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Minh N. Bui, Stefan Rost, Manuel Auge, Lanqing Zhou, Christoph Friedrich, Stefan Blügel, Silvan Kretschmer, Arkady V. Krasheninnikov, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Hans C. Hofsäss, Detlev Grützmacher, Beata E. Kardynał

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[Optical Properties of MoSe<sub>2</sub> Monolayer Implanted with Ultra-Low-Energy Cr Ions](https://mdr.nims.go.jp/datasets/8dafa80f-e70c-403e-8a7c-2cccf8089b53)

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Optical Properties of MoSe2 Monolayer Implanted with Ultra-Low-Energy Cr IonsOptical Properties of MoSe2 Monolayer Implanted with Ultra-Low-Energy Cr IonsMinh N. Bui,* Stefan Rost, Manuel Auge, Lanqing Zhou, Christoph Friedrich, Stefan Blügel,Silvan Kretschmer, Arkady V. Krasheninnikov, Kenji Watanabe, Takashi Taniguchi, Hans C. Hofsäss,Detlev Grützmacher, and Beata E. Kardynał*Cite This: ACS Appl. Mater. Interfaces 2023, 15, 35321−35331 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: This paper explores the optical properties of anexfoliated MoSe2 monolayer implanted with Cr+ ions, acceleratedto 25 eV. Photoluminescence of the implanted MoSe2 reveals anemission line from Cr-related defects that is present only underweak electron doping. Unlike band-to-band transition, the Cr-introduced emission is characterized by nonzero activation energy,long lifetimes, and weak response to the magnetic field. Torationalize the experimental results and get insights into the atomicstructure of the defects, we modeled the Cr-ion irradiation processusing ab initio molecular dynamics simulations followed by theelectronic structure calculations of the system with defects. The experimental and theoretical results suggest that the recombinationof electrons on the acceptors, which could be introduced by the Cr implantation-induced defects, with the valence band holes is themost likely origin of the low-energy emission. Our results demonstrate the potential of low-energy ion implantation as a tool to tailorthe properties of two-dimensional (2D) materials by doping.KEYWORDS: transition-metal dichalcogenide monolayer, ultra-low-energy ion implantation, MoSe2, van der Waals heterostructure,photoluminescence, molecular dynamics, density functional theory1. INTRODUCTIONThe properties of semiconductors, especially atomically thinmonolayer (ML) semiconductors, depend strongly on thetypes and densities of defects in their crystal lattices. The mosttechnologically relevant defects are dopants, i.e., foreign atomsin substitutional positions in the crystal lattice. Shallowdopants introduce free electrons or holes into the conductionor valence band and thus change the semiconductorconductivity. As such, they facilitate the fabrication of p−njunctions, which underpins most active optoelectronic devices.Doping with transition-metal atoms has been shown tointroduce ferromagnetic order in p-doped semiconductors.1Impurity atoms can also trap electrons or holes or bindexcitons. Radiative recombination involving such states can bedetected as sub-band-gap photoluminescence (PL). Singleforeign atom that bind excitons have been explored for singlephoton sources.2 Alternatively, if the dopant atom has afunctionality of a spin qubit, the bound excitons provide anoptical readout of its state.3,4 The binding of excitons to thedopant atoms depends not only on electron and hole massesbut also on the dielectric constant of the semiconductors.Because of that, excitonic effects in bulk semiconductors areonly observed at cryogenic temperatures. Foreign atoms canalso act as color centers in semiconductors and insulators. Spinqubits based on the color centers have been realized indiamond5,6 or SiC.7,8In two-dimensional (2D) semiconducting transition-metaldichalcogenides (TMDs), which feature weak electrostaticscreening, substitutional atoms tend to introduce deep levels inthe band gaps.9 While excitons have considerable bindingenergies, they are predicted to be very weakly bound toindividual doping atoms.10 Optical transitions involving defectstates have been observed,11 with defects identified asvacancies.11−14 The transition responsible for the PL wasfound to occur between the hybridized defect states and 2Dlattice electronic states.Among several methods of doping bulk semiconductors, ionimplantation offers the highest flexibility in choosing implantedelements. Ion energies of tens of keV are used for implantationsince functional layers can be even a hundred nanometersbelow the surface. High-energy ion implantation has been usedReceived: April 14, 2023Accepted: June 23, 2023Published: July 11, 2023Research Articlewww.acsami.org© 2023 The Authors. Published byAmerican Chemical Society35321https://doi.org/10.1021/acsami.3c05366ACS Appl. Mater. Interfaces 2023, 15, 35321−35331Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on July 29, 2023 at 02:05:08 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Minh+N.+Bui"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Stefan+Rost"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Manuel+Auge"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Lanqing+Zhou"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Christoph+Friedrich"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Stefan+Blu%CC%88gel"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Silvan+Kretschmer"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Silvan+Kretschmer"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Arkady+V.+Krasheninnikov"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kenji+Watanabe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Taniguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hans+C.+Hofsa%CC%88ss"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Detlev+Gru%CC%88tzmacher"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Detlev+Gru%CC%88tzmacher"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Beata+E.+Kardyna%C5%82"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acsami.3c05366&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=abs1&ref=pdfhttps://pubs.acs.org/toc/aamick/15/29?ref=pdfhttps://pubs.acs.org/toc/aamick/15/29?ref=pdfhttps://pubs.acs.org/toc/aamick/15/29?ref=pdfhttps://pubs.acs.org/toc/aamick/15/29?ref=pdfwww.acsami.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acsami.3c05366?