Trion-Induced Negative Photoconductivity in Monolayer Mos[Subscript 2]

Trion-Induced Negative Photoconductivity in Monolayer MoS[subscript 2] The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Lui, C.H., et al. "Trion-Induced Negative Photoconductivity in Monolayer MoS[subscript 2]." Phys. Rev. Lett. 113, 166801 (October 2014). © 2014 American Physical Society As Published http://dx.doi.org/10.1103/PhysRevLett.113.166801 Publisher American Physical Society Version Final published version Citable link http://hdl.handle.net/1721.1/91027 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. week ending PRL 113, 166801 (2014) PHYSICAL REVIEW LETTERS 17 OCTOBER 2014 Trion-Induced Negative Photoconductivity in Monolayer MoS2 C. H. Lui,1 A. J. Frenzel,1,2 D. V. Pilon,1 Y.-H. Lee,3,4 X. Ling,3 G. M. Akselrod,1 J. Kong,3 and N. Gedik1,* 1Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 2Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA 3Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 4Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan (Received 19 June 2014; published 16 October 2014) Optical excitation typically enhances electrical conduction and low-frequency radiation absorption in semiconductors. We, however, observe a pronounced transient decrease of conductivity in doped monolayer molybdenum disulfide (MoS2), a two-dimensional (2D) semiconductor, using ultrafast optical- pump terahertz-probe spectroscopy. In particular, the conductivity is reduced to only 30% of its equilibrium value at high pump fluence. This anomalous phenomenon arises from the strong many-body interactions in the 2D system, where photoexcited electron-hole pairs join the doping-induced charges to form trions, bound states of two electrons and one hole. The resultant increase of the carrier effective mass substantially diminishes the conductivity. DOI: 10.1103/PhysRevLett.113.166801 PACS numbers: 73.50.Pz, 71.35.Pq, 78.47.D-, 78.67.-n Atomically thin transition metal dichalcogenides (TMDs, found in multilayer and bulk MoS2, and in other conven- e.g., MoS2,MoSe2,WS2,WSe2) are two-dimensional (2D) tional semiconductors (e.g., Si, Ge, and GaAs) [26,27]. semiconductors with rich physical properties, including Our results reflect the strong many-body interactions in high mechanical strength and flexibility, strong photolumi- monolayer MoS2, where photoexcited carriers form trions nescence (PL) [1,2], superior (opto)electronic performance with the original free electrons. The resultant increase of [3,4], and intriguing coupled spin-valley physics [5–8]. effective mass significantly diminishes the carrier mobility These remarkable properties make TMD materials promi- and conductivity. This interaction-driven photoreduction of sing for developing next-generation (opto)electronics and conductivity represents an intrinsic property of monolayer (pseudo)spintronics. One distinctive feature of monolayer MoS2 crystals, in contrast to the negative photoconductiv- TMDs is the strong quantum confinement and reduced ity arising from trapping or spatial transfer of charges in dielectric screening in the strict 2D limit. The resultant strong some semiconductor systems with high defect density or Coulomb interactions cause photoexcited electron-hole pairs heterogeneous structure [28,29]. to form tightly bound excitons, which govern the optical To reveal the intrinsic conductive properties of monolayer and electronic properties of the materials [9–22].Insamples MoS2, we used time-domain terahertz spectroscopy. This with excess charges, the excitons can capture additional approach avoids the complication of electrical contacts in chargestoformtrions(chargedexcitons).Thesetrionspossess transport experiments and, when applied with femtosecond exceptionally high dissociation energies (20–50 meV), and laser excitation, can probe short-lived excited carriers prior their concentration and spin-valley configuration can be to trapping or recombination [26,27]. Our experiment controlled by electrical gate and optical helicity, respectively employed a 5-kHz Ti:sapphire amplifier system that gen- [12,23–25]. Although these strong many-body effects have erates laser pulses with 1.55-eV photon energy and 90-fs been observed in 2D TMDs, their influence on the materials’ pulse duration. The laser was split into two beams. One beam intrinsic conductive behavior and implications for device was frequency converted through an optical parametric amp- applications have not been explored thus far. lifier and/or second harmonic generation for pumping the In this Letter, we report the first experimental signature samples. The other beam generated terahertz pulses by of a profound trionic effect on the conductive properties optical rectification in a ZnTe crystal for probing the samples. of atomically thin TMD materials. By using time-resolved By using electro-optic sampling detection, we could map out terahertz spectroscopy [26,27], we have observed an the whole waveform of the terahertz