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Controlled Gas Molecules Doping of Monolayer MoS via Atomic Layer Deposited AlO Films 2

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Yuanzheng Li, Xinshu Li, Heyu Chen, Jia Shi, Qiuyu Shang, Shuai Zhang, Xiaohui Qiu, Zheng Liu, Qing Zhang, Haiyang Xu, Weizhen Liu, Xinfeng Liu, and Yichun Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08893 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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ACS Applied Materials & Interfaces

Controlled Gas Molecules Doping of Monolayer MoS2 via Atomic Layer Deposited Al2O3 Films Yuanzheng Li,†,‡,ǁ Xinshu Li,†,ǁ Heyu Chen,†,ǁ Jia Shi,‡ Qiuyu Shang, § Shuai Zhang,‡ ⊥ Xiaohui Qiu,‡ Zheng Liu, Qing Zhang,§ Haiyang Xu,†,ǁ* Weizhen Liu,†,ǁ* Xinfeng Liu, ‡* and Yichun Liu†,ǁ †

Centre for Advanced Optoelectronic Functional Materials Research and Key Laboratory of UV-Emitting Materials and Technology (Northeast Normal University), Ministry of Education, Changchun 130024, China Email: [email protected] and [email protected] ‡

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China Email: [email protected]

§

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China

ǁ

National Demonstration Center for Experimental Physics Education (Northeast Normal University), Changchun 130024, China ⊥

Center for Programmable Materials, School of Materials Science and Engineering,

Nanyang Technological University, Singapore 639798, Singapore.

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ACS Paragon Plus Environment


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ABSTRACT: MoS2 as atomically thin semiconductor is highly sensitive to ambient atmosphere (e.g., oxygen, moisture etc.) in optical and electrical properties. Here we report a controlled gas molecules doping of monolayer MoS2 via atomic layer deposited Al2O3 films. The deposited Al2O3 films, in the shape of nanospheres, can effectively control the contact areas between ambient atmosphere and MoS2 that allows precise modulation of gas molecules doping. By analyzing photoluminescence (PL) emission spectra of MoS2 with different thickness of Al2O3, the doped carrier concentration is estimated at ~2.7×1013 cm−2 based on the mass action model. Moreover, time-dependent PL measurements indicate an incremental stability of single layer MoS2 as the thickness of Al2O3 capping layer increase. Effective control of gas molecules doping in monolayer MoS2 provide us a valuable insight into the applications of MoS2 based optical and electrical devices.

KEYWORDS: Monolayer MoS2, Atomic layer deposition (ALD), Gas molecules doping, Al2O3, Stability

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Molybdenum disulfide (MoS2) has become one of the most widely studied materials over recent years due to its excellent optical and electrical properties.1-3 Bulk MoS2 whose layers hold together by weak van der Waals interactions can be separated easily to form single layer MoS2 film and also exhibits fascinating layer-number dependent electronic and optical properties.4,5 Different from its bulk counterparts, monolayer MoS2 reveals unique optical and electrical characteristics such as strong excitonic binding, large specific surface area, remarkable room temperature high mobility and so forth.6-8 Single layer MoS2 is promising for diverse potential applications in optoelectronic devices such as field effect transistors (FETs), photovoltaic cells, and light emitting diodes (LEDs).9-11 However, monolayer MoS2 as atomically thin semiconductor is highly sensitive to ambient atmosphere (e.g., oxygen, moisture etc.) in optical and electrical properties, severely limiting its industrial or practical applications in the future. The main cause is that gas molecules acting as carrier acceptors can drain free electrons from MoS2 thin films via some defect-sites or edges and then change the optical and electrical properties of MoS2.12,13 To minimize influence of gas molecules, previous works have been devoted to improving the stability of monolayer MoS2 films with the help of covering protective layers such as Al2O3 and so forth.14,15 On the other hand, the instability of monolayer MoS2 can also be utilized to modify its physical and chemical properties. Most recently, some works have been reported on manipulating the optical properties of monolayer MoS2 based on controlling its carrier concentration, such as electrical gate bias tuning,8 chemical organic-molecule doping,16 graphene quantum 3



