Ultrafast Carrier Dynamics in Few Layer Colloidal Molybdenum

Subscriber access provided by IDAHO STATE UNIV

C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Ultrafast Carrier Dynamics in Few Layer Colloidal Molybdenum Disulfide Probed by Broadband Transient Absorption Spectroscopy Pieter Schiettecatte, Pieter Geiregat, and Zeger Hens J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Ultrafast Carrier Dynamics in Few Layer Colloidal Molybdenum Disulfide Probed by Broadband Transient Absorption Spectroscopy Pieter Schiettecatte,†,‡ Pieter Geiregat,†,‡ and Zeger Hens∗,†,‡ Physics and Chemistry of Nanostructures, Ghent University, Gent, Belgium, and Center for Nano and Biophotonics, Ghent University, Gent, Belgium E-mail: [email protected]

Abstract Insights in the photophysics of MoS2 flakes made by exfoliation or chemical vapor deposition (CVD) have advanced the use of these materials in a broad range of applications. More recently, colloidal synthesis has been developed as an inexpensive, scalable and highly-tunable alternative to CVD for the production of MoS2 and other transition metal dichalcogenides (TMDs). Here, we present a comprehensive study on the charge carrier relaxation in colloidal MoS2 sheets using transient absorption spectroscopy at visible and near infrared wavelengths. We show that the transient absorbance after photoexcitation originates from a reduced oscillator strength around the direct gap and a redshift of the entire absorbance and we attribute both features to state filling and band-gap renormalization, respectively. In particular the signatures of state-filling exhibit a sub-picosecond decay, which reflects the trapping of hole carriers ∗

To whom correspondence should be addressed Physics and Chemistry of Nanostructures, Ghent University, Gent, Belgium ‡ Center for Nano and Biophotonics, Ghent University, Gent, Belgium †

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

in midgap states. The relaxation of the bandgap renormalization, on the other hand, takes several tens of picoseconds; a process that we assign to a series of charge-carrier recombination and capture events following the initial hole trapping. Since studies on CVD grown MoS2 point toward a highly similar relaxation of photogenerated charge carriers, we conclude that colloidal synthesis yields MoS2 nanosheets of comparable quality as state-of-the-art CVD, even if both production methods involve an entirely different chemistry. This indicates that TMDs made with both approaches may benefit from similar defect passivation strategies to slow down charge carrier trapping and enhance the exciton lifetime.

Introduction Ultrathin two-dimensional (2D) group VI transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2 ) have attracted much attention in opto-electronics due to their extraordinary electronic properties. 1–5 As bulk materials, they feature a layered crystal structure consisting of stacked monolayers weakly bound by Van der Waals interactions. 3 The combination of these weak out-of-plane forces with strong in-plane chemical bonds enable ultrathin mono-and few-layered TMDs to be produced using top-down exfoliation methods, 6–8 vacuum deposition approaches, such as chemical vapour deposition (CVD), 9 and, more recently, solution-based wet-chemistry methods. 10–12 While vacuum deposition approaches have arisen as efficient methods to produce large area monolayer films, high cost and substrate dependence limit the application potential of CVD grown TMDs. 13 Liquid phase exfoliation, either solvent or surfactant assisted, could cope with these issues, yet the thus obtained TMDs are highly non-uniform, which hampers their incorporation in large area devices. 14 In this respect, colloidal synthesis can offer an inexpensive, scalable and flexible alternative production process for TMDs in a similar way as for semiconductor nanocrystals. 15–17 Over the past years, much work has been devoted to understand the photophysics of MoS2 2


Page 2 of 22

Page 3 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


flakes made by exfoliation or CVD. These studies highlighted the central role of the enhanced many-body Coulomb interactions, leading to strongly bound excitons and excitonic molecules, which result from the two-dimensional character of these materials and the reduced dielectric screening by the low-permittivity environment. 18–25 Such studies not only contributed to a deeper understanding of TMDs, but also advanced the use of MoS2 in a broad range of applications in fields as diverse as opto-electronics, 26,27 and photocatalysis. 28 On the other hand, the photophysical properties of MoS2 produced by direct colloidal synthesis are still unexplored. However, if colloidal synthesis is to become a true alternative to produce TMDs, the opto-electronic characteristics of colloidal TMDs must be understood and benchmarked relative to CVD-grown and exfoliated TMDs. Here, we address charge carrier relaxation in photoexcited colloidal MoS2 sheets by means of femtosecond transient absorption (TA) spectroscopy at visible and near infrared wavelengths. We show that the transient absorbance after photoexcitation results from the joint effect of a reduced oscillator strength of the so-called A and B exciton and a redshift of the entire absorbance spectrum; two features we assign to state filling and band-gap renormalization, respectively. By a deconvolution of the absorbance transient, we show that in particular the A exciton bleach exhibits a rapid, sub-picosecond component that we attribute to the trapping of holes in midgap states. The bandgap renormalization decays on much longer timescales that reflect the eventual relaxation of the charge carrier distribution by subsequent carrier capture events. Since state-of-the-art CVD grown MoS2 exhibits a similar relaxation behavior, 19,29 we conclude that colloidal synthesis yields MoS2 nanosheets of comparable quality, even if an entirely different chemistry is involved in the TMD production. This illustrates that TMDs made with both production methods may benefit from similar defect passivation strategies.

