Improving the Photoelectric Characteristics of MoS 2 Thin Films by Doping Rare Earth Element Erbium

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1 Meng nd M Nnoscle Reserch Letters (2016) 11:513 DOI /s NANO EXPRESS Improving the Photoelectric Chrcteristics of MoS 2 Thin Films y Doping Rre Erth Element Erium Miofei Meng nd Xiying M * Open Access Astrct We investigted the surfce morphologies, crystl structures, nd opticl chrcteristics of rre erth element erium (Er)-doped MoS 2 (Er: MoS 2 ) thin films fricted on Si sustrtes vi chemicl vpor deposition (CVD). The surfce mopogrphy, crystlline structure, light sorption property, nd the photoelectronic chrcteristics of the Er: MoS 2 films were studied. The results indicte tht doping mkes the crystllinity of MoS 2 films etter thn tht of the undoped film. Menwhile, the electron moility nd conductivity of the Er-doped MoS 2 films increse out one order of mgnitude, nd the current-voltge (I-V) nd the photoelectric response chrcteristics of the Er:MoS 2 /Si heterojunction increse significntly. Moreover, Er-doped MoS 2 films exhiit strong light sorption nd photoluminescence in the visile light rnge t room temperture; the intensity is enhnced y out twice tht of the undoped film. The results indicte tht the doping of MoS 2 with Er cn significntly improve the photoelectric chrcteristics nd cn e used to fricte highly efficient luminescence nd optoelectronic devices. Keywords: MoS 2 film, Er doping, Chemicl vpor deposition, Photoelectric chrcteristics, Light sorption, Photoluminescence Bckground Lyered qusi-two dimensionl (2D) chlcogenide mterils hve ttrcted gret interest due to their excellent opticl, electricl, ctlysis, nd luriction chrcteristics [1 3]. Especilly, 2D molydenum disulfide (MoS 2 ) hs een widely studied nd pplied in field-effect trnsistors [4, 5] nd energy hrvesting [6, 7]. It hs een found tht monolyer of MoS 2 hs direct ndgp of 1.8 ev when it is stripped from ulk mteril tht hs n indirect ndgp of 1.29 ev [8, 9]. This lrge chnge in the energy nd holds gret potentil for pplictions of MoS 2 in optoelectronic fields, such s red photodiodes nd photodetectors [10, 11]. However, the efficiency of photoluminescence (PL) nd photoelectric conversion of 2D MoS 2 [12] re reltively low. Reserchers hve explored mny venues to improve the PL intensity nd the response rte of MoS 2 films. For exmple, Singh et l. found tht gold nnoprticles my impose n * Correspondence: mxy@mil.usts.edu.cn Suzhou University of Science nd Technology, Kerui Rod No. 1, Goxin section, Suzhou , Jingsu, Chin ovious p-doping effect in single-lyer nd i-lyer MoS 2 smples, resulting in enhnced PL [13]. Rre erth elements (REE) re ctive elements tht hve een widely dded in optoelectronic devices to improve PL nd photoelectric conversion efficiency [14, 15]. So fr, there hve een few reports for REE-doped MoS 2. Herein, we report study of the doping effects of rre erth element Er on the surfce morphologies, crystl structures, nd opticl chrcteristics of MoS 2 thin films. Pure MoS 2 nd Er: MoS 2 smples were fricted on Si sustrtes y chemicl vpor deposition (CVD). Additionlly, we systemticlly nlyzed the surfce morphologies, structures, nd opticl sorption chrcteristics of the smples. Methods Er (NO 3 ) 3 5H 2 O (99.9%) nd MoS 2 powder (AR, 99%) regents were used s the precursor mterils. A mixed solution comprising 1-g nlyticl grde MoS 2 micro powder, 1-g nlyticl grde erium nitrte penthydrte (Er(NO 3 ) 3 5H 2 O) crystls, nd 200 ml of diluted sulfuric cid (H 2 SO 4 ) ws formed y mixing the ove mentioned components for 5 min, The Author(s) Open Access This rticle is distriuted under the terms of the Cretive Commons Attriution 4.0 Interntionl License ( which permits unrestricted use, distriution, nd reproduction in ny medium, provided you give pproprite credit to the originl uthor(s) nd the source, provide link to the Cretive Commons license, nd indicte if chnges were mde.

