# Fileset

[nanomaterials-12-04425.pdf](https://mdr.nims.go.jp/filesets/f2e6e7f5-16f1-4775-8a39-adeb417a1220/download)

## Creator

Anna Di Renzo, Onur Çakıroğlu, Felix Carrascoso, Hao Li, Giuseppe Gigli, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Carmen Munuera, Aurora Rizzo, Andres Castellanos-Gomez, Rosanna Mastria, Riccardo Frisenda

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Enhanced Field-Effect Control of Single-Layer WS2 Optical Features by hBN Full Encapsulation](https://mdr.nims.go.jp/datasets/c3100c06-b8ea-4a03-8d51-f6ada559856a)

## Fulltext

Enhanced Field-Effect Control of Single-Layer WS2 Optical Features by hBN Full EncapsulationCitation: Di Renzo, A.; Çakıroğlu, O.;Carrascoso, F.; Li, H.; Gigli, G.;Watanabe, K.; Taniguchi, T.; Munuera,C.; Rizzo, A.; Castellanos-Gomez, A.;et al. Enhanced Field-Effect Controlof Single-Layer WS2 Optical Featuresby hBN Full Encapsulation.Nanomaterials 2022, 12, 4425. https://doi.org/10.3390/nano12244425Academic Editor: Elias StathatosReceived: 14 November 2022Accepted: 8 December 2022Published: 12 December 2022Publisher’s Note: MDPI stays neutralwith regard to jurisdictional claims inpublished maps and institutional affil-iations.Copyright: © 2022 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).nanomaterialsArticleEnhanced Field-Effect Control of Single-Layer WS2 OpticalFeatures by hBN Full EncapsulationAnna Di Renzo 1,2, Onur Çakıroğlu 3, Felix Carrascoso 3, Hao Li 3, Giuseppe Gigli 1,2, Kenji Watanabe 4 ,Takashi Taniguchi 5, Carmen Munuera 3 , Aurora Rizzo 2, Andres Castellanos-Gomez 3,* , Rosanna Mastria 2,*and Riccardo Frisenda 3,6,*1 Department of Mathematics and Physics “Ennio De Giorgi”, University of Salento, Via Arnesano,73100 Lecce, Italy2 National Research Council, Institute of Nanotechnology (CNR-NANOTEC), Via Monteroni, 73100 Lecce, Italy3 Materials Science Factory, Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), E-28049 Madrid, Spain4 Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki,Tsukuba 305-0044, Japan5 International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki,Tsukuba 305-0044, Japan6 Physics Department, Sapienza University of Rome, Piazzale Aldo Moro 5, 00185 Rome, Italy* Correspondence: andres.castellanos@csic.es (A.C.-G.); rosanna.mastria@nanotec.cnr.it (R.M.);riccardo.frisenda@uniroma1.it (R.F.)Abstract: The field-effect control of the electrical and optical properties of two-dimensional (2D) vander Waals semiconductors (vdW) is one important aspect of this novel class of materials. Thanksto their reduced thickness and decreased screening, electric fields can easily penetrate in a 2Dsemiconductor and thus modulate their charge density and their properties. In literature, the fieldeffect is routinely used to fabricate atomically thin field-effect transistors based on 2D semiconductors.Apart from the tuning of the electrical transport, it has been demonstrated that the field effect canalso be used to modulate the excitonic optical emission of 2D transition metal dichalcogenides suchas MoS2 or WSe2. In this paper, we present some recent experiments on the field-effect control ofthe optical and excitonic properties of the monolayer WS2. Using the deterministic transfer of vander Waals materials, we fabricate planar single-layer WS2 devices contacted by a gold electrodeand partially sandwiched between two insulating hexagonal boron nitride (hBN) flakes. Thanksto the planar nature of the device, we can optically access both the hBN encapsulated and theunencapsulated WS2 regions and compare the field-effect control of the exciton population in thetwo cases. We find that the encapsulation strongly increases the range of tunability of the opticalemission of WS2, allowing us to tune the photoluminescence emission from excitons-dominatedto trions-dominated. We also discuss how the full encapsulation of WS2 with hBN helps reducespurious hysteretic effects in the field-effect control of the optical properties, similar to what has beenreported for 2D vdW field-effect transistors.Keywords: van der Waals materials; WS2; hBN; photoluminescence; excitons1. IntroductionIn recent years, atomically thin semiconducting transition metal dichalcogenides(2D-TMDs) emerged as an appealing platform for the room-temperature implementation ofexcitonic systems. The reduced dimensionality, which characterises 2D-TMDs in the mono-layer regime, results in a strong confinement of the charge carriers and reduced dielectricscreening, inducing a strong Coulomb interaction with reported exciton binding energyup to 1 eV [1–3]. This allows for the room temperature optical response to be dominatedby the physics of excitons [4]. These peculiarities have attracted a great deal of interestand are underpinning several appealing phenomena, including many-body states [5,6],Nanomaterials 2022, 12, 4425. https://doi.org/10.3390/nano12244425 https://www.mdpi.com/journal/nanomaterialshttps://doi.org/10.3390/nano12244425https://doi.org/10.3390/nano12244425https://creativecommons.org/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/nanomaterialshttps://www.mdpi.comhttps://orcid.org/0000-0003-3701-8119https://orcid.org/0000-0001-8524-9304https://orcid.org/0000-0002-3384-3405https://orcid.org/0000-0003-1728-7354https://doi.org/10.3390/nano12244425https://www.mdpi.com/journal/nanomaterialshttps://www.mdpi.com/article/10.3390/nano12244425?type=check_update&version=1Nanomaterials 2022, 12, 4425 2 of 9spatially separated interlayer excitons in 2D heterostructures [7,8] and high-temperatureexciton condensation in these systems [9]. The atomically thin nature of 2D-TMDs andtheir extreme sensitivity to the surrounding conditions also offer an unprecedented play-ground to influence and control exciton dynamics through external stimuli, such as strainengineering [10,11], the control of the dielectric