3



Experimental Section Chemicals. The following chemicals were used in this work: oleic acid (OA, 90%, Alfa Aesar), oleylamine (OLA, 80-90%, Acros Organics), carbon disulfide (CS2 , ≥ 99%, Sigma Aldrich), molybdenum(V) chloride (MoCl5 , 99.99%, Sigma Aldrich) 1,2-dichlorobenzene (oDCB, 99%, Acros Organics).

Colloidal synthesis of MoS2 . The synthesis of MoS2 nanosheets was carrier out based on the protocol published by Son et. al. 11 To prepare the precursor solution, MoCl5 (18 mg, 0.0675 mmol), OA (750 µL) and OLA (5 mL) were mixed together (a mixture henceforth denoted as M1) and degassed under vacuum. At the same time, OLA (5 mL) was loaded into another three-neck round bottom flask (henceforth denoted as M2) and degassed while heating. Both solutions were flushed with consecutive nitrogen-vacuum cycles to remove traces of residual volatile components. M2 was subsequently heated to 320 °C under an inert nitrogen atmosphere. Meanwhile CS2 (0.1 mL, in excess) was introduced in M1 at room temperature. This viscous precursor solution was rapidly introduced in a pre-flushed syringe and injected over a period of 30 minutes into into M2. After injection, the reaction mixture was left at 320 °C for another 90 minutes. It was cooled down using a pre-heated water bath and a small amount of o-DCB, typically 4 mL was added to ensure proper miscibility. The MoS2 sheets were precipitated by adding butanol as a non-solvent, collected by centrifugation (10000 g) and redispersed in o-DCB under mild sonication. Afterwards, the sample was subjected to successive washing steps with ethanol as the non-solvent and o-DCB as the solvent. Samples were stored in o-DCB for further analysis.

Basic characterization of MoS2 . Bright field transmission electron microscopy (TEM) images were taken on a Cs-corrected JEOL 2200-FS TEM operated at an acceleration voltage of 200 kV. Absorption spectra were recorded on a PerkinElmer Lambda 950 spectrometer. For Raman analysis, a KAISER dispersive RXNI spectrometer (operating at 532 nm, 1 cm−1

4


Page 4 of 22

Page 5 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


spectral resolution) was used. X-ray diffraction (XRD) measurement were carried out on a Thermo Scientific ARM X0 tra X-ray diffractometer with the Cu Kα line as primary source. Atomic force microscopy (AFM) was done on a Bruker Dimension Edge with a NCHV probe using tapping mode in air.

Transient absorption spectroscopy. We recorded transient absorption spectra by exciting dispersions of MoS2 nanosheets using 110 femtosecond pump pulses with varying wavelength created from the 800 nm fundamental of a 1 kHz laser system (Spitfire Ace, Spectra Physics) through non-linear conversion in an OPA (Light Conversion, TOPAS). Probe pulses in the visible and near infrared window were generated by passing the 800 nm fundamental through a thin CaF2 or YAG crystal, respectively. Both crystals are translated along 1 axis to avoid photo-damage from the fundamental. The pulses were delayed relative to the pump using a delay stage with a maximum delay of 6 nanoseconds. The probe spectrum in our experiments covers the UV-VIS window from 350 nm up to 700 nm and the near infrared window from 850 nm up to 1300 nm. The sheets were dispersed in DCB and continuously stirred to avoid charging or photo-degradation. Photon fluxes were determined using a Thorlabs CCD camera beam profiler and a thermal power sensor.

Results and Discussion Few-Layer MoS2 Sheets - Materials Characteristics Colloidal MoS2 nanosheets were synthesized based on the protocol proposed by Son et. al., 11 see Experimental Section and Supporting Information S1. Figure 1 provides an overview of the most relevant material characteristics of the MoS2 nanosheets studied in this work. According to the transmission electron microscopy (TEM) image shown in Figure 1a, the synthesis method yields two-dimensional, partly folded MoS2 nanosheets. After casting such an MoS2 dispersion on a silicon substrate, these nanosheets could be imaged by atomic force