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dots doping,17 as well as abovementioned gas molecules doping.12,13 As can be seen, control of the doping degree is an effective method to control its optical properties. However, using external electric field to control carrier concentration may be not an energy-saving proposal. Though combining of MoS2 with electron-donor (accepter) materials could provide a valid solution of tunable optical properties for single-layer MoS2 films, this method limited by traditional “drop-cast” technology are not enough to achieve relatively precise doping effect, especially for relatively diminutive doped ranges. For gas molecules doping, it relies on the pressure of ambient atmosphere to control the doping level,13 restricting its practical applications. Therefore, seeking for a controllable and effective doping method is strongly desired for the applications of MoS2 in optoelectronic device field.

Generally, the thickness of atomic layer deposited thin films can be accurately controlled through optimizing the proportion of precursor, purging gas injections, as well as the number of growth-cycles. Moreover, we have also found the atomic layer deposited Al2O3 films are not able to form a uniform and continuous thin film on MoS2 surfaces but in the shape of nanospheres. For this reason, controlling the growth thickness of Al2O3 layers could be regarded as a valid method to change the contact areas between gas molecules and MoS2, allowing the manipulation of gas molecules doping. This method could provide a possibility for accurate and facile control of gas molecules doping in MoS2 as well as other TMDC systems.

In this work, the Al2O3 films are synthesized on monolayer MoS2 surfaces from 4 4


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to 24 nm using atomic layer deposition (ALD) method. Our results show that controlling the growth thickness of Al2O3 films can achieve a tunable gas molecules doping of monolayer MoS2. Relying on the comprehensive analyses of PL spectra, the changed carrier concentration is estimated as ~2.7×1013 cm−2 based on mass action model. Moreover, time-dependent photoluminescence (TDPL) measurements are performed and show an increased stability of monolayer MoS2 as the Al2O3 capping layers thicken. Our method can serve as an appropriate supplement for fundamental studies and technical applications of TMDC materials in the future

Monolayer MoS2 films are synthesized by chemical vapor deposition (CVD) and more detailed experiments are supplied by our previous work.18 Figure 1a shows an optical microscope image of monolayer MoS2 film on an oxide-on-silicon substrate with oxide films of 300 nm thick. Corresponding Raman spectra (Horiba, Evolution) of monolayer MoS2 thin film are demonstrated with two characteristic Raman peaks (see Figure 1b), namely in-plane mode (E12g) at 384.7 cm-1 and out-of-plane mode (A1g) at 403.3 cm-1, calibrated by Si LO phonon mode at 520.7 cm-1. The continuous-wave laser, with a 488 nm excitation wavelength and ~1 mW power, is focused to a ~1 µm spot in our measurements. It can be seen that the frequency discrepancy (∆ω) between the two Raman modes is about 18.6 cm-1, which is well agreement with the ∆ω value of monolayer MoS2 reported by other studies.19-22 Given a further study to determine the uniform of the monolayer MoS2 films, the Raman-scattering mapping of the E12g mode at 384.7 cm-1 and A1g mode at 403.3 cm-1 are performed, and the relevant results are shown in Figure 1c and d, respectively. The 5



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homogeneous intensity distribution of E12g and A1g modes clearly demonstrate that the obtained MoS2 film is a spatially uniform within the monolayer limit.