environment [12] and the application of anelectric field [13].In the key challenge of manipulating the exciton physics of 2D-TMDs, electric-fieldcontrol has been established as a promising and straightforward approach to modulatecharge density in atomically thin materials and, thus, an effective tool to control boththe electrical and the optical properties of 2D-TMDs. Indeed, due to the high excitonsbinding energies that allow for stable excitons at room temperature, the tuning of chargedensity in 2D-TMDs results in the generation of charged exciton bounded states apartfrom the neutral ones [14]. As a result, intrinsically n-doped 2D-TMDs exhibit not only thecharge-neutral exciton feature (X0), but also a lower energy resonance corresponding tonegative trions (X−) that consist of two electrons and one hole bound together throughCoulomb interactions [14]. Apart from the intrinsic doping, these optical features canbe further effectively modulated by electrical doping [15,16]. The effective tuning of thecharge carrier’s population of 2D-TMDs through an electric field relies on the quality of theinterface between the 2D-TMD monolayer and the gate dielectric oxide (usually SiO2) ina field-effect transistor (FET) device configuration. Indeed, the presence of trap states atthe 2D-TMD/SiO2 interface strongly impacts the stability of the device operation and theoverall device performances, giving rise to threshold voltage instability in FETs [17–19].Herein, we have investigated the role of the hBN encapsulation in improving the gate-dependent optical properties of TMDs that are single-layer integrated in a single-electrodeback-gated device, with a particular focus on the optical proprieties of WS2. Interestingly,we found that similarly to FETs, the field-effect control of the optical properties of the single-layer WS2 can also be strongly affected by traps, which introduce gate voltage hysteresis inthe optical response and reduce the tunability of the excitons population. In this paper, wedemonstrate that the full encapsulation of WS2 greatly enhances the optical tunability ofthe WS2 photoluminescence (PL) in response to an external electric-field provided by theback-gate, resulting in an improved modulation of the X0 and X− PL peaks intensity andin an increase in the energy separation between the X0 and X− PL peaks. In addition, wefound that full encapsulation of the WS2 with insulating hexagonal boron nitride (hBN) alsoimproves the field-effect control by reducing spurious hysteretic effects, with a beneficialeffect on the tunability of the optical emission of WS2 thanks to the effective decouplingof the charge traps mediated by the hBN interlayer [20]. In general, hBN encapsulation isshown to lead to an enhancement of 2D WS2 optical quality by offering protection againstunwanted doping contributions from substrates and chemicals, or physical adsorbatesfrom the environment, resulting in cleaner spectra characterised by sharper emission fromneutral and charged excitons [21].2. Results and DiscussionsFigure 1a shows an optical microscopy picture of the fabricated device used to in-vestigate the electric-field dependence of the exciton features of a bare and encapsulatedWS2 single layer (1L-WS2). The WS2-based device consists of a partially encapsulatedhBN/single-layer WS2/hBN heterostructure layered on top of a SiO2/Si substrate (SiO2thickness 290 nm). We used a single gold electrode geometry in which the electrode is indirect contact with the WS2 monolayer, and the doping density can be varied by applyinga voltage between the Si back-gate and the gold electrode (Figure 1b). The single-layerWS2 and hBN flakes were mechanically exfoliated (see Supplementary Materials, section‘Sample Fabrication’) and sequentially transferred by an all-dry deterministic transfer pro-cedure on the SiO2/Si substrate in contact with a pre-patterned Au electrode by using apolydimethylsiloxane stamp (Gel-Film WF × 4 6.0 mil by Gel-Pak) [22,23]. In particular,a bottom hBN flake (thickness~40 nm) was placed in close vicinity of the gold electrodeNanomaterials 2022, 12, 4425 3 of 9and then the WS2 single layer was transferred, bridging the bottom hBN flake and thegold contact. Finally, the top hBN flake (thickness~20 nm) was positioned on top of the1L-WS2/bottom hBN stack, covering only part of the WS2 flake (Figure 1a). This planardevice geometry provides a great advantage to have, in the same device, two regions: theencapsulated (highlighted by the green arrow in Figure 1b) and the unencapsulated one(purple arrow in Figure 1b), both belonging to the same 1L-WS2 flake. This enables a directunderstanding of the encapsulation effect and helps avoid possible WS2 flake-to-flake variations.Nanomaterials 2022, 12, x FOR PEER REVIEW 3 of 9   procedure on the SiO2/Si substrate in contact with a pre-patterned Au electrode by using a polydimethylsiloxane stamp (Gel-Film WF × 4 6.0 mil by Gel-Pak) [22,23]. In particular, a bottom hBN flake (thickness~40 nm) was placed in close vicinity of the gold electrode and then the WS2 single layer was transferred, bridging the bottom hBN flake and the gold contact. Finally, the top hBN flake (thickness~20 nm) was positioned on top of the 1L-WS2/bottom hBN stack, covering only part of the WS2 flake (Figure 1a). This planar device geometry provides a great advantage to have, in the same device, two regions: the encapsulated (highlighted by the green arrow in Figure 1b) and the unencapsulated one (purple arrow in Figure 1b), both belonging to the same 1L-WS2 flake. This enables a direct understanding of the encapsulation effect and helps avoid possible WS2 flake-to-flake variations.  