flake, asdepicted in Fig. 4(d). Two emission peaks were observed in the simula-tion as well. Evidently, these peaks displayed a migration pattern simi-lar to that observed for the treated flake, wherein as the wavelengthwas altered from 830 to 1040 nm, both peak A and peak B simulta-neously move toward the thicker region. After the collection of moreFIG. 3. Investigation of post-growth thinning process of tellurene flakes via hydrogen plasma. (a) Schematic illustration of plasma-assisted post-growth thinning processemployed on tellurene flakes, with the thinning process based on the reaction between Te atoms and hydrogen protons in plasma gas. (b) Monitoring thickness change andetching rate from consecutive treatments, where the inset demonstrates the constant etching rate from the average etch thickness of each treatment with tens duration. Theerror bars represent statistical variation from 10 flakes for each treatment group. (c) Raman spectra of the processed tellurene flake with the thickness decreasing from 74 to6.8 nm.Applied Physics Reviews ARTICLE pubs.aip.org/aip/areAppl. Phys. Rev. 12, 011414 (2025); doi: 10.1063/5.0218276 12, 011414-6VC Author(s) 2025 27 August 2025 00:09:57pubs.aip.org/aip/areSHG profiles (Fig. S13), the simulated and experimental results exhibita positive linear relationship between peak location and wavelength,following thickness-dependent behavior. Notably, despite the differentroot mean square value of roughness (RMS) in the regions of A and Bpeaks, the larger RMS B peak (3.816 0.11 nm) and smaller RMS Apeak (2.656 0.16nm) both exhibit the same SHG behavior inresponse to the excitation wavelength. The location of peaks suggeststhat the roughness does not change the optimized thickness foremission. Meanwhile, comparing the migration distance of peaksbetween two sets of profiles reveals that the roughened surface oftreated samples induced by plasma etching shows no major disparityfrom the smooth flake, in terms of its thickness-dependent sensitivityto changes in the excitation wavelength. As shown in Fig. 4(e), the ratioof intensities from the two peaks reveals that the treated flake has greatagreement with the ratio obtained from simulated results of thesmooth flake, further elucidating the thickness-dependent nominalFIG. 4. SHG behavior of tapered tellurene flakes obtained with the post-growth thinning process. (a) AFM image of a tapered-surface tellurene flake after plasma treatment. (b)SHG mapping images of the treated flake shown in (a) under the excitation wavelength at 830, 900, 970, and 1040 nm, where the flake shape has been highlighted by whitedot lines. (c) Experimental height profiles and wavelength-dependent SHG intensity profiles extracted along black and white dashed lines in (a) and (b), respectively. (d)Analytical SHG profiles at corresponding wavelengths based on the mean height values of treated surface in (c), where discrepancy in the SHG peak could be attributed to theexperimental error introduced during the manual extraction of the profile lines. (e) Comparison of experimental and analytical SHG variation between the local maxima undervarious wavelengths.Applied Physics Reviews ARTICLE pubs.aip.org/aip/areAppl. Phys. Rev. 12, 011414 (2025); doi: 10.1063/5.0218276 12, 011414-7VC Author(s) 2025 27 August 2025 00:09:57pubs.aip.org/aip/areSHG emission relationship. Therefore, the roughness on the treatedflake does not appear to cause deviations of SHG emission induced bylight scattering and trapping. Consequently, the corresponding lightabsorption, phase matching, and light propagation are relatively unin-fluenced by the surface roughness during the SHG emission.61,62 Theemission efficiency of a treated flake is equivalent to the optimized per-formance from the untreated flake of the various thickness under cor-responding wavelengths.In summary, this work provided a comprehensive investigationinto the SHG performance of 2D tellurene flakes fabricated by con-fined space PVD growth, in addition to the SHG performance of thepost-process plasma treatment. The tellurene flakes exhibited extraor-dinary SHG emission and had strong dependence on the flake thick-ness and excitation wavelength. In addition, the application of plasmatreatment effectively reduced flake thickness, enabling tellurene toovercome the limitation of thickness and wavelength interaction.Tellurene’s SHG emission is experimentally observed to be about twoorders of magnitudes greater than monolayer TMDC counterparts at alarger thickness. This finding offers a fundamental concept to produceall-round 2D tellurium flakes with high SHG emission at various wave-lengths. As a result, the thickness engineered tellurene serves as anexcellent nano-scaled NLO material, which will find valuable applica-tions in NLO devices, such as electro-optical switches, frequency con-verters, phase matching, and light signal modulators.SUPPLEMENTARY MATERIALSee the supplementary material for the following: Note 1 indicat-ing the sample fabrication and characterization methodology; Note 2detailing Raman characterization; Note 3 demonstrating the mecha-nism behind SHG behavior; Note 4 elaborating FDTD simulation forSHG; Figures S1–S3 showing thickness, angle, and temperature-dependent Raman spectra; Fig. S4 comparing the second-order suscep-tibility among different samples; Figs. S5–S6 exhibiting transmission,reflection, and absorption spectra and extracted refractive indices; Figs.S7–S8 presenting thickness-dependent SHG in reflection and trans-mission modes; Fig. S9 showing electric field in SHG simulation; Figs.S10–13 showing Raman and SHG results after the treatment; and Fig.S14 illustrating the SHGmeasurement setup.ACKNOWLEDGMENTSThe authors acknowledge funding support from ANU Ph.D.student scholarship, Australian Research Council (ARC; Grant Nos.DP220102219, DP240101011, LE200100032) and ARC Centre ofExcellence in Quantum Computation and Communication Technology(Grant No. CE170100012), and National Heart Foundation (ARIES ID:35852). B. L. would like to acknowledge the support from Centre ofAdvanced Microscopy (CAM), Australian National University.AUTHOR DECLARATIONSConflict of InterestThe authors have no conflicts to disclose.Author ContributionsBoqing Liu: Conceptualization (equal); Methodology (equal); Writing– original draft (equal); Writing – review & editing (equal). ZongfuYu: Supervision (supporting). Yuerui Lu: Conceptualization (equal);Supervision (equal); Writing – review & editing (equal). Kun Liang:Conceptualization (equal); Methodology (equal); Writing – originaldraft (supporting). Qingyi Zhou: Investigation (supporting);Methodology (equal); Writing – original draft (supporting); Writing –review & editing (supporting). Ahmed Raza Khan: Investigation (sup-porting); Writing – original draft (supporting). zhuoyuan Lu:Investigation (supporting). Tanju Yildirim: Writing – original draft(supporting). Xueqian Sun: Investigation (supporting). SharidyaRahman: Investigation (supporting). 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