Subsequently, Al2O3 layers with thickness ranging from 4 to 24 nm are prepared on the monolayer MoS2 surfaces via ALD method. An intuitive view of color-contrast from these samples is observed from purple to blue as thickness of Al2O3 films increase, as shown in Figure 1e. Corresponding growth-dynamic process of Al2O3 layers on monolayer MoS2 is schematically demonstrated in Figure 1f. In atomic layer deposited process, a typical deposition cycle of Al2O3 consists of alternating precursor and purging gas injections (trimethylaluminum (TMA)/N2/H2O/N2 injections for 0.03/30/0.03/30 seconds in our work). The relatively longer purging time ensure the adequate removal of residual precursors. However, monolayer MoS2 with graphene-like structure, lacking effective dangling bonds, is difficult to react with TMA during the synthesis process of Al2O3 layers.14 Furthermore, due to MoS2 with low surface energy, it maybe induces the amorphous Al2O3 films to be in the shape of “spherical structure” on MoS2 surface, just as water droplets on the lotus leaf. To confirm this assumption and monitor the morphology of Al2O3 films, atomic force microscope (AFM, Bruker, Dimension Icon) is performed. A typical morphology image of monolayer MoS2 with 24 nm Al2O3 films is demonstrated in Figure 1g. Indeed, the synthesized Al2O3 are in the shape of loose and unordered nanospheres, which is good agreement with our assumption. With thickness of Al2O3 films increasing, the coverage on MoS2 surface gradually enlarges (see Supporting Information Figure S1), which is well comparable with our hypothesis. Whether these 6


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Al2O3 nanospheres could effectively modulate the gas molecules doping of monolayer MoS2, we have established adequate experiments to clarify it as follows.

Generally speaking, the electron concentration of MoS2 is connected with its optical properties, such as PL intensity, peak-position.16,17 We firstly have performed photoluminescence (PL) measurements for monolayer MoS2 with different thicknesses Al2O3 capping layers, as shown in Figure 2a. The two characteristic peaks, respectively specified as exciton B and A, result from the valence band splitting induced by spin–orbital coupling interactions.5 Both PL intensity and peak-position exhibit obviously changes as thicknesses of Al2O3 films augment. More details are as follows. Figure 2b illustrates that the PL mappings of MoS2 with 4 nm, 12 nm and 20 nm Al2O3 films, which are measured at 660 nm, 674 nm and 690 nm, respectively. An intuitive view of the PL red-shifting and uniform distributions of PL intensity can be seen from the three mapping images. For a quantitative analysis, the integrated intensity of exciton A, extracting from Figure 2a, tends to monotonously decrease before 16 nm Al2O3 layers, and then presents an abnormal enhancement as the thickness of Al2O3 layers further increase (see Figure 2c). What is the underlying mechanism for intensity attenuation and anomalous enhancement? As mentioned above, the contact areas between gas molecules and monolayer MoS2 gradually decrease as the thickness of Al2O3 gradually increases. Moreover, gas molecules as carrier acceptors can drain surface electrons of MoS2, causing a p-type doping. That means the weaken gas molecules adsorption would suppress surface electrons transfer and supply an n-type doping for MoS2. 7



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Figure 3a schematically demonstrates the interactions between gas molecules and monolayer MoS2. With monolayer MoS2 limit, a p/n-type doping usually could induce a blue/red shifting of in-plane Raman mode (A1g) due to the changes of electron-phonon interaction, but the peak-position of out-of-plane Raman mode (E12g) is almost unaffected.23,24 This result is well agreement with our experimental data of Raman spectra (see Figure 3b and 3c). More important is the increased electron concentration in monolayer MoS2 limit will improve the weight of trions (negatively charged excitons) to induce the whole PL quenching and red-shifting.16,17,25 Underlying reason is that the trions, with lower energy level than exciton A, usually tend to release energy via some nonradiative relaxation channels,26 leading to a huge decline in radiative recombination efficiency. Hence, as the degree of n-type doping increases, PL attenuation and red-shifting are well explained in this study. In addition, through comprehensive analysis of Raman spectra, both surface strain (pressure) effect and dielectric screening of Al2O3 films are ruled out in our work (see Supporting Information Figure S2 for more details). To conclude, the increment of electron concentrations arise from reduced gas molecules doping in our work.