Figure 1. (a) Optical microscope images of the WS2-based device, and the scale bar corresponds to 20 μm. (b) Schematics of the system highlights to show the two different WS2 regions that can be experimentally accessed. (c) Photoluminescence spectrum of the hBN full encapsulated WS2 monolayer fitted to two peaks corresponding to the emission from neutral excitons (orange curve) and trions (green curve). The monolayer thickness of the WS2 flake was confirmed through differential reflectance measurements before the heterostructure fabrication [24], as well as Raman spectroscopy (see Supplementary Materials, section ‘Raman characterization of the sample’ and Figure S1) and by PL spectroscopy of the WS2 monolayer. As expected, both the differential reflectance and the PL spectrum are characterised by a main feature at 2.01 eV (respectively, in the two cases, a dip and a peak) that is indicative of a single-layer WS2 flake [24,25]. Figure 1c shows the PL spectrum of the encapsulated WS2 recorded after the device fabrication, which is dominated by a prominent peak located at 2.001 eV, slightly red-shifted from the unencapsulated 1L-WS2, due to the change in the refractive index of the substrate from SiO2 to hBN [21]. With a closer inspection, one can see that the PL peak is skewed toward lower energies, showing a clear shoulder. In fact, the experimental data can be fitted to two Gaussian peaks centred, respectively, at 2.001 eV and 1.974 eV. These two features are excitonic in nature and can be assigned, respectively, to the recombination of excitons and trions at the K point in the band structure of the single-layer WS2 where the direct bandgap is located [25]. To investigate the electric-field modulation of the optical properties of the WS2 single layer and to reveal the influence of hBN encapsulation, we measured PL emission upon shining a focused laser beam on top of the bare WS2 or on the fully encapsulated regions, while sweeping the back-gate voltage (Vg) between 40 V and −40 V. In this study, all the PL measurements were performed at room temperature using a 532 nm laser with a low excitation power of 4 μW, and a laser spot diameter of 1 μm. Figure 2a shows the recorded Figure 1. (a) Optical microscope images of the WS2-based device, and the scale bar correspondsto 20 µm. (b) Schematics of the system highlights to show the two different WS2 regions that canbe experimentally accessed. (c) Photoluminescence spectrum of the hBN full encapsulated WS2monolayer fitted to two peaks corresponding to the emission from neutral excitons (orange curve)and trions (green curve).The monolayer thickness of the WS2 flake was confirmed through differential re-flectance measurements before the heterostructure fabrication [24], as well as Ramanspectroscopy (see Supplementary Materials, section ‘Raman characterization of the sample’and Figure S1) and by PL spectroscopy of the WS2 monolayer. As expected, both thedifferential reflectance and the PL spectrum are characterised by a main feature at 2.01 eV(respectively, in the two cases, a dip and a peak) that is indicative of a single-layer WS2flake [24,25]. Figure 1c shows the PL spectrum of the encapsulated WS2 recorded after thedevice fabrication, which is dominated by a prominent peak located at 2.001 eV, slightlyred-shifted from the unencapsulated 1L-WS2, due to the change in the refractive index ofthe substrate from SiO2 to hBN [21]. With a closer inspection, one can see that the PL peakis skewed toward lower energies, showing a clear shoulder. In fact, the experimental datacan be fitted to two Gaussian peaks centred, respectively, at 2.001 eV and 1.974 eV. Thesetwo features are excitonic in nature and can be assigned, respectively, to the recombinationof excitons and trions at the K point in the band structure of the single-layer WS2 where thedirect bandgap is located [25].To investigate the electric-field modulation of the optical properties of the WS2 singlelayer and to reveal the influence of hBN encapsulation, we measured PL emission uponshining a focused laser beam on top of the bare WS2 or on the fully encapsulated regions,while sweeping the back-gate voltage (Vg) between 40 V and −40 V. In this study, all thePL measurements were performed at room temperature using a 532 nm laser with a lowexcitation power of 4 µW, and a laser spot diameter of 1 µm. Figure 2a shows the recordedPL spectra of the bare 1L-WS2 acquired at different values of Vg, between −40 V and 40 V.Similar to Figure 1c, the emission feature of the 1L-WS2 consists of a peak skewed towardlower energy ascribed to the radiative recombination of neutral excitons and negative trions.As can be observed from the plot, the total integrated PL intensity of the bare 1L-WS2 isNanomaterials 2022, 12, 4425 4 of 9weakly sensitive to the Vg, being enhanced at the negative Vg and reduced at the positiveVg. This behaviour is expected for a semiconductor characterised by an intrinsic n-dopingas is the case of the 1L-WS2 [15,26]. The two-components fits of the 40 V, 20 V, 0 V, 20 Vand −40 V PL spectra, presented in Figure 2b, show the influence of Vg on the X0 and X−peaks energies, and on the total integrated intensity two peaks [15,27]. In addition, weinvestigated the modulation of the 1L-WS2 excitonic properties by analysing the differentialreflectance spectra as a function of the Vg (see Supplementary Materials, section ‘Gatedependent differential reflectance spectroscopy’).Nanomaterials 2022, 12, x FOR PEER REVIEW 4 of 9   PL spectra of the bare 1L-WS2 acquired at different values of Vg, between −40 V and 40 V. Similar to Figure 1c, the emission feature of the 1L-WS2 consists of a peak skewed toward lower energy ascribed to the radiative recombination of neutral excitons and negative trions. As can be observed from the plot, the total integrated PL intensity of the bare 1L-WS2 is weakly sensitive to the Vg, being enhanced at the negative Vg and reduced at the positive Vg. This behaviour is expected for a semiconductor characterised by an intrinsic n-doping as is the case of the 1L-WS2 [15,26]. The two-components fits of the 40 V, 20 V, 0 V, 20 V and −40 V PL spectra, presented in Figure 2b, show the influence of Vg on the X0 and X− peaks energies, and on the total integrated intensity two peaks [15,27]. In addition, we investigated the modulation of the 1L-WS2 excitonic properties by analysing the differential reflectance spectra as a function of the Vg (see Supplementary Materials, section ‘Gate dependent differential reflectance spectroscopy’).  