2 Meng nd M Nnoscle Reserch Letters (2016) 11:513 Pge 2 of 5 followed which the solution ws mintined t 70 C vi wter th. The CVD system consisted of horizontl qurtz tue furnce, vcuum system, n intke system, nd wter th. The Si sustrtes were plced in the center of the furnce, nd susequently, the pressure in the furnce ws reduced to 10 2 P nd the furnce ws heted up to 650 C for 20 min. Ar gs ws introduced into the mixed solution t flow rte of 25 sccm, crrying Er 3+ nd MoS 2 molecules into the furnce. Furthermore, to investigte the mteril properties of MoS 2 films, some pure MoS 2 smples were deposited on the Si sustrtes y the sme method. The surfce morphologies nd crystlline structures of the thin films were chrcterized using tomic force microscope (AFM) nd X-ry diffrction (XRD). The electricl properties of the thin films were nlyzed y Hll Effect Mesurement System (HMS-3000, Ecopi, Anyng, South Kore). The ultrviolet-visile (UV-vis) sorption spectr nd photoluminescence properties of the smples were investigted y UV-vis spectrophotometer (Shimdzu UV-3600) nd fluorescence spectrophotometer t room temperture. Photocurrent current-voltge (I-V) curves of the doped nd undoped MoS 2 /Si heterojunction were investigted y semiconductor nlysis system (Keithley 4200). Results nd Discussion The AFM imges of the pure MoS 2 nd Er: MoS 2 thin films on the Si sustrtes re shown in Fig. 1. The surfce of the pure MoS 2 film in Fig. 1 is continuous film with n verge thickness out 25 nm, nd some quntum dots round 20 nm re uniformly scttered on the Si sustrte. The Er: MoS 2 film shown in Fig. 1 is lrge fluctution film composed of compct quntum dots with uniform color, nd the verge thickness is out 50 nm. For the sme deposition conditions nd time, the density nd size of the quntum dots in Er: MoS 2 film increse remrkly resulting from the ctlytic ction of Er 3+ on the deposition course. The crystl structures of the synthesized smples were chrcterized y using the X-ry diffrction (XRD) technique, s shown in Fig. 2. For the pure MoS 2 smple, there re four shrp diffrction peks locted t 14.7, 47.8, 54.6, nd 56.4, corresponding to the (002), (105), (106), nd (110) crystl plnes of MoS 2, respectively, showing tht the film is chrcterized y polycrystl structure. In the Er: MoS 2 film, the position of the ove four diffrction peks is lmost the sme s tht of pure MoS 2. Besides, there re two more peks t 29.5 nd 44.8, corresponding to the (004) nd (009) plnes, respectively. No diffrction peks from elementl Er is oserved, indicting tht the Er doping does not chnge the crystl structure of the MoS 2 film. Er toms were Fig. 1 AFM imges of MoS 2 smples. The MoS 2 film. The doped in MoS 2 film in the wy of sustitution doping, nd Mo tom ws replced y Er element. By doping, the diffrction peks of MoS 2 crystl incresed nd the diffrction intensity ws enhnced, showing tht doping improved the crystllinity of the MoS 2 films. The surfce J-V properties, crrier moilities, nd Hll coefficients of the MoS 2 nd Er: MoS 2 smples were Intensity(.u.) (002) (002) ()MoS 2 (110) (105) (009) (106) (004) (105) Fig. 2 XRD ptterns of the MoS 2 film nd the for the diffrction ngle in the rnge of 10 ~60