However, the weakened doping effect is not adequate to explain the enlarged PL intensity when thickness of Al2O3 film exceeds 16 nm. To explore the underlying mechanism of the PL enhancement, we focus on the synthesized process of Al2O3 layers. In the first place, a quite low vacuum (about 3 milli-Torr) and preheating of 373 K should be obtained before the beginning of deposited Al2O3 and hold over 60 minutes. That means the adsorbed gas molecules on sample surface, mainly 8


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containing moisture molecules and oxygen molecules, will be almost wiped off. During the whole growth process of Al2O3 layers, Al2O3 nanospheres (see Figure 1g) are increasingly deposited on MoS2 surface, gradually insulating MoS2 films from ambient atmosphere and weaken gas molecules doping. Note that deposition rate of Al2O3 is relatively slow (~ 0.1 nm/min) and a heater with ~423 K is performed on the bottom of samples to provide sufficient energy for the chemical reaction between TMA and H2O (see Figure 1f). The MoS2 films therefore will sustain a long baking-time at 423 K during preparation process of Al2O3, which maybe induce that some defect-states (such as sulfur or molybdenum vacancies etc.) are created to localize or trap free excitons. On the basis of previous study,13 these free excitons localized by defects, namely bound excitons (Xbe), could change the optical properties of MoS2. The bound exciton is quite stable and could even avoid nonradiative recombination.12 As a result, the bound excitons could possess higher radiative recombination rate and quantum efficiency than free excitons. Therefore, the increased proportion of bound excitons could induce PL intensity enhancement. Herein, existence of bound excitons is confirmed in our experiment based on the time-resolved photoluminescence (TRPL) spectra (see Supporting Information, Figure S3 for more details).

To quantify electron concentrations by PL spectra, band diagrams of exciton A (XA), exciton B (XB), trion (X-) and bound exciton (Xbe) are schematically shown in Figure 4a. PL multi-peak deconvolution fittings of monolayer MoS2 with 4 nm, 12 nm and 20 nm Al2O3 films are respectively shown in Figure 4b; these PL spectra are well 9



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reproduced by the sum of these four peak components. We first consider the relation between exction A and bound exciton and accept a rough estimate to quantitatively calculate it as follows. In traditional semiconductors system, the lifetime of bound excitons is at least two orders of magnitude larger than that of free exciton.27,28 Otherwise, the relation between PL lifetime and internal quantum-efficiency (IQE) can be expressed as follows:29 1/τ = krad + knon

(1)

ƞIQE = krad/( krad + knon)

(2)

where krad and knon represent the radiative and nonradiative recombination rates, respectively. Since bound excitons with longer lifetime could even avoid nonradiative recombination, for simplicity, we approximately consider only the knon of bound exciton is significantly reduced but its krad stays the same in contrast to free exciton. Based on equation (1) and (2), we deduce that the IQE of bound exciton is about 100 times as large as that of exciton A. In term of this result as well as the PL weight of bound exciton extracted from Figure 4b, we find that the number of bound exciton is far lesser than exciton A. We therefore neglect the contributions of bound excitons on the calculation of electron concentration. As for negatively charged trion, it usually consists of an exciton and an electron (or an exciton and a hole) via coulomb interactions in a high electron concentration environment as well as strong quantum confinement system. In monolayer MoS2, the quasi-particle of trion is composed of an exciton A and an electron and therefore corresponding population of exciton (NX) and trion (NX′) can be expressed as:16 10


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dNX/dt = G-[Γex + ktr(n)] Nx

