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Ultrafast Charge Transfer Cascade in a Mixed-Dimensionality Nanoscale Trilayer
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ACS Nano

Cite this: ACS Nano 2024, 18, 11, 8190–8198
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https://doi.org/10.1021/acsnano.3c12179
Published March 11, 2024

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Abstract

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Innovation in optoelectronic semiconductor devices is driven by a fundamental understanding of how to move charges and/or excitons (electron–hole pairs) in specified directions for doing useful work, e.g., for making fuels or electricity. The diverse and tunable electronic and optical properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs) and one-dimensional (1D) semiconducting single-walled carbon nanotubes (s-SWCNTs) make them good quantum confined model systems for fundamental studies of charge and exciton transfer across heterointerfaces. Here we demonstrate a mixed-dimensionality 2D/1D/2D MoS2/SWCNT/WSe2 heterotrilayer that enables ultrafast photoinduced exciton dissociation, followed by charge diffusion and slow recombination. Importantly, the heterotrilayer serves to double charge carrier yield relative to a MoS2/SWCNT heterobilayer and also demonstrates the ability of the separated charges to overcome interlayer exciton binding energies to diffuse from one TMDC/SWCNT interface to the other 2D/1D interface, resulting in Coulombically unbound charges. Interestingly, the heterotrilayer also appears to enable efficient hole transfer from SWCNTs to WSe2, which is not observed in the identically prepared WSe2/SWCNT heterobilayer, suggesting that increasing the complexity of nanoscale trilayers may modify dynamic pathways. Our work suggests ”mixed-dimensionality” TMDC/SWCNT based heterotrilayers as both interesting model systems for mechanistic studies of carrier dynamics at nanoscale heterointerfaces and for potential applications in advanced optoelectronic systems.

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Note Added after ASAP Publication

This paper was published ASAP on March 9, 2024, without all the changes made. The corrected version was posted on March 19, 2024.