3 Meng nd M Nnoscle Reserch Letters (2016) 11:513 Pge 3 of 5 mesured using Hll Effect mesurement system vi the four mesured points on the smples t drk condition, s shown in Fig. 3. The currents of the smples show liner dependency on the pplied voltge, reveling tht the films hve good conductivity. The slopes of the J-V curves show the resistivity of the MoS 2 smples. The curve of the Er: MoS 2 film hs good linerity nd smll slope, with the films showing significnt reduction in resistivity when Er ions re doped. According to the eqution for clcultion of moility: σ = nqμ (σ is conductivity, n is electron concentrtion, q is electron chrge, μ is moility), the electron motilities in the MoS 2 nd Er: MoS 2 films re cm 2 /Vs nd cm 2 /Vs, respectively. Note tht the moility vlue for the MoS 2 film is oviously improved y doping Er 3+. Furthermore, According to the eqution for the Hll coefficients: ε y = R H J x B z (ε y is electric field intensity, R H is Hll coefficients, J x is current density, B z is mgnetic induction intensity), the Hll coefficients of the MoS 2 nd Er: MoS 2 films re cm 3 /C nd cm 3 /C, respectively, showing tht the films re p-type semiconductors. The J-V curves in the MoS 2 film show significnt decrese in resistivity fter Er doping. Good conductive properties cn reduce the surfce het loss in the photodetector, therey incresing the lifetime nd frequency response of the MoS 2 photovoltic device. Figure 4 shows the sorption spectr of the pure MoS 2 nd Er: MoS 2 films in the visile light rnge. Clerly, the sorption of the Er: MoS 2 film is enhnced significntly y doping Er, ttriuting to the sorptions of Er ions nd the impurity energy level in the ndgp of MoS 2 y doping Er. Additionlly, few mximum vlues emerge t 475, 578, 670, nd 735 nm in the sorption spectr, showing tht the film hs strong light sorption in these wvends. The sorption pek t Intensity(.u.) ()MoS wvelength(nm) Fig. 4 The sorption spectr of the MoS 2 film nd the 670 nm corresponds to ndgp width of 1.85 ev in the MoS 2 film, close to the energy gp of monolyer of MoS 2, 1.80 ev. MoS 2 films hve strong sorption t 735 nm, which cn e considered s the opticl sorption edge, corresponding to ndgp width of 1.69 ev. Therefore, the doping of Er significntly improves the light sorption nd does not chnge the position of the sorption pek. The increse of the light sorption of Er 3+ -doped MoS 2 film cn e improved y the photoelectric trnsformtion nd photovoltic effect of MoS 2 semiconductor devices. Figure 5 shows the photoluminescence spectr of the MoS 2 nd Er: MoS 2 film excited y 360 nm light t room temperture. In the pure MoS 2 film, n ovious PL pek is centered t 693 nm, coinciding with the intrinsic rditive trnsition photoluminescence of the single-lyer MoS 2. In the Er: MoS 2 film, two significntly enhnced PL peks re locted t 394 nd 693 nm ech. The pek t 394 nm is due to the trnsitions from the 2 H 11/2 energy level to the ground stte 4 I 15/2 [16 18]. 80 ()MoS 2 60 ()Er:MoS A Voltge (V) J (na/cm 2 ) B Fig. 3 The surfce J-V chrcteristic curves of the MoS 2 film nd the intensity(.u.) 6.0x x x10 7 ()MoS2 4.5x10 7 () Er:MoS 4.0x x x x x x x x wvelength(nm) Fig. 5 The photoluminescence spectr of the MoS 2 film nd the