(3)

dNX′/dt = ktr(n) Nx - Γtr NX′

(4)

where n is the thickness of Al2O3 layers, ktr(n) is the formation rate of the trions from the excitons, and G is the generation rate of excitons. The emission rates of excitons (Γex) and trions (Γtr) are respectively 0.002 ps−1and 0.02 ps−1, obtained from the previous studies.30 Based on the above results, PL weight of excitons and trions can be also derived since PL intensity is proportional to the populations of excitons and trions. The changed electron concentration as a function of PL weight of trions is obtained using mass action model (see Supporting Information Note 1 for more details) as follows:16 ூ೉ష ூ೟೚೟ೌ೗

≈

ସ×ଵ଴షభర ௡೐ ଵାସ×ଵ଴షభర ௡೐

(5)

The PL intensity weight of trions calculated from equation (5) is shown in Figure 4(c). The electron concentration exhibits a monotonously augmented tendency with PL weight of trions gradually increasing, which is good agreement with our experimental results. The inset of Figure 4c illustrates PL weight of trions as a function of Al2O3 growth thickness. The weight of trions does not abidingly increase with the growth thickness of Al2O3 films but presents a slight saturated after 8 nm, which may be caused by two reasons: (1) reduced gas molecules doping with more pyknotic Al2O3 films; (2) finite number of donor defect-states. To sum up, a new relationship between Al2O3 films thickness and electron concentration based on mass action model is illustrated in Figure 4d. The difference of electron concentration (2.7×1013 cm−2), 11



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between bare monolayer MoS2 and with 24 nm Al2O3 layers, is well comparable with other doping methods.16,17 Modulation of carrier concentration can be achieved in a relatively large ranges by controlling the thicknesses of atomic layer deposited Al2O3 films, prospectively promoting the applications of 2D TMDCs in the future optoelectronic field.

In recent years, some works have demonstrated capping passivation layer can provide an excellent stability for monolayer MoS2 films. For example, J. Late et al. reported that absorption of moisture on the surface of MoS2 can be effectively blocked by 30 nm Si3N4 capping layer.15 Besides, W. Park et al. confirmed the organic polymer such as polymethyl methacrylate (PMMA) could be also regarded as ideal passivation layer to protect MoS2.31 In our study, due to the decrement of contact areas between ambient atmosphere and MoS2 films the stability of monolayer MoS2 films may be improved in some extent as the thicknesses of Al2O3 capping layers gradually increase. To clarify this hypothesis, the time-dependent PL spectra of monolayer MoS2 with 0 nm, 8 nm, 16 nm and 24 nm Al2O3 capping layers are performed and illustrated in Figure 5, which are respectively measured at fresh, 20 and 40 days later. We note in bare monolayer MoS2, the PL intensity signal of exciton A decays to about one-third of initial intensity after 40 days air-exposed, and meanwhile the peak-position red shift (Figure 5a). As for capping 8 nm and 16 nm Al2O3 layer, though these two PL intensities are still decreasing, the rate of PL attenuation with air-exposed time exhibits a slight decreased tendency when the capping layer gets thicker (see Figure 5b and 5c). While the capping layer reach 24 12


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nm, despite the monolayer MoS2 suffers 40 days air-exposed treatment, its PL intensity and peak-position still reveal ignorable variations (see Figure 5d). Indeed, a thicker Al2O3 layers could effectively insulate MoS2 from ambient atmosphere and avoid gas molecule doping as well as surface oxidation. On the other side, the enhancive stability in optical properties of MoS2 could also affirm the existence of gas molecule doping effect. Control of Al2O3 capping layers on MoS2 surface not only could supply a tunable gas molecules doping for MoS2, but provide a valid protection for MoS2 film in optical properties as well as carrier concentration.

In summary, we have demonstrated a possible in tuning gas molecules doping of monolayer MoS2 by atomic layer deposited Al2O3 capping films. As the thicknesses of Al2O3 capping films increase, the electrons transfer from MoS2 to gas molecules is remarkable suppressed, which induces the PL intensity attenuation and red-shifting. Based on the analysis of PL spectra, the doped electron concentration is estimated as 2.7×1013 cm-2 using mass action model. TDPL measurements show a high stability of MoS2 with 24 nm Al2O3 films, in contrast to bare MoS2. Our results not only provide a possibility for accurately tuning the gas molecules doping in monolayer MoS2, but also expand the horizons for using atomically thin MoS2 and other TMDC materials in the future applications.