Figure 2. (a) Photoluminescence spectra of the 1L-WS2 unencapsulated region recorded at gate voltages between −40 V and 40 V. Inset: schematic of the probed region on the 1L-WS2-based device. (b) Photoluminescence spectra recorded at −40 V, −20 V, 0 V, 20 V and 40 V gate voltages fitted each to two peaks coming from exciton (orange curve) and trion (green curve) emission. Differently from the bare 1L-WS2, the PL features of the hBN fully encapsulated WS2 single layer are more strongly affected from the electric-field, as shown in Figure 3a [20]. The total integrated intensity shows an abrupt decrease by sweeping the Vg from −40 V to 40 V, indicating the strong tunability of this parameter in the encapsulated case compared to the unencapsulated one. As can be seen in the fits of Figure 3b, at negative Vg the X0 peak is dominant, whereas at positive Vg it becomes almost undetectable due to the strong charge carrier injection given by the electrical doping [28,29]. In fact, at 40 V the X− peak remains the only prominent feature in the PL spectrum, thus showing an inversion of the excitonic population from excitons to trions. The shift of both the X0 and X− is also considerably enhanced thanks to the improved dielectric and traps environment provided by the hBN encapsulation, with a redshift of 30 meV of the X− peak and a blueshift of 16 meV of the X0 peak at Vg = 40 V [30,31]. Figure 2. (a) Photoluminescence spectra of the 1L-WS2 unencapsulated region recorded at gatevoltages between −40 V and 40 V. Inset: schematic of the probed region on the 1L-WS2-based device.(b) Photoluminescence spectra recorded at −40 V, −20 V, 0 V, 20 V and 40 V gate voltages fitted eachto two peaks coming from exciton (orange curve) and trion (green curve) emission.Differently from the bare 1L-WS2, the PL features of the hBN fully encapsulated WS2single layer are more strongly affected from the electric-field, as shown in Figure 3a [20].The total integrated intensity shows an abrupt decrease by sweeping the Vg from −40 V to40 V, indicating the strong tunability of this parameter in the encapsulated case comparedto the unencapsulated one. As can be seen in the fits of Figure 3b, at negative Vg the X0peak is dominant, whereas at positive Vg it becomes almost undetectable due to the strongcharge carrier injection given by the electrical doping [28,29]. In fact, at 40 V the X− peakremains the only prominent feature in the PL spectrum, thus showing an inversion ofthe excitonic population from excitons to trions. The shift of both the X0 and X− is alsoconsiderably enhanced thanks to the improved dielectric and traps environment providedby the hBN encapsulation, with a redshift of 30 meV of the X− peak and a blueshift of16 meV of the X0 peak at Vg = 40 V [30,31].Figure 4 shows the results of the two-peaks fits of the PL spectra of the bare 1L-WS2and the hBN fully encapsulated 1L-WS2 as a function of Vg. The intensity of the peaks isreported in panel a and the centre of the peaks in panel b. The peaks intensity shows, inboth the unencapsulated and encapsulated cases, a decrease when going toward positivegate voltages, but this decrease is shallower in the first case and more abrupt in thesecond case. Focusing on the neutral exciton, we can observe that its intensity is reducedby 50% in the unencapsulated WS2 by sweeping the voltage from negative to positive.Nevertheless, the X0 peak is always more intense than the X− peak in the full voltage range.On the other hand, the X0 peak intensity of the neutral exciton is reduced by 99% in theNanomaterials 2022, 12, 4425 5 of 9encapsulated sample. Moreover, the encapsulated case shows an inversion of the dominantpeak for voltages larger than 20 V, indicating that for these voltages, the generation andrecombination of trions is favoured over the neutral excitons.Nanomaterials 2022, 12, x FOR PEER REVIEW 5 of 9    Figure 3. (a) Photoluminescence spectra of the 1L-WS2 hBN-encapsulated region recorded at gate voltages between −40 V and 40 V. Inset: schematic of the 1L-WS2 sample and of the probed region. (b) Photoluminescence spectra recorded at −40 V, −20 V, 0 V, 20 V and 40 V gate voltages fitted each to two peaks coming from exciton (orange curve) and trion (green curve) emission. Figure 4 shows the results of the two-peaks fits of the PL spectra of the bare 1L-WS2 and the hBN fully encapsulated 1L-WS2 as a function of Vg. The intensity of the peaks is reported in panel a and the centre of the peaks in panel b. The peaks intensity shows, in both the unencapsulated and encapsulated cases, a decrease when going toward positive gate voltages, but this decrease is shallower in the first case and more abrupt in the second case. Focusing on the neutral exciton, we can observe that its intensity is reduced by 50% in the unencapsulated WS2 by sweeping the voltage from negative to positive. Nevertheless, the X0 peak is always more intense than the X− peak in the full voltage range. On the other hand, the X0 peak intensity of the neutral exciton is reduced by 99% in the encapsulated sample. Moreover, the encapsulated case shows an inversion of the dominant peak for voltages larger than 20 V, indicating that for these voltages, the generation and recombination of trions is favoured over the neutral excitons.  