Optoelectronic semiconductor devices rely on the balance between photoexcited charge generation, charge recombination, and charge extraction. (1−3) Quantum-confined low-dimensional materials such as two-dimensional (2D) transition metal dichalcogenides (TMDCs) and one-dimensional (1D) single-walled carbon nanotubes (SWCNTs) have enhanced electron–hole Coulomb interactions, weak dielectric screening, and large exciton binding energies. (3−12) Large exciton binding energies can correlate with short exciton lifetimes and increased difficulty in separating electrons and holes to generate an electrical current for photovoltaics, photodetectors, and sensors or chemical bonds in solar fuel schemes. (2,3,8,11−13) The diverse and tunable electronic and optical properties of TMDCs and semiconducting SWCNTs (s-SWCNTs) make them good quantum-confined model systems for fundamental studies on charge and exciton transfer across heterointerfaces, with relevance for applications in photovoltaics, quantum information processing, and solar fuels. (3,5,7,10,14−16) Recent studies of TMDC heterobilayers have shown charge recombination lifetimes of ca. 30–110 ps, and TMDC/organic heterojunctions have achieved a charge separated state lifetime of ca. 5 ns in a MoS2/pentacene bilayer, demonstrating that charge separation in 2D heterostructures is an efficient strategy to overcome large exciton binding energies. (3,8,17)
In analogy to the multistep charge transfer cascade found in the photosynthesis reaction center, more complex low-dimensional heterostructures provide opportunities for directing charge flow and lengthening charge separation lifetimes. (13,18) For example, TMDC hetero-trilayer interfaces have even longer lifetimes, up to ca. 1 ns, than their heterobilayer counterparts. As these heterostructures become more complex, it can be difficult to predict the range of motion and mechanism(s) for generating long-lived charges across the relevant interfaces. (1,3,5,8,14,16) For example, in an MoS2/WS2/MoSe2 Type II heterotrilayer, Ceballos et al. predicted a cascading charge transfer effect, where photoexcited electrons in MoSe2 would transfer to MoS2 via WS2. (19) However, electrons excited in MoSe2 transferred directly to MoS2 without populating the WS2. Furthermore, the seemingly ubiquitous existence of Coulomb-bound “interlayer” excitons in TMDC heterostructures, while useful for certain applications, can limit the ultimate lifetime of separated charges and impede the generation and extraction of free uncorrelated electrons and holes.
Sulas-Kern et al. recently demonstrated that a “mixed-dimensionality” heterobilayer combining TMDCs with s-SWCNTs resulted in charge-separated lifetimes on the microsecond time scale, surpassing TMDC/TMDC and TMDC/organic charge lifetimes. (3,5,19−21) Despite the success of this heterostructure in producing exceptionally long-lived charge carriers, fundamental questions still exist such as (1) if and/or how the electron–hole pair at the donor–acceptor interface can escape from the mutual “interlayer” Coulombic attraction and (2) the roles both in and out of plane carrier delocalization and/or diffusion have on stabilizing long-lived charges. (3,19,22,23) Here we use transient absorption (TA) spectroscopy to track exciton dissociation and charge diffusion in a mixed-dimensionality 2D/1D/2D MoS2/SWCNT/WSe2 heterotrilayer. Upon photoexcitation of the trilayer, we observe ultrafast electron transfer to MoS2 and hole transfer to WSe2, with microsecond charge recombination lifetimes. Interestingly, the trilayer architecture appears to facilitate ultrafast hole transfer to WSe2 (<200 fs), whereas this charge transfer reaction is not as efficient in identically prepared WSe2/SWCNT heterobilayers. By tracking multiple well-resolved spectral signatures of charge carriers in each semiconductor layer, we can experimentally monitor and simulate charge diffusion away from the site of charge generation, confirming the separation of electron–hole pairs at the TMDC/SWCNT interface and subsequent diffusion to the other 2D/1D interface. This behavior contrasts with the dominance of Coulomb-bound “interlayer” excitons in TMDC/TMDC heterobilayers or trilayers. Our results highlight the distinct photophysics and potential technological advantages of “mixed-dimensionality” heterostructures.

Results and Discussion

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MoS2 and WSe2 Bilayers

To understand the exciton and charge dynamics in our MoS2/SWCNT/WSe2 heterotrilayer, we first studied individual bilayers made from combining s-SWCNTs with each TMDC monolayer. Monolayer TMDCs are used throughout this paper and characterized via Raman spectroscopy; see Figure 1c–e. Both MoS2/SWCNT and WSe2/SWCNT bilayers were obtained by spray coating a thin film of (6,5) SWCNTs onto the surface of the respective neat TMDC monolayer. (1,16,24) The (6,5) SWCNT film thickness was estimated to be ca. 7 nm. (16,24) Steady state absorption and Raman spectroscopy were used to confirm the successful assembly of both bilayers (Figure 1c,d). (5,25−32)

Figure 1

Figure 1. (A) Predicted energy level diagram of MoS2/SWCNT and WSe2/SWCNT bilayers (top); Calculated thermodynamic driving forces for electron and hole transfer at the MoS2/SWCNT and WSe2/SWCNT interfaces (bottom). (B) Raman spectra of (6,5) SWCNT film, MoS2 monolayer, MoS2/SWCNT bilayer, WSe2 monolayer, and the MoS2/SWCNT/WSe2 trilayer. (C) Absorbance spectra for MoS2 monolayer, (6,5) SWCNT film, and the MoS2/SWCNT heterojunction (top); Schematic of MoS2/SWCNT bilayer (bottom). (D) Absorbance spectra of WSe2 monolayer, (6,5) SWCNT film, and the WSe2/SWCNT bilayer (top); Schematic of the WSe2/SWCNT bilayer. (E) Absorbance spectra of (6,5) SWCNT film, MoS2/SWCNT bilayer, and MoS2/SWCNT/WSe2 trilayer (top); Schematic of the MoS2/SWCNT/WSe2 trilayer (bottom).