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://www.acsami.org?ref=pdfhttps://www.acsami.org?ref=pdfhttps://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://acsopenscience.org/open-access/licensing-options/to modify 2D materials,15−17 but its efficiency is low in thiscase, as most atoms go through the 2D target.18 Moreover, theions penetrating through the ML can cause undesirable effects,e.g., trapped charges in the substrate. Implantation into 2Dmaterials has the highest implantation efficiency with ionenergies in the range of tens of eV. At these energies, theimplantation efficiency and threshold energy depend on theions’ mass and also chemical properties.19 Ultra-low-energy ionimplantation20,21 has recently been demonstrated to beefficient in doping graphene using 40 eV Mn ions22,23 or inSe ion implantation into MoS2 with an ion energy of 20eV.24,25 The ratio of the replaced S atoms with Se in the topsublattice was sufficient to form a Janus compoundMoS2−2xSe2x as indicated by Raman spectroscopy and thetransmission electron microscopy imaging.Here, we study the optical properties of molybdenumdiselenide (MoSe2) ML implanted with 25 eV 52Cr+ ions. Sub-band-gap defect-induced PL emission was observed only at thelow n-doping level and with saturation behavior, characteristicof defects with low density. Ab initio molecular dynamics(MD) simulations of the implantation process were performed,and possible configurations of the Cr atoms in the MoSe2 MLlattice were outlined to understand the atomic structure of theimplanted MLs. The optical properties of the MoSe2 ML withsuch defects were calculated using density functional theory(DFT). The most probable defect configurations wereidentified by combining experimental data and theoreticalcalculations.2. RESULTS2.1. Sample Preparation and Ion Implantation. Asample for ion implantation was prepared by mechanicalexfoliation of the MoSe2, graphite (Gr), and hexagonal boronnitride (hBN) flakes and their sequential transfer onto the Si/SiO2 substrate with pre-patterned Ti/Au contacts. The use ofthe dry viscoelastic transfer technique26 ensured that thesurface of the ML was sufficiently clean for the implantation.The MoSe2 ML has to be grounded during the implantation.An electric contact to the ML was provided by placing themultilayer part of the exfoliated MoSe2 flake on a Ti/Au metalcontact. The ML was placed atop a graphite gate connected toanother Ti/Au contact. The ML was separated from the gatewith an hBN flake. Once completed, the device was implantedwith 52Cr+ ions at 25 eV and a fluence of 3 × 1012 cm−2(equivalent to 0.003 Cr per ML MoSe2 unit cell, using theMoSe2 ML in-plane lattice constant 3.32 Å27). Theimplantation was performed with the device heated to 220°C. Following the implantation and initial characterization,another hBN flake was deposited on the ML MoSe2 for fullencapsulation, which protects the ML from interactions withthe environment during the optical measurement and reducesthe inhomogeneous broadening in the PL linewidth.28 Thedevice was annealed at 150 °C to improve the interfacebetween the constituent layers.28 The complete device isshown in Figure 1. More details are available in Section 5.2.2. Optical Spectroscopy Data. Figure 2a shows PLspectra of weakly electron-doped pristine and Cr-implantedMoSe2 MLs, measured at 10 K with the same laser power of 1μW. Both spectra show similar features around the band-gaptransitions, with an emission line from neutral excitons (X) andnegative trions (X−). The red shift of these transitions in theCr-implanted sample (Figure 2a) is most likely due to adifference in the dielectric environment or strain between thesamples. The level of implantation is too low to expect changesin the band gap.29 Significant homogeneous broadening of theX line was determined by the Voigt function fitting. It resultedin the Lorentzian width of nearly 7 meV for the implantedsample, compared to about 2 meV for the pristine sample,suggesting a much shorter lifetime of the former. The mostsignificant difference between the samples is a broad emissionat around 1.51 eV, which we label D.The relative intensity of the D peak compared to X− and Xdepends on the excitation power P (Figure 2b). It is the mostintense line at low laser excitation powers (P < 1 μW) butsaturates as the laser power increases, while X− and X continueto grow linearly. The saturating behavior of the D intensity,shown in Figure 2c, can be expressed phenomenologically as+I PP Psat (1)with saturation power Psat ≈ 10 μW. The saturating behavior isexpected when the exciton generation rate exceeds therecombination rate of the states responsible for D. Thesaturation threshold depends on the density of states and thelifetime of the recombining carriers.11,13 The low threshold forD is consistent with the low implantation level. The lifetime ofcarriers was measured from time-resolved PL. The decay of thepopulation of the excited states contributing to the D peakafter a pulsed excitation can be seen in Figure 2d. As can beexpected from measuring an ensemble of emitters, the decay isFigure 1. Cr-implanted MoSe2 ML with hBN encapsulation and graphite backgate. (a) Micrograph of the finished device. The ML part of theexfoliated MoSe2 flake is encapsulated between two thin hBN flakes. The few-layer graphite backgate and the thick part of MoSe2 flake makecontacts with the two Ti/Au lines to the right. (b) Schematic diagram of the device cross section. Backgate voltage Vg can be applied to the graphitebackgate via the Au contact, while the MoSe2 flake is grounded via the other Au contact.ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c05366ACS Appl. Mater. Interfaces 2023, 15, 35321−3533135322https://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig1&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c05366?