Figure 4. (a) Normalised integrated PL intensities of the X and X−peaks versus gate voltage on the 1LWS2 (left panel) and the hBN/WS2/hBN region (right panel). (b) Emission energy peaks of X and X− versus gate voltage on the 1L-WS2 (left panel) and the hBN/WS2/hBN region (right panel). Figure 3. (a) Photoluminescence spectra of the 1L-WS2 hBN-encapsulated region recorded at gatevoltages between −40 V and 40 V. Inset: schematic of the 1L-WS2 sample and of the probed region.(b) Photoluminescence spectra recorded at −40 V, −20 V, 0 V, 20 V and 40 V gate voltages fitted eachto two peaks coming from exciton (orange curve) and trion (green curve) emission.Nanomaterials 2022, 12, x FOR PEER REVIEW 5 of 9    Figure 3. (a) Photoluminescence spectra of the 1L-WS2 hBN-encapsulated region recorded at gate voltages between −40 V and 40 V. Inset: schematic of the 1L-WS2 sample and of the probed region. (b) Photoluminescence spectra recorded at −40 V, −20 V, 0 V, 20 V and 40 V gate voltages fitted each to two peaks coming from exciton (orange curve) and trion (green curve) emission. Figure 4 shows the results of the two-peaks fits of the PL spectra of the bare 1L-WS2 and the hBN fully encapsulated 1L-WS2 as a function of Vg. The intensity of the peaks is reported in panel a and the centre of the peaks in panel b. The peaks intensity shows, in both the unencapsulated and encapsulated cases, a decrease when going toward positive gate voltages, but this decrease is shallower in the first case and more abrupt in the second case. Focusing on the neutral exciton, we can observe that its intensity is reduced by 50% in the unencapsulated WS2 by sweeping the voltage from negative to positive. Nevertheless, the X0 peak is always more intense than the X− peak in the full voltage range. On the other hand, the X0 peak intensity of the neutral exciton is reduced by 99% in the encapsulated sample. Moreover, the encapsulated case shows an inversion of the dominant peak for voltages larger than 20 V, indicating that for these voltages, the generation and recombination of trions is favoured over the neutral excitons.  Figure 4. (a) Normalised integrated PL intensities of the X and X−peaks versus gate voltage on the 1LWS2 (left panel) and the hBN/WS2/hBN region (right panel). (b) Emission energy peaks of X and X− versus gate voltage on the 1L-WS2 (left panel) and the hBN/WS2/hBN region (right panel). Figure 4. (a) Normalised integrated PL intensities of the X and X−peaks versus gate voltage on the1LWS2 (left panel) and the hBN/WS2/hBN region (right panel). (b) Emission energy peaks of X andX− versus gate voltage on the 1L-WS2 (left panel) and the hBN/WS2/hBN region (right panel).The energies of the peaks in the two 1L-WS2 regions also show analogous behaviour,with the encapsulated case showing a much stronger dependence on Vg than the unencap-sulated one, in which the peak’s energies appear almost constant. The sweep of Vg from−40 V to 40 V induces, in the encapsulated case, a redshift of almost 30 meV for the X-peakposition and a slight blueshift 16 meV for the X0 peak. In addition, the energy splittingbetween the X0 and X− remarkably increases in the encapsulated case when sweeping Vgfrom −40 V to 40 V, resulting in an energy separation that goes from 20 meV to 66 meV,which in this latter case is about five times higher than in the bare 1L-WS2. Interestingly,the full encapsulation enables improved field-effect control considering that, in previousNanomaterials 2022, 12, 4425 6 of 9reports that exploited the electric-field control of the optical properties of the bare WS2,a maximum splitting of 34 meV had been achieved only by using very high operationalvoltages [27,31]. It is worth noting that both the X0 and X− peaks are initially redshiftedbecause of the encapsulation, as previously reported [32]. In the literature, the energysplitting between the X and X− has been defined as the dissociation energy of trions,which, in the case of constant trion binding energy, is mainly dependent on the Fermi-levelposition [16]. The energy splitting thus increases when increasing the electron doping, asa consequence of the rising of the Fermi level (see Figure 4b) [15,33]. On the other hand,the blueshift of the exciton energy is ascribed to a reduction in the exciton binding energyresulting from electron doping [16,34]. Thus, the enhanced energy splitting between the X0and X− reported in the case of the fully hBN encapsulated WS2 represents clear evidenceof the more effective electrical doping of the 1L-WS2 and the enhanced field-effect controlenabled by the hBN full encapsulation.To gain insight into the field-effect control of the optical properties of the single-layer WS2 and to further investigate the advantage of the hBN encapsulation, we alsoperformed a full gate sweep measurement, which consists of forward and reverse sweeps,first from 0 V to 50 V, then from 50 V to −50 V and finally sweeping back from −50 Vto 0 V, with a gate voltage changing rate of 0.6 V/s, and focusing on the emission of theneutral exciton X0. Figure 5a shows the intensity of the X0 peaks in the two 1L-WS2 whenperforming this full gate sweep. As can be seen, the curves do not overlap in the forwardand backward Vg sweep. Nevertheless, this non-ideal hysteretic behaviour is remarkablyreduced when considering the PL emission of the hBN/WS2/hBN region. In detail, theencapsulation by few-layers hBN induces a 52% reduction in the hysteresis thanks to theeffective decoupling of the charge traps and the atmosphere adsorbates from the 1L-WS2.However, even with hBN encapsulation, the hysteresis is not completely suppressed. Thiscould be due to the presence of residual impurities (i.e., PDMS residues and/or watermolecules) unintentionally introduced during the fabrication process, or to defects andtraps intrinsic to the WS2 crystal [20].Finally, we have investigated the optical hysteresis at different Vg sweep rates. Figure 