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The thermodynamic driving force for free carrier generation depends on the free energy difference between initially photoexcited excitons and the final separated charge carriers (electron and hole). In a type II heterojunction, the “donor” has an electron affinity and ionization potential closer to vacuum than the “acceptor”. (3) Figure 1a depicts the expected Type II heterojunction energy level offset for the MoS2/SWCNT and WSe2/SWCNT bilayers, where the conduction band (CB) and valence band (VB) energies of MoS2 and WSe2 are taken from theoretical and experimental literature reports (vide infra and SI, Table S3). We estimate the exciton dissociation driving force for each Type II bilayer using the following equation:
ΔGET/HT=|IPD−EAA|−Eopt,D/opt,A
(1)
where ΔGET/HT is the change in free energy following electron transfer (ET) or hole transfer (HT), IPD and EAA are the ionization potential (IP) of the donor and electron affinity (EA) of the acceptor, respectively, and Eopt,D and Eopt,A refer to the optical band gap of the donor or acceptor, respectively. (3,16) Using optical band gap energies from absorption spectra (Figure 1c–e) and EA/IP values shown in Figure 1a, we estimate that exciton dissociation in MoS2 via hole transfer to (6,5) SWCNTs is thermodynamically favorable by ca. 700 meV and electron transfer from the SWCNTs to MoS2 is favorable by ca. 70 meV. (3,15,16) Uncertainty in ΔGET and ΔGHT for WSe2 is higher due to slight variations in the wavelength for the WSe2 A exciton peak across different samples and a higher variability (and smaller number) of reported experimental and computed CB/VB values of WSe2 compared to the more widely studied MoS2. (7) To account for these variations, we used experimental data captured from steady state absorption to calculate Eopt,WSe2, resulting in a ΔGET of −490 meV and a ΔGHT of 0 meV. Theoretically, exciton dissociation at the interface can occur for both bilayers in both directions, but the hole transfer from (6,5) s-SWCNTs to WSe2 is the least energetically favorable pathway.
Transient absorption (TA) spectroscopy is used to elucidate the charge transfer dynamics at the MoS2/SWCNT and WSe2/SWCNT interfaces. The narrow and energetically distinct absorbance peaks of MoS2, WSe2 and (6,5) SWCNTs, highlighted in Figure 1e, allow for selective photoexcitation and probing of each material, making these ideal model systems for optical studies of excited-state dynamics at the TMDC heterointerface. Photoexcitation of the SWCNT S11 absorption at 1000 nm only allows for charge transfer from SWCNT to either/both TMDC, as the 1000 nm photons are too low in energy for direct excitation of MoS2 or WSe2 and energy transfer would be significantly uphill.
Figure 2 shows long-lived TA spectra of both bilayers following 1000 nm excitation of the s-SWCNT S11 excitonic transition. In the MoS2/SWCNT bilayer, we see charge separation that persists well beyond the 5 ns window of the TA experiment, consistent with previous MoS2/SWCNT studies, the Type II energetic alignment and the thermodynamic spontaneity for photoinduced electron transfer expected from Figure 1a. (3,5,25) In the near-infrared (NIR) range of photoexcited neat MoS2 monolayers there is no TA signal, since all MoS2 excitonic features occur in the visible range. However, clear signatures of an SWCNT charge population in the MoS2/SWCNT bilayer indicate photoinduced electron transfer from SWCNTs to MoS2 (Figure 2a). The well-documented signature is the SWCNT trion (X+) induced absorption (IA) at 1175 nm and the accompanying 1000 nm ground-state bleach (GSB) of the S11 transition. (5,16) These NIR features are complemented by long-lived features in the visible range, which we can attribute to the MoS2 A and B excitons (bleaches at 660 and 610 nm, respectively) and to the SWCNT S22 transition (575 nm bleach). Figure 2c demonstrates long-lived carrier separation in the MoS2 heterobilayer compared to the neat SWCNTs via charge-related kinetics at the 1175 nm SWCNT trion-induced absorption. We note the presence of a second pulse in the TA kinetics, around 10 ps, due to reflection off of the air-free holder sapphire windows. This second pulse is present in all our samples and it should not be mistaken for a delayed rise of a given population. The effect of the second pulse is explained and addressed in the Supporting Information.