5



Figure 1: Overview of the most relevant material characteristics of the MoS2 nanosheets studied in this work. (a) Low resolution TEM image. (b) AFM analysis accompanied with a height histogram (N =85) (c) (black) XRD diffractogram and (red) a reference pattern for 2H-MoS2 according to JCPDS 75-1539. (d) Raman spectrum of colloidal MoS2 nanosheets, indicating the A1g and E2g vibrational modes of 2H-MoS2 . (e) Linear absorbance spectrum A0 of a dispersion of MoS2 nanosheets in o-DCB, indicating the characteristic A, B and C transitions. microscopy (AFM). As shown in Figure 1b, such an AFM analysis yields the distribution of the nanosheet thickness, which measures on average 2.5 nm and has a standard deviation of ± 0.5 nm. These figures indicate that the MoS2 sheets analyzed here consist of approximately 3 ± 1 MoS2 monolayers. The x-ray powder diffractogram corresponds to that of hexagonal (2H) MoS2 , see Figure 1c. The finding that the synthesis yields few-layer, 2H MoS2 is corroborated by the Raman spectrum, which shows the characteristic A1g and E2g vibrations of hexagonal MoS2 separated by 23±1 cm−1 , a Raman shift that is representative of few-layered MoS2 nanosheets. 30,31 Furthermore, as can be seen in Figure 1e, three features characteristic of 2H MoS2 show up in the linear absorbance spectrum A0 of a dispersion containing these few-layer MoS2

6


Page 6 of 22

Page 7 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2: (a) 2D wavelength-delay map of ∆A measured on colloidal MoS2 nanosheets after photo-excitation with 400 nm (¯hω=3.1 eV), 110 fs pulses (fluence: 470 µJ/cm2 ). The full black line represents the linear absorbance spectrum A0 . The position of (vertical white lines) the A and B exciton and the C band have been indicated. (b) Transient absorbance (normalized with respect to the bleach maximum) at the wavelength of the B exciton, after photo-excitation at 400 nm using an excitation fluence of 235 µJ/cm2 . The markers represent experimental data, the solid line is a fit to a combination of a biexponential decay and a fixed background. (inset) Decay rates obtained from biexponential fits to similar transient absorbance decay traces recorded with different pump fluences. Average values of the fast (circles) and slow (squares) component have been indicated. nanosheets. The features labeled A and B, at approximately 665 nm (1.87 eV) and 615 nm (2.02 eV), respectively, correspond to transitions between the spin-orbit split valence-band maxima and the conduction-band minimum at the K point. 1,32,33 As outlined in Supporting Information S2, the wavelength assigned to these resonances was obtained from a derivative analysis of the linear absorbance spectrum. The prominent feature C at around 420 nm, on the other hand, originates from transitions between the valence and conduction band along the direction from K to Γ, where these bands are mostly parallel. 34

Transient Absorption Spectroscopy – Band-Gap Renormalization Figure 2a shows a 2D transient absorption (TA) map covering the wavelength range 400– 750 nm (3.1–1.65 eV) and pump delays up to 2.5 ns. We identified two different regimes

7



in this ∆A map. At early pump-probe delays, ∆A features two short-lived bleach features (∆A < 0) close to the A and B exciton resonances, which disappear within 2 ps after photoexcitation. At longer time delays, ∆A shows multiple bands that decay on a similar time scale. An analysis of the transient absorbance around, for example, the spectral region of the B exciton provides a more quantitative view on both regimes. As shown in Figure 2b, the decay of ∆A exhibits two different rates that can be well described using a fit to a bi-exponential. Importantly, the corresponding decay rates are largely independent of the pump fluence and amount to 3.0 ± 0.3 ps−1 and 0.058 ± 0.003 ps−1 , see inset Figure 2b and Supporting Information S3.

Figure 3: (a) Comparison of (black) transient absorbance spectrum (400 nm, 470 µJ/cm2 photo-excitation) at a delay of 10 ps and (red) the derivative dA0 /dE of the linear absorbance spectrum. The spectral positions of the A and B position are indicated in solid vertical lines. (b) Transient absorbance spectra recorded at a delay of 10 ps for different pump excitation energies, including (blue) h ¯ ω=3.1 eV (400 nm), (orange) h ¯ ω = 2.07 eV (600 nm) and (red) h ¯ ω = 1.88 eV (660 nm). Focusing on the second, more slowly decaying regime, we noticed that the alternation between bleach and photoinduced absorption (∆A > 0) at a given delay closely resembles the derivative of the absorbance spectrum. For comparison, Figure 3a represents a spectral cut of the TA spectrum after a pump-probe delay of 10 ps and the derivative dA0 /dE of the linear absorbance spectrum to the photon energy E. Both show a similar behavior where features in the TA spectrum coincide with local minima and maxima in dA0 /dE. It is well-known that small spectral shifts δE give rise to a transient absorbance ∆A that mimicks dA0 /dE, 8