5b,cshow the neutral exciton PL peak intensity recorded from the bare 1L-WS2 and hBN/WS2/hBNobtained applying two different back-gate sweep rates, namely 0.6 V/s and 1.6 V/s, forthe full gate sweep measurement. We observe the overall trend of reduced hysteresis asthe sweep rate decreases; however, the behaviour is different in the bare and the fullyencapsulated 1L-WS2 regions. While the bare 1L-WS2 exhibits a hysteresis that is almostnot influenced by the speed, showing only a slight reduction estimated in 19% of thehysteresis voltage window, in the hBN/WS2/hBN the hysteresis at 0.6 V/s sweep rateis significantly lower if compared to the 1.6 V/s rate, showing a reduction of 59%. Thissuggests that, in the latter case, the traps present in the system are more short-lived andless detrimental for the overall optical tunability and field-effect control.Nanomaterials 2022, 12, 4425 7 of 9Nanomaterials 2022, 12, x FOR PEER REVIEW 7 of 9     Figure 5. (a) Comparison of the gate-dependent optical hysteresis of the X0 peak integrated intensity in 1L-WS2 (magenta curve) and hBN/1L-WS2/hBN (green curve). (b–c) Comparison of the hysteresis with different back-gate voltage sweep rates for the X0 intensity in 1L-WS2 (b) and in hBN/WS2/hBN (c) at 0.6 V/s and at 1.6 V/s. 3. Conclusions In conclusion, we demonstrated the tuning of excitonic emission in the monolayer WS2 exploiting an externally applied electric field in a single-electrode device. We considered both the bare and hBN fully encapsulated WS2 single layer and we found that the hBN encapsulation greatly enhances the optical tunability in response to the electric field. In detail, the PL response of the full hBN encapsulated WS2 monolayer changes dramatically, resulting in an increase of the energy splitting between neutral excitons and trions from 20 meV at −40 V up to 66 meV for voltages of 40 V, as the Fermi level rises. In addition, the enhanced effect of doping enabled by the full encapsulation allows us to fully suppress the neutral exciton generation and recombination at positive voltages, resulting in a PL spectrum completely dominated by the trions emission. The hBN encapsulation is also effective at reducing the hysteresis observed in the voltage-dependent PL, providing adequate protection from the trap states that can be present, for example, at the interface between SiO2 and WS2. These findings demonstrate that field-effect control combined with hBN encapsulation represents a practical approach to control and/or enhance the radiative excitonic emission in single-layer WS2 at room temperature toward improved exciton-based optoelectronic applications. Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1: Sample Fabrication, Raman characterization of the sample, Gate dependent differential reflectance spectroscopy. References [35–41] are cited in the supplementary materials. Author Contributions: Conceptualization, R.F. and A.C.-G.; methodology, R.F., A.C.-G., C.M., A.D.R., H.L. and F.C.; software, R.F., O.Ç. and A.D.R.; validation, A.D.R., R.F. and A.C.-G.; formal analysis, A.D.R.; investigation, A.D.R.; resources, A.C.-G., T.T. and K.W.; data curation, A.D.R. and Figure 5. (a) Comparison of the gate-dependent optical hysteresis of the X0 peak integrated intensityin 1L-WS2 (magenta curve) and hBN/1L-WS2/hBN (green curve). (b,c) Comparison of the hysteresiswith different back-gate voltage sweep rates for the X0 intensity in 1L-WS2 (b) and in hBN/WS2/hBN(c) at 0.6 V/s and at 1.6 V/s.3. ConclusionsIn conclusion, we demonstrated the tuning of excitonic emission in the monolayer WS2exploiting an externally applied electric field in a single-electrode device. We consideredboth the bare and hBN fully encapsulated WS2 single layer and we found that the hBNencapsulation greatly enhances the optical tunability in response to the electric field. Indetail, the PL response of the full hBN encapsulated WS2 monolayer changes dramatically,resulting in an increase of the energy splitting between neutral excitons and trions from20 meV at −40 V up to 66 meV for voltages of 40 V, as the Fermi level rises. In addition, theenhanced effect of doping enabled by the full encapsulation allows us to fully suppressthe neutral exciton generation and recombination at positive voltages, resulting in a PLspectrum completely dominated by the trions emission. The hBN encapsulation is alsoeffective at reducing the hysteresis observed in the voltage-dependent PL, providingadequate protection from the trap states that can be present, for example, at the interfacebetween SiO2 and WS2. These findings demonstrate that field-effect control combinedwith hBN encapsulation represents a practical approach to control and/or enhance theradiative excitonic emission in single-layer WS2 at room temperature toward improvedexciton-based optoelectronic applications.Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/nano12244425/s1: Sample Fabrication, Raman characterizationof the sample, Gate dependent differential reflectance spectroscopy. References [35–41] are cited inthe supplementary materials.Author Contributions: Conceptualization, R.F. and A.C.-G.; methodology, R.F., A.C.-G., C.M., A.D.R.,H.L. and F.C.; software, R.F., O.Ç. and A.D.R.; validation, A.D.R., R.F. and A.C.-G.; formal analysis,A.D.R.; investigation, A.D.R.; resources, A.C.-G., T.T. and K.W.; data curation, A.D.R. and R.F.;https://www.mdpi.com/article/10.3390/nano12244425/s1https://www.mdpi.com/article/10.3390/nano12244425/s1Nanomaterials 2022, 12, 4425 8 of 9writing-original draft preparation, A.D.R., R.M. and A.R.; writing-review and editing, R.F., A.C.-G.,C.M., A.D.R. and R.M.; visualization, A.D.R., R.F. and A.C.-G.; supervision, R.F., A.C.-G., R.M. andA.R.; project administration, R.F. and A.C.-G.; funding acquisition, A.C.