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Interlayer orientation-dependent light absorption and emission in monolayer semiconductor stacks.

Heo H, Sung JH, Cha S, Jang BG, Kim JY, Jin G, Lee D, Ahn JH, Lee MJ, Shim JH, Choi H, Jo MH - Nat Commun (2015)

Bottom Line: Two-dimensional stacks of dissimilar hexagonal monolayers exhibit unusual electronic, photonic and photovoltaic responses that arise from substantial interlayer excitations.However, this rotation-dependent excitation is largely unknown, due to lack in control over the relative monolayer rotations, thereby leading to momentum-mismatched interlayer excitations.Our study suggests that the interlayer rotational attributes determine tunable interlayer excitation as a new set of basis for investigating optical phenomena in a two-dimensional hexagonal monolayer system.

View Article: PubMed Central - PubMed

Affiliation: 1] Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), 77 Cheongam-Ro, Pohang 790-784, Korea [2] Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Pohang 790-784, Korea.

ABSTRACT
Two-dimensional stacks of dissimilar hexagonal monolayers exhibit unusual electronic, photonic and photovoltaic responses that arise from substantial interlayer excitations. Interband excitation phenomena in individual hexagonal monolayer occur in states at band edges (valleys) in the hexagonal momentum space; therefore, low-energy interlayer excitation in the hexagonal monolayer stacks can be directed by the two-dimensional rotational degree of each monolayer crystal. However, this rotation-dependent excitation is largely unknown, due to lack in control over the relative monolayer rotations, thereby leading to momentum-mismatched interlayer excitations. Here, we report that light absorption and emission in MoS2/WS2 monolayer stacks can be tunable from indirect- to direct-gap transitions in both spectral and dynamic characteristics, when the constituent monolayer crystals are coherently stacked without in-plane rotation misfit. Our study suggests that the interlayer rotational attributes determine tunable interlayer excitation as a new set of basis for investigating optical phenomena in a two-dimensional hexagonal monolayer system.

No MeSH data available.


Related in: MedlinePlus

Ultrafast time-resolved exciton dynamics of coherently/randomly stacked bilayers.(a) Top panel: comparison of transient dynamics of interlayer excitons in coherently stacked and randomly stacked bilayers. Both bilayers are pumped by the same laser fluence of 24 μJ cm−2, and measured at the same probe photon energy of 1.6 eV. Bottom panel: intralayer exciton dynamics of MoS2 (blue, 1.87 eV) and WS2 (red, 1.96 eV) measured in the coherently stacked bilayer with the same pump fluence of 24 μJ cm−2. (b) The pump-fluence-dependent maximum ΔT/T0 of the coherently stacked bilayer. (c) Spectrally resolved ΔT/T0 at the direct interlayer exciton state of ∼1.6 eV (the dashed line) is shown. The data are measured at Δt≈0 ps with a pump fluence of 32 μJ cm−2. (d) Schematic illustration of a pump-induced exciton transient as a function of pump–probe delay Δt (from left to right). Immediately after the 3.1-eV pump excitation (left), electrons are rapidly transferred in MoS2 and holes are rapidly transferred in WS2 (middle). Then the interlayer recombination is measured by setting the probe photon energy of 1.6 eV (right).
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f3: Ultrafast time-resolved exciton dynamics of coherently/randomly stacked bilayers.(a) Top panel: comparison of transient dynamics of interlayer excitons in coherently stacked and randomly stacked bilayers. Both bilayers are pumped by the same laser fluence of 24 μJ cm−2, and measured at the same probe photon energy of 1.6 eV. Bottom panel: intralayer exciton dynamics of MoS2 (blue, 1.87 eV) and WS2 (red, 1.96 eV) measured in the coherently stacked bilayer with the same pump fluence of 24 μJ cm−2. (b) The pump-fluence-dependent maximum ΔT/T0 of the coherently stacked bilayer. (c) Spectrally resolved ΔT/T0 at the direct interlayer exciton state of ∼1.6 eV (the dashed line) is shown. The data are measured at Δt≈0 ps with a pump fluence of 32 μJ cm−2. (d) Schematic illustration of a pump-induced exciton transient as a function of pump–probe delay Δt (from left to right). Immediately after the 3.1-eV pump excitation (left), electrons are rapidly transferred in MoS2 and holes are rapidly transferred in WS2 (middle). Then the interlayer recombination is measured by setting the probe photon energy of 1.6 eV (right).