Page 8 of 22

Page 9 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a correspondence from which δE can be readily deduced, see Supporting Information S4. 35 The deviations from this one-to-one correlation apparent in Figure 3a at high photon energy (short wavelengths) are likely caused by light scattering contributing to A0 but not to ∆A, which leads to an overestimation of positive and an underestimation of negative slopes. As scattering is less important at longer wavelengths, one indeed sees a better resemblance between ∆A and dA0 /dE at photon energies around the A exciton, from which we can estimate a spectral redshift of around 20 meV for this specific example. This value lines up with recent reports of redshifts observed in TMD systems. 36 To further support the interpretation that the long-delay ∆A spectra merely reflect spectral shifts, we pumped a dispersion of MoS2 nanosheets using different excitation energies. As can be seen in Figure 3b, we obtained highly similar spectra that show a pronounced bleach near the C transition even for excitation photon energies near the B (¯ hω=2.07 eV) and A (¯ hω=1.88 eV) exciton. As different single particle states give rise to the A exciton, the B exciton and the C band, such a behavior cannot be explained by a state filling argument. 32 We thus conclude that at delays longer than 2 ps, the transient absorbance after photoexcitation is mainly dominated by a spectral shift of the transition energies. This finding is consistent with the current literature on CVD grown and exfoliated TMDs, where a similar behaviour was observed and assigned to a photo-induced renormalization of the band-gap, which concomitantly shifts transition energies. 18,19,36

Transient Absorption Spectroscopy – Exciton Dynamics Figure 4a shows the ∆A map obtained in the wavelength range around the A and the B exciton after pumping at 400 nm. The aforementioned short-lived bleach bands centered at ∼ 655 nm and ∼ 610 nm can be clearly distinguished. Interestingly, these wavelengths appear slightly blue-shifted relative to the wavelengths of the A and B exciton that we extracted from the linear absorbance spectrum (see Figure 1e). Both bleach bands build up within a few hundred femtoseconds after photo-excitation and decay within 1-2 picoseconds 9



Figure 4: (a) (top) ∆A delay-wavelength map after photoexcitation with a 400 nm (¯hω=3.1 eV) pump pulse (fluence: 470 µJ/cm2 ). (middle) ∆A spectrum after a 1 ps delay taken from the ∆A map. The wavelength of the bleach features around the A (655 nm) and B (610 nm) exciton have been indicated. (bottom) Evolution of ∆A with pump-probe delay at the wavelengths indicated. (b) Parameters obtained from fitting the ∆A map to a sum of two Gaussians and a background as a function of the pump-probe delay (markers), including (top) evolution of the area, (middle) energy shift, and (bottom) line-width of the Gaussians that describe the A (red) and B (blue) feature. The dashed line is a fit to an exponential decay and a fixed background. (see Figure 4a). After this rapid picosecond decay, the transient absorbance of both bands reaches a value in line with the respective spectral shift bands, which yield a net photoinduced absorbance at around the A exciton and a remaining bleach at the B exciton (see Figure 2a). To better quantify the net bleach of the A and B exciton band, we fit the excited state absorbance A? (λ, t) = A0 (λ) + ∆A(λ, t) to a sum of two Gaussians that account for the A 10


Page 10 of 22

Page 11 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and B exciton bands and a polynomial background that describes any residual absorption of the broad C feature and scattering. A more elaborate description of this fitting procedure can be found in Supporting Information S5. Figure 4b summarizes the main results of this analysis by plotting the evolution of the area, spectral position and width of the Gaussians used to describe both exciton bands. We analyzed the time-dependence of this parameters through fits to a multi-exponential decay, which yield a set of fitting parameters that have been listed in Supporting Information S5. It can be seen that photo-excitation rapidly induces a notable decrease of the area of both Gaussians, hereby reflecting a true bleach of both exciton bands. We assign these bleaches to state-filling brought about by the presence of a photo-generated carrier population at the band edges. 37 During the next picosecond, the area of the Gaussian used to describe the A exciton band quickly recovers with a rate constant of 2.1 ps−1 (Figure 4b, stage I), while the area of the B band hardly changes and recovers with a notable slower rate of 0.1ps−1 . Concomitantly, peaks broaden and the spectra shift to lower energies. The recovery of the A exciton bleach, the ingrowth of a spectral red shift and the rapid changes in line-width take place on a remarkably similar time scale with matching rate constants as indicated in Figure 4b, stage I. These processes line up with the rapid rate constant already identified in the transient absorbance at 610 nm (see Figure 2b) and reflect a carrier loss process. At longer time delays (Figure 4b, stage II), the netto A exciton bleach, the changes in line-width and the spectral shift recover at a much slower rate, defined by two rate constants in the order of 0.1 ps−1 and 0.01 ps−1 . The aforementioned B bleach band recovers at a rate of 0.1 ps−1 over the entire time window. In the case of CVD grown MoS2 , TA spectra were presented in the literature that strongly resemble the ∆A maps shown in Figures 2a and 4a. 19 In that case, however, the initial bleach features were assigned to the broadening of the exciton transitions, rather than to a net bleach. To ascertain the initial accumulation of charge carriers after photo-excitation, we therefore extended our analysis to infrared wavelengths. At those wavelengths, neither spectral shifts nor line broadening can affect the transient absorbance since A0 will be zero