-G., G.G., T.T. and K.W. Allauthors have read and agreed to the published version of the manuscript.Funding: This work was supported by the European Research Council (ERC) through the project2D-TOPSENSE (GA 755655), the European Union’s Horizon 2020 research and innovation programunder the grant agreement 956,813 (2Exciting), the EU FLAG-ERA through the project To2Dox (JTC-2019-009), the Comunidad de Madrid through the project CAIRO-CM project (Y2020/NMT-6661)and the Spanish Ministry of Science and Innovation through the projects PID2020-118078RB-I00,FISR-CNR national project “TECNOMED-Tecnopolo di nanotecnologia e fotonica per la medicina diprecisione”-CUP B83B17000010001, by “TecnoMed Puglia-Tecnopolo per la medicina di Precisione-Regione Puglia”-CUP B84I18000540002. K.W. and T.T. acknowledge support from the JSPS KAKENHI(Grant Numbers 19H05790, 20H00354 and 21H05233).Data Availability Statement: The data presented in this study are in the paper and/or theSupplemental Information. Additional data related to this paper may be requested from R.F. (ric-cardo.frisenda@uniroma1.it).Conflicts of Interest: The authors declare no conflict of interest.References1. Qiu, D.Y.; da Jornada, F.H.; Louie, S. G Optical Spectrum of MoS2: Many-Body Effects and Diversity of Exciton. Phys. Rev. Lett.2013, 111, 216805. [CrossRef] [PubMed]2. Cheiwchanchamnangij, T.; Lambrecht, W.R.L. Quasiparticle Band Structure Calculation of Monolayer, Bilayer, and Bulk MoS2.Phys. Rev. B Condens. Matter Mater. Phys. 2012, 85, 205302. [CrossRef]3. Ramasubramaniam, A.A. Large Excitonic Effects in Monolayers of Molybdenum and Tungsten Dichalcogenides. Phys. Rev. BCondens. Matter Mater. Phys. 2012, 86, 115409. [CrossRef]4. Mueller, T.; Malic, E. Exciton Physics and Device Application of Two-Dimensional Transition Metal Dichalcogenide Semiconduc-tors. Npj 2D Mater. Appl. 2018, 2, 2. [CrossRef]5. Van Tuan, D.; Scharf, B.; Wang, Z.; Shan, J.; Mak, K.F.; Žutić, I.; Dery, H. Probing Many-Body Interactions in MonolayerTransition-Metal Dichalcogenides. Phys. Rev. B 2019, 99, 085301. [CrossRef]6. Xiao, K.; Yan, T.; Liu, Q.; Yang, S.; Kan, C.; Duan, R.; Liu, Z.; Cui, X. Many-Body Effect on Optical Properties of MonolayerMolybdenum Diselenide. J. Phys. Chem. Lett. 2021, 12, 2555. [CrossRef]7. Rivera, P.; Schaibley, J.R.; Jones, A.M.; Ross, J.S.; Wu, S.; Aivazian, G.; Klement, P.; Seyler, K.; Clark, G.; Ghimire, N.J.; et al.Observation of Long-Lived Interlayer Excitons in Monolayer MoSe2-WSe2 Heterostructures. Nat. Commun. 2015, 6, 6242.[CrossRef]8. Chiu, M.H.; Li, M.Y.; Zhang, W.; Hsu, W.T.; Chang, W.H.; Terrones, M.; Terrones, H.; Li, L.J. Spectroscopic Signatures for InterlayerCoupling in MoS2-WSe2 van Der Waals Stacking. ACS Nano 2014, 8, 9649. [CrossRef]9. Wang, Z.; Rhodes, D.A.; Watanabe, K.; Taniguchi, T.; Hone, J.C.; Shan, J.; Mak, K.F. Evidence of High-Temperature ExcitonCondensation in Two-Dimensional Atomic Double Layers. Nature 2019, 574, 76. [CrossRef]10. Carrascoso, F.; Li, H.; Frisenda, R.; Castellanos-Gomez, A. Strain Engineering in Single-, Bi- and Tri-Layer MoS2, MoSe2, WS2 andWSe2. Nano Res. 2021, 14, 1698. [CrossRef]11. Carrascoso, F.; Frisenda, R.; Castellanos-Gomez, A. Biaxial versus Uniaxial Strain Tuning of Single-Layer MoS2. Nano Mater. Sci.2022, 4, 44. [CrossRef]12. Peimyoo, N.; Wu, H.Y.; Escolar, J.; de Sanctis, A.; Prando, G.; Vollmer, F.; Withers, F.; Riis-Jensen, A.C.; Craciun, M.F.; Thygesen,K.S.; et al. Engineering Dielectric Screening for Potential-Well Arrays of Excitons in 2D Materials. ACS Appl. Mater. Interfaces2020, 12, 55134. [CrossRef] [PubMed]13. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A.A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147.[CrossRef] [PubMed]14. Ross, J.S.; Wu, S.; Yu, H.; Ghimire, N.J.; Jones, A.M.; Aivazian, G.; Yan, J.; Mandrus, D.G.; Xiao, D.; Yao, W.; et al. Electrical Controlof Neutral and Charged Excitons in a Monolayer Semiconductor. Nat. Commun. 2013, 4, 1474. [CrossRef]15. Mak, K.F.; He, K.; Lee, C.; Lee, G.H.; Hone, J.; Heinz, T.F.; Shan, J. Tightly Bound Trions in Monolayer MoS2. Nat. Mater. 2013,12, 207. [CrossRef] [PubMed]16. Shang, J.; Shen, X.; Cong, C.; Peimyoo, N.; Cao, B.; Eginligil, M.; Yu, T. Observation of excitonic fine structure in a 2D transition-metal dichalcogenide semiconductor. ACS Nano 2015, 9, 647. [CrossRef]17. Egginger, M.; Bauer, S.; Schwödiauer, R.; Neugebauer, H.; Sariciftci, N.S. Current versus gate voltage hysteresis in organic fieldeffect transistors. Mon. Fur Chem. 2009, 140, 735. [CrossRef]18. Ghatak, S.; Pal, A.N.; Ghosh, A. Nature of Electronic States in Atomically Thin MoS2 Field-Effect Transistors. ACS Nano 2011,5, 7707. [CrossRef]http://doi.org/10.1103/PhysRevLett.111.216805http://www.ncbi.nlm.nih.gov/pubmed/24313514http://doi.org/10.1103/PhysRevB.85.205302http://doi.org/10.1103/PhysRevB.86.115409http://doi.org/10.1038/s41699-018-0074-2http://doi.org/10.1103/PhysRevB.99.085301http://doi.org/10.1021/acs.jpclett.1c00320http://doi.org/10.1038/ncomms7242http://doi.org/10.1021/nn504229zhttp://doi.org/10.1038/s41586-019-1591-7http://doi.org/10.1007/s12274-020-2918-2http://doi.org/10.1016/j.nanoms.2021.03.001http://doi.org/10.1021/acsami.0c14696http://www.ncbi.nlm.nih.gov/pubmed/33232104http://doi.org/10.1038/nnano.2010.279http://www.ncbi.nlm.nih.gov/pubmed/21278752http://doi.org/10.1038/ncomms2498http://doi.org/10.1038/nmat3505http://www.ncbi.nlm.nih.gov/pubmed/23202371http://doi.org/10.1021/nn5059908http://doi.org/10.1007/s00706-009-0149-zhttp://doi.org/10.1021/nn202852jNanomaterials 2022, 12, 4425 9 of 919. Guo, Y.; Wei, X.; Shu, J.; Liu, B.; Yin, J.; Guan, C.; Han, Y.; Gao, S.; Chen, Q. Charge Trapping at the MoS2-SiO2 Interface and ItsEffects on the Characteristics of MoS2 Metal-Oxide-Semiconductor Field Effect Transistors. Appl. Phys. Lett. 2015, 106, 103109.