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ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Materials and methods; schematic diagram and AFM images of the hetero-structure comprising MoS2 and Al2O3; detailed analysis of Raman spectra; time-resolved PL spectra; detail for mass action model. Corresponding Author *Email: [email protected], [email protected] and [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the NSFC for Excellent Young Scholars (No. 51422201), the Program of the NSFC (No. 61505026, 61604037, 61574031, 11604044 and 51602028), “111” Project (No. B13013), Fund from Jilin Province (No. 20160520009JH, 20160520115JH and 20160520114JH), Fundamental Research Funds for the Central Universities (No. 2412017FZ010 and 2412016KJ017). X.F.L thanks support from Ministry of Science and Technology (No.2016YFA0200700 and 2017YFA0205004), National Natural Science Foundation of China (No.21673054), Key Research Program of Frontier Science, CAS (No.QYZDB-SSW-SYS031) and Open Project of Key Laboratory for UV-emitting Materials and Technology of Ministry of Education (130028699). Z. L thanks Singapore National Research Foundation under NRF RF Award No. NRF-RF2013-08.

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Figures and captions

Figure 1 (a) Optical image of the as-prepared monolayer MoS2. A scratch is made onto the left corner to clearly identify the difference between MoS2 and substrate. Scale bar inside: 100 µm. (b) Typical Raman spectra of monolayer MoS2 on Si/SiO2 substrate. Inset is the magnification for the two peaks at range of ~370 to 420 cm-1. (c) Raman peak intensity mapping of in-plane mode at 384.7 cm-1 and (d) out-of-plane at 403.3 cm-1 from the red-dashed rectangular region show in (a). Scale bar: 20 µm. (e) Optical graphs of varying growth-thicknesses of Al2O3 on monolayer MoS2 films. (f) Schematic diagram of the Al2O3 layer synthesized by ALD on monolayer MoS2 films. (g) AFM images of monolayer MoS2 with 24 nm Al2O3. Scale bar: 1 µm.

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Figure 2 (a) PL measurement of monolayer MoS2 with varying growth thicknesses of Al2O3 films. (b) PL intensity mapping of monolayer MoS2 with 4 nm, 12 nm, 24 nm Al2O3 films at 660 nm, 674 nm, and 690 nm, respectively. Scale bar: 1µm. (c) The PL integrated intensity versus growth thicknesses of Al2O3.

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Figure 3 (a) Sketch of gas molecules doping in monolayer MoS2 regime. (b) Raman spectra of monolayer MoS2 with varying thicknesses of Al2O3 dielectric layer. (c) The peak position of out-of-plane mode (red quadrate) and in-plane mode (blue rhombus) as a function of Al2O3 growth thickness, respectively.

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Figure 4 (a) Schematics of the exciton-related radiative transition at the K point in the Brillioun zone. ∆, εX, εX-, εbe denote the valence band splitting, binding energy of the neutral exciton XA, trion X- and bound exciton Xbe, respectively. (b) The PL multi-peak deconvolution fittings of monolayer MoS2 films with 4 nm, 12 nm, and 20 nm thickness Al2O3. (c) Calculations of charge density (ne) based on the law of mass action model; the insets are the PL weight of trions as a function of the growth thicknesses of Al2O3. (d) The charge density (ne) as a function of the growth thicknesses of Al2O3.

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Figure 5 Time dependent PL spectra of monolayer MoS2 with 0 nm (a), 8 nm (b), 16 nm (c) and 24 nm (d) Al2O3 films, which are measured at fresh, 20 days later and 40 days later, respectively.

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Controlled Gas Molecules Doping of Monolayer ... - ACS Publications

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