11



Figure 5: (a) 2D transient time delay-wavelength map upon 400 nm (¯hω=3.1 eV), 110 fs, 470 µJ/cm2 photoexcitation in the wavelength range from 900 nm to 1.3 µm. b) Temporal dependence of the transient probed at 1.25 µm. The dashed line is a fit to a combination of an exponential decay and a fixed background. at wavelengths longer than the indirect gap of 1.2 eV, i.e., 1000 nm. 3 Opposite from this prediction, the experimental ∆A spectrum shown in Figure 5a exhibits a featureless yet pronounced photo-induced absorption in the entire wavelength range between 900 and 1300 nm. Moreover, as shown in Figure 5b, the transient absorbance at a fixed probe wavelength features a decay with a fast stage decaying at a rate of 1.6ps−1 and two slower stages decaying with rates of 0.2ps−1 and 0.01ps−1 , as was obtained by a fit to three exponential functions and a fixed background, see Table S1. Importantly, the observation of sub-bandgap photoinduced absorption confirms that ∆A is not a mere combination of broadened and shifted exciton lines. Typically, this increased absorption is attributed to intraband transitions that involve conduction-band electrons or valence-band holes. 23 In that case, the sub-bandgap transient probes changes in the population of these charge-carriers, hence the similar decay behaviour of ∆A above and below the MoS2 band gap.

A Model for Charge Carrier Decay in Colloidal MoS2 Based on the acquired data and analysis, we propose that photo-excited carriers in colloidal MoS2 nanosheets decay through a model as outlined in Figure 6. This involves cooling of photoexcited carriers towards the band-edge extrema, followed by the trapping of one of the band-edge carriers and a subsequent series of slower non-radiative capture events. Cooling 12


Page 12 of 22

Page 13 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6: Schematic representation of the proposed photo-excited carrier decay in colloidal MoS2 . Upon cooling towards the band-edge extrema, (left) photoexcited holes are captured by mid-gap defect states, (right) followed by a series of slower recombination events involving trapping of conduction band electrons and an eventual remaining hole population. The socalled A and B exciton states, originating from transitions between the spin-orbit split valence band and the conduction-band minimum, are indicated for clarity. induces the net initial bleach of the A and B exciton absorption and a notable build-up of the sub-bandgap photoinduced absorption. Accordingly, the rapid loss of this net bleach reflects the depletion of band-edge carriers. Given the fluence independent, first order decay, we attribute this depletion to a defect-related trapping processes rather than to a higher order Auger mechanism as was previously observed in exfoliated MoS2 flakes. 20,38 While the different analyses of the TA spectra confirm the presence of this rapid trapping process, estimated decay rates range from 1.6 to 3 ps−1 depending on the probe wavelength and the analysis method. Discrepancies are caused by the transient dynamics in the visible being compounded by simultaneous spectral shifts and line-width broadening effects, while those in the near infrared are not. Even so, the presence of a rapid decay of the A exciton bleach, during which the area of the B exciton band hardly changes, suggests that hole carriers are being trapped, since both transitions share the same conduction-band states, yet different valence band states. Note that within this model, the rapid loss of a hole population also 13



accounts for the first, rapid decay component of the sub-bandgap photo-induced absorption. The second stage in the transient absorbance predominantly involves the relaxation of the spectral shift or band-gap renormalization created by substantial carrier loss after photoexcitation; a phenomena which takes place over two distinctive time regimes, with estimated decay rates of ∼ 0.1ps−1 and ∼ 0.01ps−1 . Note that the sub-bandgap transient shows a highly similar decay behaviour. According to the proposed model, this mirrors the non-radiative recombination of remaining carrier populations through a series of processes which bring the photo-excited MoS2 nanosheets back to equilibrium. For one thing, this involves the relaxation of a remaining electron population, as advocated by the presence of a ∼ 0.1 ps−1 decay on both the A and B exciton bleach bands – both transitions share the same conduction band states. The ∼ 0.01 ps−1 relaxation appears to be associated with the decay of eventual remaining hole carriers due to its absence from the B exciton trace. This interpretation lines up with a model proposed by Wang et. al. 23 who found a similar sequence of first order processes in exfoliated MoS2 and with the work by Docherty et al. 39 on CVD grown MoS2 and WSe2 where it was shown that one type of carrier is predominantly captured at surface states in the first few picoseconds, while the other carrier is captured over a much longer time scale. Again, this interpretation accounts for the concomitant slower decay components in the sub-bandgap photo-induced absorption. The fact that this recombination is also observed for resonant pumping, see Supporting Information S6, suggests that carriers are trapped by in-gap defect states. While no STM studies on defects have been performed on colloidally synthesized 2D TMDs, vacancies, edge states and grain boundaries 42 are prominent defects in CVD grown and exfoliated MoS2 flakes. 40,41 Sulfur vacancies, for example, have a low formation energy and lead to a mid-gap defect state.