[CrossRef]20. Lee, C.; Rathi, S.; Khan, M.A.; Lim, D.; Kim, Y.; Yun, S.J.; Youn, D.H.; Watanabe, K.; Taniguchi, T.; Kim, G.H. Comparison ofTrapped Charges and Hysteresis Behavior in HBN Encapsulated Single MoS2 Flake Based Field Effect Transistors on SiO2 andHBN Substrates. Nanotechnology 2018, 29, 335202. [CrossRef]21. Wierzbowski, J.; Klein, J.; Sigger, F.; Straubinger, C.; Kremser, M.; Taniguchi, T.; Watanabe, K.; Wurstbauer, U.; Holleitner, A.W.;Kaniber, M.; et al. Direct Exciton Emission from Atomically Thin Transition Metal Dichalcogenide Heterostructures Near theLifetime Limit. Sci. Rep. 2017, 7, 12383. [CrossRef] [PubMed]22. Castellanos-Gomez, A.; Buscema, M.; Molenaar, R.; Singh, V.; Janssen, L.; van der Zant, H.S.J.; Steele, G.A. Deterministic Transferof Two-Dimensional Materials by All-Dry Viscoelastic Stamping. 2D Mater. 2014, 1, 011002. [CrossRef]23. Zhao, Q.; Wang, T.; Ryu, Y.K.; Frisenda, R.; Castellanos-Gomez, A.J. An Inexpensive System for the Deterministic Transfer of 2DMaterials. J. Phys. Mater. 2020, 3, 016001. [CrossRef]24. Niu, Y.; Gonzalez-Abad, S.; Frisenda, R.; Marauhn, P.; Drüppel, M.; Gant, P.; Schmidt, R.; Taghavi, N.S.; Barcons, D.; Molina-Mendoza, A.J.; et al. Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS2, MoSe2, WS2 andWSe2. Nanomaterials 2018, 8, 725. [CrossRef] [PubMed]25. Cong, C.; Shang, J.; Wang, Y.; Yu, T. Optical Properties of 2D Semiconductor WS2. Adv. Opt. Mater. 2018, 6, 1700767. [CrossRef]26. Ayari, A.; Cobas, E.; Ogundadegbe, O.; Fuhrer, M.S. Realization and Electrical Characterization of Ultrathin Crystals of LayeredTransition-Metal Dichalcogenides. J. Appl. Phys. 2007, 101, 014507. [CrossRef]27. Plechinger, G.; Nagler, P.; Kraus, J.; Paradiso, N.; Strunk, C.; Schüller, C.; Korn, T. Identification of excitons, trions and biexcitonsin single-layer WS2. Phys. Status Solidi Rapid Res. Lett. 2015, 9, 457. [CrossRef]28. Illarionov, Y.Y.; Knobloch, T.; Jech, M.; Lanza, M.; Akinwande, D.; Vexler, M.I.; Mueller, T.; Lemme, M.C.; Fiori, G.; Schwierz, F.;et al. Insulators for 2D nanoelectronics: The gap to bridge. Nat. Commun. 2020, 11, 3385. [CrossRef]29. Illarionov, Y.Y.; Rzepa, G.; Waltl, M.; Knobloch, T.; Grill, A.; Furchi, M.M.; Mueller, T.; Grasser, T. The role of charge trapping inMoS2/SiO2 and MoS2/hBN field-effect transistors. 2D Mater. 2016, 3, 035004. [CrossRef]30. Cavalcante, L.S.R.; da Costa, D.R.; Farias, G.A.; Reichman, D.R.; Chaves, A. Stark Shift of Excitons and Trions in Two-DimensionalMaterials. Phys. Rev. B 2018, 98, 245309. [CrossRef]31. Zhu, B.; Chen, X.; Cui, X. Exciton Binding Energy of Monolayer WS2. Sci. Rep. 2015, 5, 9218. [CrossRef] [PubMed]32. Sun, Z.; Beaumariage, J.; Xu, K.; Liang, J.; Hou, S.; Forrest, S.R.; Fullerton-Shirey, S.K.; Snoke, D.W. Electric-Field-Induced OpticalHysteresis in Single-Layer WSe2. Appl. Phys. Lett. 2019, 115, 161103. [CrossRef]33. Huard, V.; Cox, R.T.; Saminadayar, K.; Arnoult, A.; Tatarenko, S. Bound states in optical absorption of semiconductor quantumwells containing a two-dimensional electron Gas. Phys. Rev. Lett. 2000, 84, 187. [CrossRef] [PubMed]34. Zhang, C.; Wang, H.; Chan, W.; Manolatou, C.; Rana, F. bsorption of Light by Excitons and Trions in Monolayers of MetalDichalcogenide Mo S2: Experiments and Theory. Phys. Rev. B Condens. Matter Mater. Phys. 2014, 89, 205436. [CrossRef]35. Taghavi, N.S.; Gant, P.; Huang, P.; Niehues, I.; Schmidt, R.; de Vasconcellos, S.M.; Bratschitsch, R.; García-Hernández, M.;Frisenda, R.; Castellanos-Gomez, A. Thickness Determination of MoS2, MoSe2, WS2 and WSe2 on Transparent Stamps Used forDeterministic Transfer of 2D Materials. Nano Res. 2019, 12, 1691. [CrossRef]36. Taniguchi, T.; Watanabe, K.J. Synthesis of high-purity boron nitride single crystals under high pressure by using Ba-Bn solvent.Cryst. Growth 2007, 303, 525. [CrossRef]37. Berkdemir, A.; Gutiérrez, H.R.; Botello-Méndez, A.R.; Perea-López, N.; Elías, A.L.; Chia, C.I.; Wang, B.; Crespi, V.H.; López-Urías,F.; Charlier, J.C.; et al. Identification of individual and few layers of WS2 using Raman Spectroscopy. Sci. Rep. 2013, 3, 1755.[CrossRef]38. Castellanos-Gomez, A.; Quereda, J.; van der Meulen, H.P.; Agraït, N.; Rubio-Bollinger, G. Spatially Resolved Optical AbsorptionSpectroscopy of Single- and Few-Layer MoS2 by Hyperspectral Imaging. Nanotechnology 2016, 27, 115705. [CrossRef]39. Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim, C.Y.; Galli, G.; Wang, F. Emerging Photoluminescence in Monolayer MoS2.Nano Lett. 2010, 10, 1271. [CrossRef]40. Zhao, W.; Ghorannevis, Z.; Chu, L.; Toh, M.; Kloc, C.; Tan, P.H.; Eda, G. Evolution of Electronic Structure in Atomically Thin-Sheets of Ws 2 and Wse2. ACS Nano 2013, 7, 791. [CrossRef]41. Shree, S.; Paradisanos, I.; Marie, X.; Robert, C.; Urbaszek, B. Guide to Optical Spectroscopy of Layered Semiconductors. Nat. Rev.Phys. 2021, 3, 39. [CrossRef]http://doi.org/10.1063/1.4914968http://doi.org/10.1088/1361-6528/aac6b0http://doi.org/10.1038/s41598-017-09739-4http://www.ncbi.nlm.nih.gov/pubmed/28959034http://doi.org/10.1088/2053-1583/1/1/011002http://doi.org/10.1088/2515-7639/ab6a72http://doi.org/10.3390/nano8090725http://www.ncbi.nlm.nih.gov/pubmed/30223445http://doi.org/10.1002/adom.201700767http://doi.org/10.1063/1.2407388http://doi.org/10.1002/pssr.201510224http://doi.org/10.1038/s41467-020-16640-8http://doi.org/10.1088/2053-1583/3/3/035004http://doi.org/10.1103/PhysRevB.98.245309http://doi.org/10.1038/srep09218http://www.ncbi.nlm.nih.gov/pubmed/25783023http://doi.org/10.1063/1.5123514http://doi.org/10.1103/PhysRevLett.84.187http://www.ncbi.nlm.nih.gov/pubmed/11015866http://doi.org/10.1103/PhysRevB.89.205436http://doi.org/10.1007/s12274-019-2424-6http://doi.org/10.1016/j.jcrysgro.2006.12.061http://doi.org/10.1038/srep01755http://doi.org/10.1088/0957-4484/27/11/115705http://doi.org/10.1021/nl903868whttp://doi.org/10.1021/nn305275hhttp://doi.org/10.1038/s42254-020-00259-1 Introduction  Results and Discussions  Conclusions  References