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asnot a single exponential. The very fast initial decay ofpopulation by about 10%, which is faster than the timeresolution of the experiment, is followed by a slower decaywith the 1/e decay time of around 14 ns. These decay times are2−3 orders of magnitude longer than the lifetime of freeexcitons in MoSe2 MLs30,31 and one order of magnitude longerthan that from the isolated, confined excitons,32 pointing to alow oscillator strength of the emitters.The doping-dependent PL from neutral and chargedexcitons shown in Figure 3a is typical of MoSe2 MLs. Whenincreasing the gate voltage, the D emission only emerges afterthe signal from the positive trion, X+, entirely disappears. Itreaches the maximum intensity around 1 V; the gate voltage ofthe transition between X and X− dominated spectra. Theblueshift of the D emission at higher gate voltage is likely to bedue to the energy renormalization due to the screening by thefree charges in the ML. Similar behavior for defect peaks wasobserved by others.33 The D line weakens strongly as the X−line intensifies with a further gate voltage increase. Thisbehavior suggests a competition between the exciton capture atthe defect state and the formation of an X−. This scenario issupported by the PL excitation (PLE) spectroscopy measure-ment (Figure 3b), which shows that the intensity of the Demission is maximum when the excitation wavelength isresonant with the energy of X around 1.637 eV. The Demission was not excited with laser resonant with X− (around1.608 eV) even though the PL spectrum, measured at the sameFigure 2. PL of Cr-implanted MoSe2 ML at 10 K. (a) PL spectra of Cr-implanted MoSe2 ML (red curve) at low n-doping (Vg = 0.8 V), plottedwith that of pristine MoSe2 ML (black). In addition to the X− and X from MoSe2 ML, the Cr-implanted sample also shows the broad D peak ataround 1.51 eV. (b) PL spectra of Cr-implanted ML under laser power ranging from 36 nW to 123 μW. Spectra are normalized to X−. Here, thesample is slightly n-doped at Vg = 0.8 V. (c) Power dependence of PL. Best-fit lines (dashed), with their standard deviations (shaded region aroundthe lines), are plotted together with the extracted intensity from PL spectra (dots). Unless explicitly shown, the error bars are smaller than the sizeof the data points. X− and X are fitted with power law I ∝ Pα, and D is fitted with the saturation curve described by eq 1. (d) Time-resolved PL ofCr-implanted MoSe2. 1/e time is around 14 ns.ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c05366ACS Appl. Mater. Interfaces 2023, 15, 35321−3533135323https://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig2&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c05366?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asdoping level, clearly shows that the ML is doped with electrons(X− emission is the strongest PL signal).Figure 4a compares gate-dependent PL at 22 and 108 K.While X− emission is the most intense line in the spectra at thelower temperature, it is very weak at the higher temperature. Xbecomes the strongest line, but D diminished less than X− andremains up to room temperature (Supporting Figure S4b). Wetrace the change of the PL signal counts from X− and D, bothnormalized to X signal counts, on the Arrhenius plot shown inFigure 4b. X− dissociates into higher-energy X and an electronat a higher temperature. Its intensity can be fitted with thestandard Arrhenius formula34=+ ( )I TIA( )(0)1 exp Ek TaB (2)where I(0) is the PL intensity at temperature 0 K, A is aproportionality constant, Ea is the activation energy for thedissociation of X−, and kB is the Boltzmann constant. Fittingthe formula to the data gives Ea ≈ 32 ± 5 meV, which isexpected for a binding energy of the trion.10,35 The D emissionintensity first increased with temperature up to around 34 Kbefore diminishing. To account for this initial increase inintensity, we assume that the trapping of carriers thatrecombine requires overcoming the activation energy. Weuse a modified multilevel model for the temperaturedependence of the D intensity34=++( )( )I T IAA( ) (0)1 exp1 expEk TEk T12a1Ba2B (3)where A1 and A2 are the proportionality constants, and Ea1 andEa2 are the activation energies for trapping and detrapping ofcarriers, respectively. Fitting of the D peak yields Ea1 ≈ 1.2 ±0.8 meV and Ea2 ≈ 30 ± 7 meV.Temperature affects not only the intensity but also the Demission energy. The temperature-dependent energy shift of Xand D lines can be described by the modified Varshnirelation13,36 asFigure 3. Doping level and excitation energy dependencies of PL. (a) Gate-dependent PL, where the backgate voltage Vg was varied from −12 to 8V to tune the doping level in the ML from p- via neutral to n-doping. The carrier concentration n is calculated using the simple parallel platecapacitor model (more details in the Supporting Note 7). (b) PLE of the D peak, taken at Vg = 0.7 V. The D peak intensity was integrated aroundits PL emission energy between 1.48 and 1.52 eV. Inset: PL spectrum under a 688 nm (1.80 eV) excitation under the gate voltage as applied for thePLE measurements.Figure 4. Temperature dependence of PL emission. (a) Gate voltage-dependent PL at 22 and 108 K. (b) Arrhenius plot of X− and D (symbols). Ateach temperature, the two peaks’ integrated intensities were acquired at the doping levels, where each emission is the brightest. The intensities werethen normalized to that of X (at the voltage where X is most intense). The best fit lines were according to eqs 2 and 3. (c) The temperature-dependent band gap of X and D (symbols). The best-fit line was according to eq 4.ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c05366ACS Appl. Mater. Interfaces 2023, 15, 35321−3533135324https://pubs.acs.org/doi/suppl/10.1021/acsami.3c05366/suppl_file/am3c05366_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c05366/suppl_file/am3c05366_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig4&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c05366?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-as=ÄÇÅÅÅÅÅÅÅÅÅÅÉÖÑÑÑÑÑÑÑÑÑÑE T E Sk T( ) (0) coth21g gB (4)where Eg(0) is the emission energy at 0 K, S is the electron−phonon coupling, and ⟨ℏω⟩ is the average phonon energy.Fitting gives ⟨ℏω⟩ = 11.1 ± 1.3 meV and S = 1.82 ± 0.14 forX, similar to the reported values (⟨ℏω⟩ ≈ 12−20 meV, S ≈2.37−39). For the D emission, the fitted ⟨ℏω⟩ and S are 10.7 ±1.5 and 0.79 ± 0.09 meV, respectively. A smaller S constantcompared to excitonic lines has been reported for vacancy-induced PL emissions from TMD MLs13,14,40 and explained asa result of the defect being decoupled from the conductionband, which varies with the temperature. A similar scenario isalso likely to be the case in our sample.To gain further insight, we measured the PL emission fromthe sample under the out-of-plane magnetic field B varyingfrom −8 to 8 T. Figure 5a shows the splitting of X and X−spectra in two circular polarization detection states under theapplied B-field. The valley splitting, caused by the Zeemaneffect41,42 and defined as= =+E E E g BZ B (5)(where Eσ+ and Eσ− are the emission energy in the detectedcircular polarization basis σ+ and σ−, respectively, g is theLande ́ g-factor, μB is the Bohr magneton) changes linearly withthe applied magnetic field (Figure 5b). The g-factors derivedfrom