14


Page 14 of 22

Page 15 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Conclusion We have presented a detailed study on the charge carrier relaxation in photoexcited colloidal MoS2 nanosheets using femtosecond transient absorption spectroscopy at visible and near infrared wavelengths. We demonstrate that the transient absorbance after photo-excitation is caused by both a reduction in oscillator strength of the transitions making up the direct gap and a shift of the entire absorbance spectrum towards lower energy. We attribute these features to state filling and band-gap renormalization, respectively. Through spectral deconvolution, we find that in particular the A exciton bleach exhibits a sub-picosecond decay, which we attribute to rapid hole capture in midgap defect states. The disappearance of spectral shifts at longer delay times reflects the relaxation of the charge carrier distribution by further charge trapping and recombination events. A comparison between the dynamics of photogenerated charge carrier in the case of colloidal MoS2 nanosheets as measured here and existing literature on CVD grown MoS2 nanosheets points towards a very similar sequence of rapid trapping of one carrier type, followed by capture of the remaining carrier on a longer time scale. Accordingly, we conclude that colloidal synthesis yields MoS2 nanosheets of comparable quality, even if an entirely different chemistry is involved in the TMD production. 19 This shows that TMDs fabricated with both approaches may benefit from similar defect passivation strategies in order to enhance the lifetime of photogenerated excitons.

Acknowledgments The authors acknowledge Jorick Maes for his help with AFM measurements and Hannes Van Avermaet for his help with XRD measurements. PS acknowledges the FWO-Vlaanderen for a fellowship (FWO-SB scholarship), PG acknowledges support from FWO Vlaanderen through a post-doctoral fellowship and ZH acknowledges the Marie-Sklodowska Curie action COMPASS (H2020-MSCA-RISE-691185), the FWO-Vlaanderen (FWO17/PRJ/380) and Ghent 15



University (GOA 01G01019) for funding.

Supporting Information Available (S1) MoS2 synthesis, (S2) determination of spectral positions of A and B resonances, (S3) biexponential fits to decay traces recorded with different pump fluences, (S4) proportionality between between ∆A and dA0 /dE, (S5) fit to the exciton dynamics, and (S6) transient absorption spectroscopy after resonant pumping. This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2 : A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805. (2) Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Emerging Device Applications for Semiconducting Two-Dimensional Transition Metal Dichalcogenides. ACS Nano 2014, 8, 1102–1120. (3) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699. (4) Bhimanapati, G. R.; Lin, Z.; Meunier, V.; Jung, Y.; Cha, J.; Das, S.; Xiao, D.; Son, Y.; Strano, M. S.; Cooper, V. R. et al. Recent Advances in Two-dimensional Materials Beyond Graphene. ACS Nano 2015, 9, 11509–11539. (5) Ferrari, A. C.; Bonaccorso, F.; Fal’Ko, V.; Novoselov, K. S.; Roche, S.; Bøggild, P.; Borini, S.; Koppens, F. H.; Palermo, V.; Pugno, N. et al. Science and Technology Roadmap for Graphene, Related Two-dimensional Crystals, and Hybrid Systems. Nanoscale 2015, 7, 4598–4810. 16