Figure 2

Figure 2. Transient absorption spectra averaged over 2–5 ns following 1000 nm excitation of (A) SWCNT (black) and MoS2/SWCNT bilayer (purple) and (B) SWCNT (black) and WSe2/SWCNT bilayer (red). Kinetic traces corresponding to the SWCNT trion (X+ or X–, depending on the transferred charge) induced absorption with 1000 nm excitation: (C) SWCNT (black) and MoS2/SWCNT bilayer (purple) and (D) SWCNT (black) and WSe2/SWCNT bilayer (red).

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In contrast to the MoS2/SWCNT bilayer, when exciting the WSe2/SWCNT bilayer at 1000 nm, the WSe2 GSB at 750 nm is absent in the TA spectra (Figure 2b), indicating that there is negligible (or low-yield) exciton dissociation via hole transfer from the SWCNTs to WSe2. However, a small trion feature is observed in the NIR region, from which we calculated a hole transfer yield of 2% after 1000 nm excitation. This low hole transfer yield could explain why we do not observe the WSe2 GSB at 750 nm, since the GSB could be embedded in the noise. The kinetics at 1175 nm change very little relative to that of the neat SWCNT, also suggesting negligible charge generation in this bilayer. This inefficient hole transfer event may result from the low thermodynamic driving force of ca. 0 meV (Figure 1a), even though the calculated band alignment is Type II. We confirmed the absence of the WSe2 GSB at 750 nm after 1000 nm excitation in this bilayer through TA experiments on two other separately prepared bilayers that incorporated CVD-grown WSe2 monolayers that were either purchased or grown in-house at NREL (Figure S1).
Interestingly, we also observe a relatively low charge generation yield in the WSe2/SWCNT bilayer after photoexcitation of the WSe2 layer at 750 nm, approximately 10% (Figure S1). We speculate on two potential mechanisms for the somewhat low efficiency of this electron transfer event. It is possible that the large predicted thermodynamic driving force of ca. 490 meV (Figure 1a) places this electron transfer event in the Marcus inverted regime, (9) although hole transfer from MoS2 to (6,5) s-SWCNTs is substantially more efficient (ca. 39% with ΔGHT = −700 meV). Fang and co-workers have also proposed the formation of interlayer excitons in WSe2/SWCNT heterojunctions, based on PL features observed at room temperature. (33) It is unclear how these interlayer excitons would manifest in our TA measurements, so this subject will be considered in future studies.

MoS2/SWCNT/WSe2 Trilayer

We prepared the MoS2/SWCNT/WSe2 trilayer by transferring a CVD-grown WSe2 monolayer onto the existing MoS2/SWCNT bilayer. (34−36) Figure 1b,e shows Raman and steady state absorption spectra comparing MoS2, the MoS2/SWCNT bilayer, and the MoS2/SWCNT/WSe2 trilayer. The peak at 250 cm–1 in the trilayer Raman spectrum is the WSe2 out-of-plane (E2g) Raman active mode and in the trilayer absorption spectra, the WSe2 A exciton peak appears at 750 nm, indicating the successful creation of the heterotrilayer. (26,28) TA spectroscopy of the trilayer shows ultrafast exciton dissociation at both TMDC/SWCNT interfaces and a resulting long-lived charge separation. Figure 3a highlights the formation of separated carriers within femtoseconds (<200 fs) following 1000 nm excitation, evidenced by the observation of both WSe2 and MoS2 bleach features, followed by charge recombination that persists beyond the nanosecond time scale.