the data are −3.69 ± 0.04 and −4.80 ± 0.03 for X andX−, respectively. The g-factor value for X is close to the onesfrom the previous experimental work,28,41−45 which arebetween −3.8 and −4.3 and well within the expected rangefrom −3.22 to −3.82 predicted by recent ab initiocalculations.46,47 The g-factor for X− is slightly higher thanfor X but similar to the values observed for samples underhigher doping level.43−45 On the other hand, the D emissionshows little change with the magnetic field (Figure 5c).Comparing the energy of photons from the D peak in bothcircular polarization gives a g-factor of about −1.18 ± 0.06.The peak position was determined by fitting the data withthree Voigt functions and then taking the maximum of thefitted line. The uncertainty here is high, partly owing to the Dpeak’s large width.2.3. First-Principles Molecular Dynamics Simulationof Cr-Ion Implantation into MoSe2 ML. To get insightsinto the defect formation process and types of defects that canappear upon impacts of energetic Cr ions, we carried out DFTMD simulations, as described below. The atomic structure of afree-standing MoSe2 rectangular slab containing 90 atoms wasfully optimized; then, a Cr atom was placed 6 Å above itssurface (Figure 6a) and a kinetic energy of 25 eV was assignedto the atom. Normal incidence was simulated; that is, theinitial velocity vector of the projectile was orientedperpendicular to the surface of the ML. The projectile wasassumed to be a neutral atom, as at such low energies and lowFigure 5. Magneto-PL measurement of Cr-implanted MoSe2 ML at 1.8 K. PL spectra acquired with out-of-plane magnetic field B (varying between−8 and 8 T) applied to the sample and excited with an H-polarized laser. The detection is set to measure either σ+ (black) or σ− (red) polarizationstates. The figure shows the polarization-resolved PL spectra of (a) X and X−, (c) D, with their Zeeman splitting ΔEZ shown in (b, d). The splittingwas calculated from the peak positions (extracted from fitting Voigt functions to the PL spectra), with the error bars representing the propagatedstandard deviation of the fit procedure. The Zeeman splitting of the three emissions reveals expected g-factors around −4 for X and X−, only around−1.18 for D.ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c05366ACS Appl. Mater. Interfaces 2023, 15, 35321−3533135325https://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig5&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c05366?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ascharge states, its neutralization must occur well before itreaches the surface. We note that DFT MD on the Born−Oppenheimer surface cannot describe the evolution of chargetransfer anyway, and the Ehrenfest dynamics48,49 should beused. 21 impact points were selected in the irreducible area ofthe primitive cell of MoSe2 (Figure 6b), and the outcomes ofthe simulations were averaged with the corresponding weights.The MD runs continued until the kinetic energy brought up bythe projectile was distributed over the whole supercell(normally after a few picoseconds); then, the system’stemperature was quenched to zero, and the atomic structurewas analyzed. Although the effects of substrate on defectgeneration in a 2D system can be significant for ions withmuch higher (keV range) energies,50−52 the role of thesubstrate should be minimal for impacts of 25 eV Cr ions ontoMoSe2, so that a free-standing slab was simulated. Spin-polarized calculations were carried out. Although computa-tionally more efficient non-spin-polarized method with acorrection for isolated atom polarization energies can beused to simulate irradiation effects,19 the account for spineffects is particularly important for Cr, as it is magnetic, whichaffects the energetics of defect configurations.Figure 6c shows the most common atomic configurationsthat appear after Cr atom impacts. These are Cr adatoms, X-sub configuration (Cr at Se sites with a Se adatom53),interstitials (the Cr atom between Mo atoms), and substitu-tional defects in Mo and Se sites, which are Cr@Mo and Cr@Se, respectively. Table 1 lists the probabilities for the defects toappear. Ion irradiation also gives rise to the sputtering of Seatoms, that is, the formation of Se vacancies (VSe), but theseevents were not so common.According to the DFT MD simulations, the most probabledefects that appear upon 25 eV Cr-ion irradiation are Cradatoms, Cr@Mo, and X-sub defects. The Cr atoms that passthrough the MoSe2 sheet will likely form adatoms attached tothe bottom of MoSe2. Self-annealing of defects at finitetemperatures at which irradiation was carried out in theexperiment can affect their concentrations in the implantedsamples. To get insight into the possible evolution of defects,we assessed the defect formation energies Ef, as donepreviously.53 For adatoms, interstitials, and X-sub defects, Efwas calculated as the energy difference between the systemwith the Cr atom and the pristine system plus isolated Cratom. For the Cr@Mo, Cr@Se, and VSe configurations, theenergies of isolated Mo and Se atoms were also taken as areference. We note that the listed defect formation energies forthe Cr@Mo, Cr@Se, and VSe cannot be used to assess theequilibrium concentrations of these defects, as the chemicalpotentials were chosen to match isolated, that is, sputtered,atoms. This can be done, though, if the chemical potentials ofthe displaced Se and Mo atoms are chosen in such a way thatthey reflect the actual experimental conditions that thepotential can be anywhere between the values correspondingto the Se- or Mo-rich limits. This would result in lowerformation energies, as the sputtered atoms would beincorporated into the lattice. It can also be assumed that thedisplaced Se atoms form Se clusters at the surface, whichwould give rise to the lowering of Cr@Se defect energies.As evident from Table 1, Ef for adatoms is lower than for theinterstitials, so that at finite temperatures, the interstitials willmost likely be “pushed away” from the Mo plane and formadatoms. We note that this result was obtained for a relativelysmall 90-atom supercell, and in the larger system, thedifference between these energies is smaller, as reportedearlier.53 Nevertheless, even for equal formation energies atzero temperature, with an account for the entropic term in theGibbs energy, the probabilities for the adatoms should behigher due to a larger configurational space. Some X-subdefects may also be converted to Cr@Se configurations,especially in the