Mentions: To further investigate the observed contrast in interlayer transitions between the coherent and random stacks, we used ultrafast time-resolved optical pump–probe spectroscopy to study the inter-ML transition dynamics. The stacks were excited using a 50-fs, 3.1-eV pump pulse, and the corresponding differential transmission data (ΔT/T0, where ΔT is the pump-induced transmission change, and T0 is the transmission without the pump) from 1.47 to 2.13 eV probe photon energy were measured as a function of pump–probe delay Δt33 (the inset of Fig. 3a; see also Supplementary Fig. 1). The two kinds of stacks were excited by the laser pump at a fluence of 24 μJ cm−2. Immediately after the pump, the signals for the interlayer exciton with 1.6 eV probe (both of coherent and random stacks) rapidly increase with ΔT/T0>0 (the upper panel of Fig. 3a). The positive ΔT/T0 originates from the reduced probe absorption by state-filling in the interlayer exciton states, that is, the Pauli-blocking effect. The kinetic origin of the increased interlayer transients is rapid charge separation into each ML; the photoexcited electrons and holes are immediately transferred into energetically favoured states in MoS2 and WS2, respectively33, as illustrated in the left and middle panels of Fig. 3d. This rapid charge transfer and resulting interlayer exciton population explain the measured PL intensity ratio (Fig. 2f,h); if the charge transfer is slow compared with the intralayer exciton lifetime of individual MLs, no interlayer PL can be expected. It also signifies that in the bilayer stacks, the charge-transfer dynamics determines the relative PL intensities between the inter- and intra-ML light emissions. In fact, the linear fluence dependence of the peak ΔT/T0 in Fig. 3b ensures that no higher-order nonlinear excitonic interactions are involved; that is, that first-order population dynamics primarily governs both the PL and the ΔT/T0 dynamics34. The spectrally resolved prominent ΔT/T0 peak at 1.6 eV measured at Δt≈0 ps (Fig. 3c) corroborates that the rapid charge separation leads to the interlayer population dynamics, rationalizing the interlayer excitonic PL in the coherent bilayer stacks. Although both stacks exhibit rapid charge separation, the corresponding decay dynamics show qualitatively different features, as indicated in bi-exponential function fits with fast (τ1) and slow (τ2) decay components. The initial decay was slower in the coherently stacked bilayer (τ1=5.3 ps) than in the randomly stacked bilayer (τ1=3.3 ps). This difference indicates that coherent stacks were less sensitive to defect-mediated recombination than were the random stacks, presumably because the coherent stacks have cleaner interfaces with fewer defects than the random stacks35. More importantly, for the slow decay component, τ2 was much faster in the coherent stacks (τ2=39 ps) than in the random ones (τ2=1.5 ns). This ultrafast timescale is comparable to direct-gap transitions in conventional III–V semiconductor superlattices36, and is qualitatively different from previous reports of random bilayer stacks of WSe2/MoS(Se)2 MLs, in which τ2 ranges from nanoseconds to microseconds, attributed to indirect-gap interlayer recombinations46. The substantially faster interlayer recombination dynamics of ∼39 ps in our coherent stacks is comparable to those typically observed in the intralayer direct-gap transitions in individual MoS2 MLs3738. This similarity strongly suggests that our interlayer transitions may be like those of direct gaps. The slower transient character (nanoseconds) in the random stacks is consistent with the observed absence of the corresponding PL at 1.5 eV. This fast recombination dynamics of τ2 excludes a possibility that the excitons are recombined radiatively from defect states, by which the corresponding decay timescale would be very long in the range of nanoseconds39. Overall, the observed contrasts in interlayer transitions of the coherent and random stacks for both spectral and dynamic features suggest critical roles of the relative rotation misfit in the 2D hexagonal k-space of the bilayer stacks.