Page 16 of 22

Page 17 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6) Frindt, R. Single Crystals of MoS2 Several Molecular Layers Thick. J. Appl. Phys. 1966, 37, 1928–1929. (7) Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M.; Chhowalla, M. Photoluminescence from Chemically Exfoliated MoS2 . Nano Lett. 2011, 11, 5111–5116. (8) Nicolosi, V.; Chhowalla, M.; Kanatzidis, M. G.; Strano, M. S.; Coleman, J. N. Liquid Exfoliation of Layered Materials. Science 2013, 340, 1226419. (9) Lee, Y.-H.; Zhang, X.-Q.; Zhang, W.; Chang, M.-T.; Lin, C.-T.; Chang, K.-D.; Yu, Y.C.; Wang, J. T.-W.; Chang, C.-S.; Li, L.-J. et al. Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition. Adv. Mater. 2012, 24, 2320–2325. (10) Mahler, B.; Hoepfner, V.; Liao, K.; Ozin, G. A. Colloidal Synthesis of 1T-WS2 and 2HWS2 Nanosheets: Applications for Photocatalytic Hydrogen Evolution. J. Am. Chem. Soc. 2014, 136, 14121–14127. (11) Son, D.; Chae, S. I.; Kim, M.; Choi, M. K.; Yang, J.; Park, K.; Kale, V. S.; Koo, J. H.; Choi, C.; Lee, M. et al. Colloidal Synthesis of Uniform-Sized Molybdenum Disulfide Nanosheets for Wafer-Scale Flexible Nonvolatile Memory. Adv. Mater. 2016, 28, 9326– 9332. (12) Sun, Y.; Fujisawa, K.; Lin, Z.; Lei, Y.; Mondschein, J. S.; Terrones, M.; Schaak, R. E. Low-Temperature Solution Synthesis of Transition Metal Dichalcogenide Alloys with Tunable Optical Properties. J. Am. Chem. Soc. 2017, 139, 11096–11105. (13) Kang, K.; Xie, S.; Huang, L.; Han, Y.; Huang, P. Y.; Mak, K. F.; Kim, C.-J.; Muller, D.; Park, J. High-Mobility Three-Atom-Thick Semiconducting Films with Wafer-Scale Homogeneity. Nature 2015, 520, 656. (14) Coleman, J. N.; Lotya, M.; ONeill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.;

17



Gaucher, A.; De, S.; Smith, R. J. et al. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568–571. (15) Tessier, M. D.; Dupont, D.; De Nolf, K.; De Roo, J.; Hens, Z. Economic and SizeTunable Synthesis of InP/ZnE (E= S, Se) Colloidal Quantum Dots. Chem. Mater. 2015, 27, 4893–4898. (16) Park, J.; An, K.; Hwang, Y.; Park, J.-G.; Noh, H.-J.; Kim, J.-Y.; Park, J.-H.; Hwang, N.-M.; Hyeon, T. Ultra-large-scale Syntheses of Monodisperse Nanocrystals. Nat. Mater. 2004, 3, 891. (17) Choi, H. W.; Zhou, T.; Singh, M.; Jabbour, G. E. Recent Developments and Directions in Printed Nanomaterials. Nanoscale 2015, 7, 3338–3355. (18) Pogna, E. A. A.; Marsili, M.; De Fazio, D.; Dal Conte, S.; Manzoni, C.; Sangalli, D.; Yoon, D.; Lombardo, A.; Ferrari, A. C.; Marini, A. et al. Photo-Induced Bandgap Renormalization Governs the Ultrafast Response of Single-Layer MoS2 . ACS Nano 2016, 10, 1182–1188. (19) Cunningham, P. D.; McCreary, K. M.; Hanbicki, A. T.; Currie, M.; Jonker, B. T.; Hayden, L. M. Charge Trapping and Exciton Dynamics in Large-Area CVD Grown MoS2 . J. Phys. Chem. C. 2016, 120, 5819–5826. (20) Kime, G.; Leontiadou, M. A.; Brent, J. R.; Savjani, N.; OBrien, P.; Binks, D. Ultrafast Charge Dynamics in Dispersions of Monolayer MoS2 Nanosheets. J. Phys. Chem. C 2017, 121, 22415–22421. (21) He, K.; Kumar, N.; Zhao, L.; Wang, Z.; Mak, K. F.; Zhao, H.; Shan, J. Tightly Bound Excitons in Monolayer WSe2 . Phys. Rev. Lett. 2014, 113, 026803. (22) Vega-Mayoral, V.; Borzda, T.; Vella, D.; Prijatelj, M.; Pogna, E. A. A.; Backes, C.;

18


Page 18 of 22

Page 19 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Coleman, J. N.; Cerullo, G.; Mihailovic, D.; Gadermaier, C. Charge Trapping and Coalescence Dynamics in Few Layer MoS2 . 2D Mater. 2017, 5, 015011. (23) Wang, H.; Zhang, C.; Rana, F. Ultrafast Dynamics of Defect-Assisted Electron–Hole Recombination in Monolayer MoS2 . Nano Lett. 2014, 15, 339–345. (24) Shi, H.; Yan, R.; Bertolazzi, S.; Brivio, J.; Gao, B.; Kis, A.; Jena, D.; Xing, H. G.; Huang, L. Exciton Dynamics in Suspended Monolayer and Few-layer MoS2 2D Crystals. ACS Nano 2013, 7, 1072–1080. (25) Hong, X.; Kim, J.; Shi, S.-F.; Zhang, Y.; Jin, C.; Sun, Y.; Tongay, S.; Wu, J.; Zhang, Y.; Wang, F. Ultrafast Charge Transfer in Atomically Thin MoS2 /WS2 Heterostructures. Nat. Nanotechnol. 2014, 9, 682. (26) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, I. V.; Kis, A. Single-layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147. (27) Lembke, D.; Bertolazzi, S.; Kis, A. Single-Layer MoS2 Electronics. Acc. Chem. Res. 2015, 48, 100–110. (28) Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L.; Jin, S. Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2 Nanosheets. J. Am. Chem. Soc. 2013, 135, 10274–10277. (29) Chi, Z.; Chen, H.; Chen, Z.; Zhao, Q.; Chen, H.; Weng, Y. Ultrafast Energy Dissipation via Coupling with Internal and External Phonons in Two-Dimensional MoS2 . ACS Nano 2018, (30) Lee, C.; Yan, H.; Brus, L. E.; Heinz, T. F.; Hone, J.; Ryu, S. Anomalous Lattice Vibrations of Single-and Few-Layer MoS2 . ACS Nano 2010, 4, 2695–2700. (31) Li, H.; Zhang, Q.; Yap, C. C. R.; Tay, B. K.; Edwin, T. H. T.; Olivier, A.; Baillargeat, D.