Mo-rich limit, when Se vacancies are present,but the energetics of this process naturally depends on theexperimental conditions, that is, the choice of Se chemicalpotential. The Cr@Se defects can also appear due to theadsorption of Cr atoms on Se vacancies, as this is energeticallyfavorable due to the saturation of dangling bonds. Thus, onecan expect that the most prolific defects in the samples are Cradatoms (or Cr clusters on top of MoSe2), X-sub, as well asCr@Mo and Cr@Se substitutional configurations.2.4. DFT Calculations of Optical Properties. Toinvestigate if Cr defects introduce states in the band gap ofthe MoSe2 ML that are optically active, we have simulatedoptical absorption spectra for MoSe2 with Cr defects in variouspositions. The simulations are based on a 5 × 5 supercell. Eachsupercell hosts one Cr defect. The unfolded band structuresare shown in the Supporting Figure S5. All defects give rise tostates in the band gap. The absorption spectrum Im[ε(ω)]with the energy-dependent macroscopic dielectric functionε(ω) has been calculated within the random-phase approx-imation in the limit k → 0. Optical matrix elements and local-field effects are taken into account. It should be pointed outthat self-energy corrections (such as GW) or electron−holeinteractions (as described by the Bethe−Salpeter equation) areneglected. Self-energy corrections and electron−hole inter-actions are known to have a partially compensating effect onFigure 6. First-principles MD simulation of the ion implantationprocess into ML MoSe2. (a) The setup for simulations of ion impacts.(b) Impact sites used in the simulation. (c) Atomic structures of thedefects likely to appear upon impacts of energetic Cr ions.Table 1. Results of DFT MD Simulations of a 25 eV Cr-IonIrradiation on a Single-Layer MoSe2ap Ef [eV]adatom 0.16 −0.85X-sub 0.41 −0.76interstitial 0.08 −0.25Cr@Mo 0.21 3.03Cr@Se 0.04 2.57VSe 0.01 5.41passed through 0.09 0.00aThe probabilities p of likely defect configurations to appear alongwith the formation energies Ef of these configurations are listed.ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c05366ACS Appl. Mater. Interfaces 2023, 15, 35321−3533135326https://pubs.acs.org/doi/suppl/10.1021/acsami.3c05366/suppl_file/am3c05366_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig6&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c05366?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asthe band gap:54 while the former tends to increase the bandgap, the electron−hole interactions make the optical band gapsmaller. Due to this compensating effect, the presenttheoretical results can be seen as approximate spectraspecifically showing the impact of the defects. However,quantitative differences between theory and experiment shouldbe expected due to the neglect of many-body effects and alsodue to the difference in the dielectric environment. Thetheoretical spectra are shown in Figure 7. Absorption belowthe band gap (around 1.6 eV) is present for all defects andoriginates from transitions involving the defect states.There is an optical transition for X-sub at 1.5 eV, which is inthe same energy range of the D emission from the PL spectrabetween the valence band and an acceptor state of X-sub. Thisstate results from the coupling of the conduction band at theK-point with the Cr defect state. The energy of this transitionis similar to that of the MoSe2 ML with a vacancy.55−58The weak transition involving a deep acceptor state at 0.9 eVis outside the spectral range of our experiments. We note thatthe coupling between the conduction band and the defect stateshifts the conduction band minimum from K toward the Γpoint (Supporting Figure S5). However, the resultingsuppression of PL would not be visible in the experimentdue to the low density of defects and only the local opening ofthe band gap.Well-defined spin-degenerate acceptor levels are alsointroduced by Cr substituting the Mo atom in the lattice(Cr@Mo). Optical transitions from this defect state into thevalence band states can be seen in Figure 7 in the rangebetween 1.4 and 1.5 eV, which is also in a similar energy rangeto the D peak PL emission. The band-to-band transition isshifted to higher energy compared to the pristine MoSe2 MLbecause of the coupling between the conduction band and thedefect state. However, similar to the X-sub configurationdiscussed above, it is unlikely to observe this blueshift in thePL spectra due to the low defect density.The coupling of the defect and conduction band results in agradual increase of the above-band-gap absorption for Crsubstitution into the Se site (Cr@Se). This defect type alsointroduces a donor state at the Fermi level and two single-spin,deep defect levels. The signal from the donor state merges withthe band-to-band absorption. Otherwise, the Cr defect at theSe site hardly affects the MoSe2 band structure. Several weakoptical transitions are present at a large range of energies(down to 500 meV below the band gap).Interstitial Cr introduces several deep defect levels in theband gap, and again the highest state couples to theconduction band shifting the conduction band minimum tothe Λ point. The absorption spectrum does not containdiscrete absorption lines but a gradually increasing absorptionfrom 1.2 eV.3. DISCUSSIONRadiative recombination of an electron (e−) bound to a defectstate with the valence band hole (h+) can explain the measuredPL. Considering that our DFT calculations do not show donorstates at high enough energy, the electron here is likely tooccupy an acceptor. In this scenario, an exciton bound to anegatively charged acceptor (A−X) dissociates into A−h+ and afree electron in the conduction band. Following radiativerecombination, A−h+ becomes neutral acceptor A0. Theoreticalmodeling of A−X indicated a binding energy of only a few meVcompared with the A0 + e− state,10 which is of the same orderof magnitude as the activation energy of the D line determinedfrom the Arrhenius plot. Among the potential defects identifiedby MD calculations, Cr@Mo, X-sub, and Cr@Se have nonzeromatrix elements for optical transitions between acceptor statesand valence band. Other configurations, e.g., interstitial Cr orSe vacancies, are unlikely to be present. Besides, neither wouldexplain the data (see Figure 7 and Supporting InformationNote 4).The measured 1/e recombination time is longer than thelifetimes reported for band-to-band and localized staterecombination in MoSe2. Low oscillator strength of thetransition can result from the spatial separation of