electric field EðtÞ in the anomalous transient decrease of terahertz conductivity in time domain, and the pump-induced change of the field doped monolayer MoS2 after femtosecond laser excitation ΔE (t; τ) at a controllable pump-probe delay time τ [30–32]. at T ¼ 4–350 K. The negative photoconductivity saturates We investigated large-area monolayer MoS2 samples at high pump fluence, where the conductivity is substan- grown by chemical vapor deposition (CVD) on sapphire tially reduced to ∼30% of its equilibrium value. This substrates [33,34]. The samples were mounted on a helium- behavior contrasts with the positive photoconductivity flow cryostat in high vacuum. We first measured their 0031-9007=14=113(16)=166801(5) 166801-1 © 2014 American Physical Society week ending PRL 113, 166801 (2014) PHYSICAL REVIEW LETTERS 17 OCTOBER 2014 (a) (b) To investigate the optoelectronic response of monolayer MoS2, we excited the sample using 3.1-eV laser pulses. Excitation at this photon energy, with fluences used in our experiment, can generate ∼1013–1014 carriers=cm2 in the MoS2 monolayer [1], which is expected to greatly enhance the terahertz absorption (e.g., see Fig. 11 in Ref. [27]). Surprisingly, our result of pump-induced change of transmission (proportional to ΔE) indicates a transient decrease of terahertz absorption in the monolayer MoS2 sample (c) (d) [Fig. 1(b)] [30]. To further explore this unusual behavior, we have measured the temporal (τ) evolution of the frac- tional change of the terahertz field, i.e., the ratio between the maximum values of waveforms ΔE (t; τ)andEðtÞ [Fig. 1(c)]. The terahertz dynamics exhibit a short component with lifetime τ1 ≈ 1 ps, followed by a long component with lifetime τ2 ≈ 42 ps. From the measured transmission spectra, we extracted the change of complex sheet conductivity Δσ (ω; τ) of monolayer MoS2 [30]. Both the real and imaginary parts of Δσ (ω; τ) exhibit negative values for all delay times FIG. 1 (color online). (a) Terahertz response of monolayer MoS2 (τ) and frequencies (ω) in our measurement range [Fig. 1(d)]. on a sapphire substrate in equilibrium conditions. The red and blue The observed negative photoconductivity is a robust lines show the terahertz electric field transmitted through areas and substantial effect in monolayer MoS2. It persists for all with and without the sample, respectively. The left inset is a incident pump fluences (F ¼ 0.4–170 μJ=cm2) and temper- zoomed-in view of the peak (dashed square). The measurement T ¼ 4–350 uncertainty at the peak is 3.5 × 10−5, corresponding to 0.06% atures ( K) in our experiment [see, for example, of the total terahertz signal. The right inset shows the terahertz the fluence dependence in Figs. 2(a)–2(b) and data for power spectra. (b) Change of terahertz transmission after pulsed T ¼ 300 K in Fig. 2(c)]. As the pump fluence increases, the excitation. The red and green lines denote, respectively, the reduction of the terahertz absorption increases and gradually equilibrium transmitted terahertz field EðtÞ and the pump-induced saturates at an almost 70% decrease of the total absorption waveform ΔE (t; τ)atτ ¼ 3 ps. The inset is a schematic of our [Fig. 2(b)], indicating that the conductivity of monolayer experiment. (c) Temporal evolution of the ratio between the MoS2 is reduced to only ∼30% of its equilibrium value. The maximum values of waveforms ΔE (t; τ)andEðtÞ. A biexpo- τ ¼ 1 τ ¼ 42 long component exhibits a considerable increase of magni- nential fit (red line) yields lifetimes 1 ps and 2 ps. The tude and relaxation time with the fluence, and becomes the inset is a zoomed-in view of the short component. (d) The extracted Δσ ω; τ dominant contribution to the overall dynamical response in pump-induced change of complex sheet conductivity ( )at – τ ¼ 3 and40ps.ThemeasurementsweremadeatT ¼ 15 K, with the saturation regime [Figs. 2(a) 2(b)]. incident pump fluence 50 μJ=cm2 and photon energy 3.1 eV. Negative photoconductivity was observed in monolayer MoS2 samples deposited on sapphire and quartz substrates, terahertz absorption in equilibrium conditions at T ¼ 15 K. with and without HfO2 or polymer electrolyte top layers. We recorded the terahertz electric field transmitted through The insensitivity to the dielectric and interfacial environ- the MoS2=substrate area, EðtÞ, and as a reference, through ment implies that the phenomenon originates from the an area without MoS2 flakes, E0ðtÞ [Fig. 1(a)]. The maxi- intrinsic property of the MoS2 material. We also examined mum value of EðtÞ was 2.1% smaller than that of E0ðtÞ. multilayer CVD MoS2 films and bulk MoS2 crystals. We This attenuation of the terahertz transmission [insets did not observe any photoreduction of conductivity in these of Fig. 1(a)] arises from intraband absorption of excess thicker samples, but only the expected photoenhancement free electrons in the doped sample. From the measured (see the Supplemental Material [30]).

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