4 Meng nd M Nnoscle Reserch Letters (2016) 11:513 Pge 4 of ()MoS I(A) U(V) Fig. 6 Photocurrent I-V curves of the doped nd undoped MoS 2 /Si heterojunction The intense pek t 693 nm is lrgely enhnced result from the direct-gp luminescence of MoS 2. It is importnt to note tht the PL intensity of the Er: MoS 2 film is lmost twice s strong s tht of the undoped film, i.e., the doping of Er in MoS 2 cn lrgely improve the sorption nd photoluminescence efficiency of MoS 2, which in turn cts s n exciting ctive center in the film. The photocurrent I-V ehvior of the MoS 2 -Si heterojunction ws otined while irrditing the surfce of the films y stndrd white light with power of 100 mw/cm 2, s shown in Fig. 6. For two smples, the current increses exponentilly with n increse in the voltge. The short-circuit currents (I SC ) of the MoS 2 nd Er: MoS 2 film smples re nd 4.35 ma, respectively, nd the open-circuit voltge (U OC ) is nd mv, respectively. Oviously, fter Er doping the short-circuit current nd open-circuit voltge oth increse significntly. This is ecuse the doped Er ions will increse light sorption, resulting in n increse in the numer of photo-generted crriers nd finlly enhncing the photocurrent response. Conclusions We hve studied the effects of Er doping on the surfce morphologies, crystlline, opticl sorption, PL, nd photoelectricl properties of MoS 2 films. We found tht the Er 3+ ions do not chnge the crystl structure of MoS 2 films ut mke the crystllinity etter. At the sme time, Er 3+ doping improves the crrier moility nd enhnces the current-voltge (I-V) chrcteristics of the MoS 2 thin films. Additionlly, Er 3+ -doped MoS 2 films exhiit stronger light sorption nd photoluminescence in the visile light rnge t room temperture. The results show tht Er 3+ -doped MoS 2 film cn e used to fricte highly efficient luminescence nd optoelectronic devices. Arevitions 2D: Qusi-two dimensionl; AFM: Atomic force microscope; CVD: Chemicl vpor deposition; Er(NO 3 ) 3 5H 2 O: Erium nitrte penthydrte; Er: Erium; Er: MoS 2 : Erium-doped MoS 2 ;H 2 SO 4 : Sulfuric cid; I SC : Short-circuit currents; I-V: Current-voltge; PL: Photoluminescence; REE: Rre erth elements; U OC : Open-circuit voltge; UV-vis: Ultrviolet-visile; XRD: X-ry diffrction Acknowledgements This work ws supported in prt y the Innovtion Progrm for Postgrdute of Suzhou University of Science nd Technology (No. SKCX15_065), the Ntionl Nturl Science Foundtion of Chin (No ), nd the Scientific Project Progrm of Suzhou City (No. SYN201511). Funding The Innovtion Progrm (No. SKCX15_065) cts s guide to the design of the study nd the collection, nlysis, nd interprettion of the dt. The others (No nd No. SYN201511) support the collection of dt nd the puliction of the study. Authors Contriutions MM prticipted in the friction of MoS 2, mesured nd nlyzed the dt, nd wrote the mnuscript. XM interpreted the dt. Both uthors red nd pproved the finl mnuscript. Authors Informtion MM is grdute student mjor in the friction of semiconductor nnometer mterils. XM is professor nd PhD degree holder specilizing in semiconductor mterils nd devices, especilly expert in nnoscled opticl-electronic mterils nd optoelectronic devices. Competing Interests The uthors declre tht they hve no competing interests. Received: 28 Septemer 2016 Accepted: 9 Novemer 2016 References 1. Muller GA, Cook JB, Kim H, Tolert SH, Dunn B (2015) High performnce pseudocpcitor sed on 2D lyered metl chlcogenide nnocrystls. Nno Lett 15: Lemke D, Bertolzzi S, Kis A (2015) Single-lyer MoS 2 electronics. Accounts Chem Res 48: Mitr U, Gupt U, De M (2013) Highly effective visile-light-induced H 2 genertion y single-lyer 1T-MoS 2 nd nnocomposite of few-lyer 2H-MoS 2 with hevily nitrogented grphene. 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