electronsand holes, as for Cr@Se or X-sub. However, since this lifetimeis longer than that of the spin dark states in WSe2, which isonly a few nanoseconds,59 this transition could also be from aspin-forbidden state. Such a state would correspond to thecharge configuration of A0 for the Cr@Se defect in the absenceof exchange interactions between electrons in the conductionband.The g-factor of the D emission is negative but much smallerthan those for X or X−. With large g-factors for electrons in thevalence band, it implies either a large g-factor for an electronon the acceptor level near the conduction band (e.g., in Cr@Mo or X-sub configuration) and valley-selective transitions orreduced g-factor of holes in the valence band. The latter couldbe caused by the hybridization of the valence band with thedefect level as in the Cr@Se configuration. Further insightwould require higher magnetic field measurements andtheoretical input.4. CONCLUSIONSIn conclusion, we demonstrated the ultra-low-energy ionimplantation of Cr ions (at 25 eV) into a MoSe2 ML. Theimplantation was performed with ions at 25 eV and an ionfluence of 3 × 1012 cm−2; the resulting material retains highoptical quality as evidenced by clear excitonic PL. ImplantedCr ions introduce an additional low-energy PL signal at around1.51 eV visible at the onset of n-doping. Molecular dynamicscalculations identified defects that can be generated byimplantation. We found that Cr atoms can substitute forFigure 7. Comparison of calculated absorption functions for pristineMoSe2 ML (black line) and MoSe2 ML with Cr in various positions ofthe crystal structure: Cr at the interstitial position (red), Cr at the Moposition (Cr@Mo�blue), Cr at the Se position with additional Seadatom (X-sub, green), and Cr at the Se position (Cr@Se, purple).Spectra are offset vertically for clarity. The vertical dotted line at about1.6 eV marks the calculated band gap of pristine MoSe2 ML.ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c05366ACS Appl. Mater. Interfaces 2023, 15, 35321−3533135327https://pubs.acs.org/doi/suppl/10.1021/acsami.3c05366/suppl_file/am3c05366_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c05366/suppl_file/am3c05366_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c05366/suppl_file/am3c05366_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?fig=fig7&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c05366?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asboth Mo and Se atoms. In the latter case, the Cr atom isslightly more likely to bind an additional Se atom than not.The defects’ stability, including interstitial Cr, depends on thepost-implantation treatment and the final configuration of Seand Mo, which are not in the lattice. DFT calculations revealedthat all of the probable defects introduce one or more defectstates in the MoSe2 band gap with nonzero matrix elements foroptical transitions. It is impossible to identify with certaintywhich defect is the origin of the D line, Cr at the Se site withthe Se adatom (X-sub), and perhaps Cr at Mo (Cr@Mo)seems to fit best with the measured data. Further experiments,for example, the implantation of Cr only into the Se sublatticeor implantation through the hBN protective layer to avoidenvironmental changes, could be considered to distinguishbetween the cases.More generally, this study shows that implantation ofheavier elements into the metal sublattice of TMD MLs ispossible without the visible loss of material quality asevidenced by the unchanged excitonic PL of gated MoSe2.However, the implantation process is complex, and simulationsof the possible outcomes are necessary to identify materialsystems of the desired properties. In the search for singlephoton emitting sites, it is also worth noting that uponimplantation with a very low fluence, it should be possible toaddress individual Cr atoms at different lattice sites.Implantation of foreign atoms might be used to introducecatalytic sites in 2D materials or to add functionalization suchas magnetism.5. METHODS5.1. Atomistic Simulation. We used DFT MD as implementedin the VASP code.60,61 The Perdew−Burke−Ernzerhof (PBE)exchange and correlation functional was employed.62 The evolutionof the system was modeled using the microcanonical ensemble. Acutoff value of 300 eV was chosen for DFT MD, and sampling overthe Brillouin zone was done using a 3 × 3 × 1 k-point mesh. The timestep was chosen to be 0.1 fs, which provided energy conservationbetter than 0.1 eV.5.2. Band Structure and Absorption Spectral Calculation.Density functional theory (DFT) simulations were performed insupercells of 5 × 5 primitive unit cells. Each was constructed withlattice constants of a = 3.28 Å and c = 12.918 Å of the hexagonallattice. An internal structure parameter of z = 0.125 was used. Thedefect systems were spatially relaxed using FLEUR63,64 until theresidual atomic forces had fallen below 5 × 10−2 eV/Å. Thesubsequent calculation of the macroscopic dielectric function inSPEX65,66 is based on the random-phase approximation67,68 andincludes local-field effects. Calculations of 2D materials with 3Dperiodic boundary conditions are computationally expensive becausethe decoupling of neighboring layers in the z direction requires largesupercells in this direction. In the case of 2D systems with defects, thecomputational cost grows considerably, particularly in the case of lowdefect concentrations, because suppressing the unwanted defect−defect coupling requires large supercells in the x and y directions. Tofacilitate the calculations of the dielectric function, we had to reducethe reciprocal cutoff radius from 4.1 to 3.6 Bohr−1 in the case of theX-sub defect system. However, this should not affect the form of therespective spectrum shown in Figure 7.The band structures presented in the Supporting Information aremade up of 320 k points along the unfolded high-symmetry path Γ−M−K−Γ. Here, “unfolded” means that the high-symmetry points referto those of the defect-free MoSe2 ML. The necessary unfolding of theband structures of the defect systems has been carried out with a newimplementation55 in the FLEUR code, adapting the techniquedescribed in ref 69 to the LAPW basis.70 In this technique, a spectralweight is assigned to each state plotted in the band structure. Theweight wn(k) for the n-th state at k of the unfolded path is given by= * · ·w C C Sk G G k( ) ( ) ( ) ( )n n nG Gk k GG, (6)where k′ = k + G″ with a suitable reciprocal lattice vector G″ thatfolds k back into the (smaller) Brillouin zone of the