Interlayer orientation-dependent light absorption and emission in monolayer semiconductor stacks.

Heo H, Sung JH, Cha S, Jang BG, Kim JY, Jin G, Lee D, Ahn JH, Lee MJ, Shim JH, Choi H, Jo MH - Nat Commun (2015)

Ultrafast time-resolved exciton dynamics of coherently/randomly stacked bilayers.(a) Top panel: comparison of transient dynamics of interlayer excitons in coherently stacked and randomly stacked bilayers. Both bilayers are pumped by the same laser fluence of 24 μJ cm−2, and measured at the same probe photon energy of 1.6 eV. Bottom panel: intralayer exciton dynamics of MoS2 (blue, 1.87 eV) and WS2 (red, 1.96 eV) measured in the coherently stacked bilayer with the same pump fluence of 24 μJ cm−2. (b) The pump-fluence-dependent maximum ΔT/T0 of the coherently stacked bilayer. (c) Spectrally resolved ΔT/T0 at the direct interlayer exciton state of ∼1.6 eV (the dashed line) is shown. The data are measured at Δt≈0 ps with a pump fluence of 32 μJ cm−2. (d) Schematic illustration of a pump-induced exciton transient as a function of pump–probe delay Δt (from left to right). Immediately after the 3.1-eV pump excitation (left), electrons are rapidly transferred in MoS2 and holes are rapidly transferred in WS2 (middle). Then the interlayer recombination is measured by setting the probe photon energy of 1.6 eV (right).
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Related In: Results  -  Collection

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f3: Ultrafast time-resolved exciton dynamics of coherently/randomly stacked bilayers.(a) Top panel: comparison of transient dynamics of interlayer excitons in coherently stacked and randomly stacked bilayers. Both bilayers are pumped by the same laser fluence of 24 μJ cm−2, and measured at the same probe photon energy of 1.6 eV. Bottom panel: intralayer exciton dynamics of MoS2 (blue, 1.87 eV) and WS2 (red, 1.96 eV) measured in the coherently stacked bilayer with the same pump fluence of 24 μJ cm−2. (b) The pump-fluence-dependent maximum ΔT/T0 of the coherently stacked bilayer. (c) Spectrally resolved ΔT/T0 at the direct interlayer exciton state of ∼1.6 eV (the dashed line) is shown. The data are measured at Δt≈0 ps with a pump fluence of 32 μJ cm−2. (d) Schematic illustration of a pump-induced exciton transient as a function of pump–probe delay Δt (from left to right). Immediately after the 3.1-eV pump excitation (left), electrons are rapidly transferred in MoS2 and holes are rapidly transferred in WS2 (middle). Then the interlayer recombination is measured by setting the probe photon energy of 1.6 eV (right).
Mentions: To further investigate the observed contrast in interlayer transitions between the coherent and random stacks, we used ultrafast time-resolved optical pump–probe spectroscopy to study the inter-ML transition dynamics. The stacks were excited using a 50-fs, 3.1-eV pump pulse, and the corresponding differential transmission data (ΔT/T0, where ΔT is the pump-induced transmission change, and T0 is the transmission without the pump) from 1.47 to 2.13 eV probe photon energy were measured as a function of pump–probe delay Δt33 (the inset of Fig. 3a; see also Supplementary Fig. 1). The two kinds of stacks were excited by the laser pump at a fluence of 24 μJ cm−2. Immediately after the pump, the signals for the interlayer exciton with 1.6 eV probe (both of coherent and random stacks) rapidly increase with ΔT/T0>0 (the upper panel of Fig. 3a). The positive ΔT/T0 originates from the reduced probe absorption by state-filling in the interlayer exciton states, that is, the Pauli-blocking effect. The kinetic origin of the increased interlayer transients is rapid charge separation into each ML; the photoexcited electrons and holes are immediately transferred into energetically favoured states in MoS2 and WS2, respectively33, as illustrated in the left and middle panels of Fig. 3d. This rapid charge transfer and resulting interlayer exciton population explain the measured PL intensity ratio (Fig. 2f,h); if the charge transfer is slow compared with the intralayer exciton lifetime of individual MLs, no interlayer PL can be expected. It also signifies that in the bilayer stacks, the charge-transfer dynamics determines the relative PL intensities between the inter- and intra-ML light emissions. In fact, the linear fluence dependence of the peak ΔT/T0 in Fig. 3b ensures that no higher-order nonlinear excitonic interactions are involved; that is, that first-order population dynamics primarily governs both the PL and the ΔT/T0 dynamics34. The spectrally resolved prominent ΔT/T0 peak at 1.6 eV measured at Δt≈0 ps (Fig. 3c) corroborates that the rapid charge separation leads to the interlayer population dynamics, rationalizing the interlayer excitonic PL in the coherent bilayer stacks. Although both stacks exhibit rapid charge separation, the corresponding decay dynamics show qualitatively different features, as indicated in bi-exponential function fits with fast (τ1) and slow (τ2) decay components. The initial decay was slower in the coherently stacked bilayer (τ1=5.3 ps) than in the randomly stacked bilayer (τ1=3.3 ps). This difference indicates that coherent stacks were less sensitive to defect-mediated recombination than were the random stacks, presumably because the coherent stacks have cleaner interfaces with fewer defects than the random stacks35. More importantly, for the slow decay component, τ2 was much faster in the coherent stacks (τ2=39 ps) than in the random ones (τ2=1.5 ns). This ultrafast timescale is comparable to direct-gap transitions in conventional III–V semiconductor superlattices36, and is qualitatively different from previous reports of random bilayer stacks of WSe2/MoS(Se)2 MLs, in which τ2 ranges from nanoseconds to microseconds, attributed to indirect-gap interlayer recombinations46. The substantially faster interlayer recombination dynamics of ∼39 ps in our coherent stacks is comparable to those typically observed in the intralayer direct-gap transitions in individual MoS2 MLs3738. This similarity strongly suggests that our interlayer transitions may be like those of direct gaps. The slower transient character (nanoseconds) in the random stacks is consistent with the observed absence of the corresponding PL at 1.5 eV. This fast recombination dynamics of τ2 excludes a possibility that the excitons are recombined radiatively from defect states, by which the corresponding decay timescale would be very long in the range of nanoseconds39. Overall, the observed contrasts in interlayer transitions of the coherent and random stacks for both spectral and dynamic features suggest critical roles of the relative rotation misfit in the 2D hexagonal k-space of the bilayer stacks.