19



From Bulk to Monolayer MoS2 : Evolution of Raman Scattering. Adv. Funct. Mater. 2012, 22, 1385–1390. (32) Qiu, D. Y.; Felipe, H.; Louie, S. G. Optical Spectrum of MoS2 : Many-Body Effects and Diversity of Exciton States. Phys. Rev. Lett. 2013, 111, 216805. (33) Cheiwchanchamnangij, T.; Lambrecht, W. R. Quasiparticle Band Structure Calculation of Monolayer, Bilayer, and Bulk MoS2 . Phys. Rev. B 2012, 85, 205302. ˇ (34) Rukelj, Z.; Strkalj, A.; Despoja, V. Optical Absorption and Transmission in a Molybdenum Disulfide Monolayer. Phys. Rev. B 2016, 94, 115428. (35) Geiregat, P.; Houtepen, A.; Justo, Y.; Grozema, F. C.; Van Thourhout, D.; Hens, Z. Coulomb Shifts upon Exciton Addition to Photoexcited PbS Colloidal Quantum Dots. J. Phys. Chem. C 2014, 118, 22284–22290. (36) Sie, E. J.; Steinhoff, A.; Gies, C.; Lui, C. H.; Ma, Q.; Rosner, M.; Schonhoff, G.; Jahnke, F.; Wehling, T. O.; Lee, Y.-H. et al. Observation of Exciton Redshift–Blueshift Crossover in Monolayer WS2. Nano Lett. 2017, 17, 4210–4216. (37) Schmitt-Rink, S.; Chemla, D. S.; Miller, D. A. B. Theory of Transient Excitonic Optical Nonlinearities in Semiconductor Quantum-Well Structures. Phys. Rev. B 1985, 32, 6601–6609. (38) Sun, D.; Rao, Y.; Reider, G. A.; Chen, G.; You, Y.; Brezin, L.; Harutyunyan, A. R.; Heinz, T. F. Observation of Rapid Exciton–Exciton Annihilation in Monolayer Molybdenum Disulfide. Nano Lett. 2014, 14, 5625–5629. (39) Docherty, C. J.; Parkinson, P.; Joyce, H. J.; Chiu, M.-H.; Chen, C.-H.; Lee, M.-Y.; Li, L.-J.; Herz, L. M.; Johnston, M. B. Ultrafast transient terahertz conductivity of monolayer MoS2 and WSe2 grown by chemical vapor deposition. ACS nano 2014, 8, 11147–11153. 20


Page 20 of 22

Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(40) Santosh, K.; Longo, R. C.; Addou, R.; Wallace, R. M.; Cho, K. Impact of Intrinsic Atomic Defects on the Electronic Structure of MoS2 Monolayers. Nanotechnology 2014, 25, 375703. (41) Zhou, W.; Zou, X.; Najmaei, S.; Liu, Z.; Shi, Y.; Kong, J.; Lou, J.; Ajayan, P. M.; Yakobson, B. I.; Idrobo, J.-C. Intrinsic Structural Defects in Monolayer Molybdenum Disulfide. Nano Lett. 2013, 13, 2615–2622. (42) Lin, Z.; Carvalho, B. R.; Khan, E.; Ruitao, L.; Rao, R.; Terrones, H.; Pimenta, M. A.;Terrones, M. Defect engineering of two-dimensional transition metal dichalcogenides. 2D Mater. 2016, 3, 022002.

21



Graphical TOC Entry Colloidal Synthesis

Ultrafast Spectroscopy

Charge Carrier Relaxation

fast hole capture

k ∼ 1 ps-1

pump

22


Page 22 of 22

Ultrafast Carrier Dynamics in Few Layer Colloidal Molybdenum

Recommend Documents