defect system.The G′ sum runs over the set of all reciprocal lattice vectors (of thedefect system) at k′, and the G̃ sum runs over the set of reciprocallattice vectors (of the pristine system) at k. The latter is a subset ofthe former. The wave functions are represented in the LAPW basis{χkG(r)} with coefficients Ckn(G) and overlap matrix SGG′(k) =⟨χkG|χkG′⟩.555.3. Sample Preparation. Si with 90 nm thick dry-thermallygrown SiO2 chips with 60 nm thick Ti/Au contacts (pre-patterned byelectron beam lithography) were used as the substrate. Before flaketransfer, the chips were cleaned in acetone and isopropanol (IPA)under bath sonication, blown dry with N2, and treated with oxygenplasma (300 W, 200 sccm for 10 min). Few-layer graphite, MoSe2(from 2Dsemiconductors) MLs, and hBN (from Takashi Taniguchiand Kenji Watanabe) multilayers were mechanically exfoliated frombulk crystal using polydimethylsiloxane (PDMS) stamps (Gel-pakDGL X4 films) and transferred onto the substrate using the dryviscoelastic transfer process.26 The process was performed in a N2-filled glovebox. After transferring the graphite (5.5 nm thick) andbottom-hBN (20 nm thick) flakes, the sample was annealed in a H2/Ar (1:10 ratio) atmosphere at 300 °C for 3 h to improve the topsurface for the subsequent MoSe2 ML transfer. After transferring thetop-hBN (15 nm thick), the sample was annealed in low vacuum (5 ×10−3 mbar) at 150 °C for 2.5 h to improve interfaces in the vdWstack. Electrical contacts, provided by the Ti/Au lines, were made tothe MoSe2 flake and the graphite backgate. After each transfer, theheterostructure surface was checked with atomic force microscopy toensure a sufficiently flat area in the stack and to obtain the flakes’thickness. MoSe2 ML’s quality was confirmed by Raman and PLspectroscopies at room temperature.715.4. Ion Implantation. Bronze tips were used to fix the sample ona holder, making contact with the sample’s Au pads and, thus, the ML.To remove volatile contamination from the sample, the samplechamber was then evacuated to 10−9 mbar for several hours. Thesample was heated to 150 °C for 10 min to remove residual volatileadsorbates and then to 220 °C during the implantation. A foil wasused as a feedstock to provide 52Cr+ ions. After extraction, the ions aredecelerated from 30 keV to 25 eV directly in front of the sample.Since the deceleration voltage is set relative to the potential of thesource anode, this energy represents the upper limit, with a tail towardlower energies. The fluence of the ions was set to 3 × 1012 cm−2. Thefluence was verified by test implantations using the Rutherfordbackscatter spectrometry (more information in the SupportingInformation Note 1). A detailed description of the source and theimplantation system can be found in the references.20,215.5. Optical Measurements. PL spectroscopy was performed at10 K (unless otherwise specified) in a He-cooled cold-finger cryostat(Cryoindustries) with a heating element (allowing a sampletemperature range from 10 to 300 K). For PL measurements, thelaser beam�688 nm (1.80 eV) from a Ti:Sa laser�is passed througha 680 ± 5 nm band-pass filter before being focused by an aspheric lens(NA = 0.47) into a spot of 1.6 μm in diameter on the sample. Unlessotherwise specified, the laser power on the sample was at 1 μW for PLexperiments. The PL signal is collected by the same lens and passedthrough a 700 nm low-pass filter before being focused by anachromatic doublet (NA = 0.24) through the entrance slit of aCzerny−Turner spectrometer, dispersed by a 600 l/mm grating ontoa CCD camera. For gate dependence and temperature dependencePL, the laser power was kept at 1 μW. For PLE, the excitation powerranged from 2 to 5 μW, and the PL intensity is normalized to thepower density for final data.For time-resolved PL, the excitation was done using a pulsed laserat 660 nm (1.88 eV) with a 200 ps pulse length, a 2.5 MHz repetitionACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c05366ACS Appl. Mater. Interfaces 2023, 15, 35321−3533135328https://pubs.acs.org/doi/suppl/10.1021/acsami.3c05366/suppl_file/am3c05366_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c05366/suppl_file/am3c05366_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c05366/suppl_file/am3c05366_si_001.pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c05366?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asrate, and a 2.8 μW average power. The PL signal is directed throughan 800 nm (1.55 eV) low-pass filter on the detection path beforeentering an avalanche photodiode with a 30 ps time resolution. Thehistogram of the time difference between the laser pulses and PLemission was acquired with a time tagger.The sample was mounted on an x−y−z Attocube stage in a Heflow cryostat (attoDRY2100) for magneto-optics measurement attemperature T = 1.8 K. A magnetic field up to ±8 T was appliedperpendicularly to the sample (Faraday configuration). The excitationlaser beam (688 nm, i.e., 1.80 eV at 4 μW) was passed through a 680nm band-pass filter and a linear polarizer in an H-configuration. Thelaser was focused by an aspheric lens (NA = 0.47) into a spot of ≈1.6μm in diameter on the sample. The emitted PL was collected with thesame objective. It was passed through a combination of λ/4, λ/2waveplates, and a linear polarizer set to pass the σ± polarized light. Itwas then propagated via a single-mode optical fiber toward theentrance slit of a Czerny−Turner spectrometer, where it wasdispersed by a 600 l/mm grating onto a CCD camera. A long-passfilter (with a 700 nm band edge) was inserted between the fiberoutput and the spectrometer entrance to remove any remaining laserlight.■ ASSOCIATED CONTENTData Availability StatementThe data supporting the findings of this study are availablewithin the paper and its Supporting Information files. Data arealso available from the corresponding author upon reasonablerequest.