Bottom Line: Two-dimensional stacks of dissimilar hexagonal monolayers exhibit unusual electronic, photonic and photovoltaic responses that arise from substantial interlayer excitations.However, this rotation-dependent excitation is largely unknown, due to lack in control over the relative monolayer rotations, thereby leading to momentum-mismatched interlayer excitations.Our study suggests that the interlayer rotational attributes determine tunable interlayer excitation as a new set of basis for investigating optical phenomena in a two-dimensional hexagonal monolayer system.

View Article: PubMed Central - PubMed

Affiliation: 1] Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), 77 Cheongam-Ro, Pohang 790-784, Korea [2] Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Pohang 790-784, Korea.

ABSTRACT
Two-dimensional stacks of dissimilar hexagonal monolayers exhibit unusual electronic, photonic and photovoltaic responses that arise from substantial interlayer excitations. Interband excitation phenomena in individual hexagonal monolayer occur in states at band edges (valleys) in the hexagonal momentum space; therefore, low-energy interlayer excitation in the hexagonal monolayer stacks can be directed by the two-dimensional rotational degree of each monolayer crystal. However, this rotation-dependent excitation is largely unknown, due to lack in control over the relative monolayer rotations, thereby leading to momentum-mismatched interlayer excitations. Here, we report that light absorption and emission in MoS2/WS2 monolayer stacks can be tunable from indirect- to direct-gap transitions in both spectral and dynamic characteristics, when the constituent monolayer crystals are coherently stacked without in-plane rotation misfit. Our study suggests that the interlayer rotational attributes determine tunable interlayer excitation as a new set of basis for investigating optical phenomena in a two-dimensional hexagonal monolayer system.

No MeSH data available.


Related in: MedlinePlus