*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsami.3c05366.Additional experimental details on the RBS measure-ment of Cr implantation on the test ta-C substrate;spatial PL maps of D, X, and X−; polarization-resolvedPL; Raman and PL spectroscopic studies of vacancies inMoSe2 ML; MD simulation and TEM studies of Crimplantation on MoS2 ML; unfolded band structure ofCr-implanted MoSe2 ML with various defect config-urations; and charge carrier density conversion from gatevoltage (PDF)■ AUTHOR INFORMATIONCorresponding AuthorsMinh N. Bui − Peter Grünberg Institute 9 (PGI-9),Forschungszentrum Jülich, 52425 Jülich, Germany;Department of Physics, RWTH Aachen University, 52074Aachen, Germany; orcid.org/0000-0003-4146-5026;Email: m.bui@fz-juelich.deBeata E. Kardynał − Peter Grünberg Institute 9 (PGI-9),Forschungszentrum Jülich, 52425 Jülich, Germany;Department of Physics, RWTH Aachen University, 52074Aachen, Germany; Email: b.kardynal@fz-juelich.deAuthorsStefan Rost − Peter Grünberg Institute 1 (PGI-1) andInstitute for Advanced Simulation 1 (IAS-1),Forschungszentrum Jülich and JARA, 52425 Jülich,Germany; Department of Physics, RWTH Aachen University,52074 Aachen, GermanyManuel Auge − II. Institute of Physics, University of Göttingen,37077 Göttingen, GermanyLanqing Zhou − Peter Grünberg Institute 9 (PGI-9),Forschungszentrum Jülich, 52425 Jülich, Germany;Department of Physics, RWTH Aachen University, 52074Aachen, GermanyChristoph Friedrich − II. Institute of Physics, University ofGöttingen, 37077 Göttingen, GermanyStefan Blügel − II. Institute of Physics, University of Göttingen,37077 Göttingen, Germany; Department of Physics, RWTHAachen University, 52074 Aachen, Germany; orcid.org/0000-0001-9987-4733Silvan Kretschmer − Institute of Ion Beam Physics andMaterials Research, Helmholtz-Zentrum Dresden-Rossendorf,01328 Dresden, Germany; orcid.org/0000-0002-5098-5763Arkady V. Krasheninnikov − Institute of Ion Beam Physicsand Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany; Department ofApplied Physics, Aalto University School of Science, 00076Aalto, Finland; orcid.org/0000-0003-0074-7588Kenji Watanabe − Research Center for Functional Materials,National Institute for Materials Science, Tsukuba 305-0044,Japan; orcid.org/0000-0003-3701-8119Takashi Taniguchi − International Center for MaterialsNanoarchitectonics, National Institute for Materials Science,Tsukuba 305-0044, Japan; orcid.org/0000-0002-1467-3105Hans C. Hofsäss − II. Institute of Physics, University ofGöttingen, 37077 Göttingen, GermanyDetlev Grützmacher − Peter Grünberg Institute 9 (PGI-9),Forschungszentrum Jülich, 52425 Jülich, GermanyComplete contact information is available at:https://pubs.acs.org/10.1021/acsami.3c05366Author ContributionsB.E.K., H.C.H., and M.N.B. conceived and designed theexperiments. S.K. and A.V.K. performed MD simulations. S.R.,C.F., and S.B. performed DFT calculations for band structuresand absorption spectra. T.T. and K.W. grew high-quality hBNcrystals. L.Z. processed the Si/SiO2 substrate with patternedmarkers and metal contacts. M.N.B. prepared the samples onthe Si/SiO2 substrate. M.A. and H.C.H. performed ionimplantation. M.N.B. and L.Z. acquired and analyzed PL andRaman data. All authors discussed the results and contributedto the writing of this manuscript.FundingVolkswagen Foundation: “Integration of Molecular Compo-nents in Functional Macroscopic Systems” initiative, grantnumbers 93425, 93427, and 93428. German ResearchFoundation (DFG): project KR 4866/8-1, and the collabo-rative research center “Chemistry of Synthetic 2D Materials”SFB-1415-417590517. Japan Society for the Promotion ofScience (JSPS): Grants-in-Aid for Scientific Research (KA-KENHI), grant numbers 19H05790 and 20H00354.NotesThe authors declare no competing financial interest.All scripts used to generate the results in this study areavailable from the corresponding author upon reasonablerequest.■ ACKNOWLEDGMENTSThis project was supported by the “Integration of MolecularComponents in Functional Macroscopic Systems” initiative ofVolkswagen Foundation (grant numbers 93425, 93427, and93428). The authors would like to thank the staff at theACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c05366ACS Appl. Mater. Interfaces 2023, 15, 35321−3533135329https://pubs.acs.org/doi/suppl/10.1021/acsami.3c05366/suppl_file/am3c05366_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c05366/suppl_file/am3c05366_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Minh+N.+Bui"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-4146-5026mailto:m.bui@fz-juelich.dehttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Beata+E.+Kardyna%C5%82"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfmailto:b.kardynal@fz-juelich.dehttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Stefan+Rost"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Manuel+Auge"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Lanqing+Zhou"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Christoph+Friedrich"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Stefan+Blu%CC%88gel"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0001-9987-4733https://orcid.org/0000-0001-9987-4733https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Silvan+Kretschmer"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-5098-5763https://orcid.org/0000-0002-5098-5763https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Arkady+V.+Krasheninnikov"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-0074-7588https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kenji+Watanabe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-3701-8119https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Taniguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-1467-3105https://orcid.org/0000-0002-1467-3105https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hans+C.+Hofsa%CC%88ss"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Detlev+Gru%CC%88tzmacher"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c05366?ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c05366?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asHelmholtz Nano Facility72 of Forschungszentrum Jülich forhelping with substrate fabrication and Felix Junge (II. Instituteof Physics, University of Göttingen, Göttingen, Germany) fororganizing the RBS data, and the authors acknowledge theusage of VESTA software73 for producing the graphics and thecomputing time granted through JARA-HPC on the super-computer JURECA at Forschungszentrum Jülich. A.V.K.acknowledges funding from the German Research Foundation(DFG), project KR 4866/8-1, and the collaborative researchcenter “Chemistry of Synthetic 2D Materials” SFB-1415-417590517. Generous grants of computer time from theTechnical University of Dresden computing cluster (TAU-RUS) and the High Performance Computing Center (HLRS)in Stuttgart, Germany, are gratefully appreciated. K.W. andT.T. acknowledge support from the JSPS KAKENHI (grantnumbers 19H05790 and 20H00354).■ REFERENCES(1) Dietl, T.; Ohno, H. Dilute Ferromagnetic Semiconductors:Physics and Spintronic Structures. Rev. Mod. 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