Figure 3

Figure 3. (A) Transient absorption spectra, at varying pump-probe time delays, for the MoS2/SWCNT/WSe2 trilayer, following 1000 nm excitation. (B) Transient absorption spectra averaged over 2–5 ns for the MoS2/SWCNT bilayer (purple), and MoS2/SWCNT/WSe2 trilayer (orange). (C) and (D) Kinetic traces at (C) 1175 nm, corresponding to the SWCNT trion (X+) induced absorption, and (D) 660 nm, corresponding to the ground state bleach of MoS2, following 1000 nm excitation.

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Figure 3a,b demonstrates that not only does photoinduced hole transfer from MoS2 to SWCNTs occur in the trilayer (similar to what is observed for the MoS2 bilayer), but the electron transfer event from WSe2 to SWCNTs, which is inefficient for the WSe2 bilayer, is observed in the trilayer. This unexpected result suggests that some ground- or excited-state properties of the trilayer change the spontaneity/efficiency of the photoinduced hole transfer event from (6,5) SWCNTs to WSe2, relative to the corresponding WSe2/SWCNT bilayer. We speculated that releasing WSe2 from its original sapphire substrate may modify the VB energy in such a way that HT is now spontaneous in the trilayer but not in the bilayer. However, photoexcitation of a “reverse bilayer” (SWCNT/WSe2), in which the SWCNTs are sprayed onto WSe2 that remains on its original substrate, refutes this idea as hole transfer from SWCNT to WSe2 is not present in the transient data (Figure S2).
The photoinduced charge transfer quantum yield (ϕ CT) is defined as
ϕCT=Ne/hNex
(2)
where Ne/h is the number of separated charges (electrons or holes) per photogenerated exciton (Nex). (1,5,25) Using the trion-induced absorption intensity as in prior studies, we estimate the charge yield in the trilayer to be 41%, which is a summation of electron and hole transfer yields at the MoS2 and WSe2 interfaces, respectively. (37) The addition of the hole-accepting WSe2 layer to MoS2/SWCNT roughly doubles the total charge transfer yield when compared to the 23% electron transfer yield in the MoS2/SWCNT bilayer. (5) This result positions the trilayer as an effective method for significantly increasing the charge yield.
Comparing the kinetics of specific spectral features between the MoS2/SWCNT bilayer and the MoS2/SWCNT/WSe2 trilayer can provide a window into the diffusive charge dynamics that occur following photoinduced exciton dissociation in the trilayer. In Figure 3c,d, pumping at 1000 nm excites only the SWCNTs, generating both the MoS2(−)/SWCNT(+) and WSe2(+)/SWCNT(−) interfacial species. Figure 3c compares the kinetics of the SWCNT trion induced absorption in the trilayer and the MoS2/SWCNT bilayer. In the trilayer, the trion transition gets its oscillator strength from both holes (i.e., X+) at the MoS2(−)/SWCNT(+) interface and electrons (i.e., X–) at the WSe2(+)/SWCNT(−) interface. In the MoS2/SWCNT bilayer, the X+ signal is long-lived and decays outside of the nanosecond window, while in the trilayer the trion signal (X+ + X–) is substantially reduced by 5 ns. This depletion of SWCNT charges could arise from two diffusive processes: (1) diffusion-limited recombination of electrons from the WSe2(+)/SWCNT(−) interface with holes from the MoS2(−)/SWCNT(+) interface within the SWCNTs; (2) diffusion of electrons away from the WSe2(+)/SWCNT(−) interface to the MoS2(−)/SWCNT(+) interface where they are injected into MoS2 (and/or the reverse process for holes at the MoS2(−)/SWCNT(+) interface). Both of these processes require SWCNT-based carriers at the SWCNT/TMDC interface to escape the Coulomb potential of the oppositely charged TMDC-based carrier.
To determine if process (2) from above is active, we track the MoS2 GSB kinetics at 660 nm (Figure 3d). 1000 nm is used unless otherwise stated. Encouragingly, the MoS2 GSB does not decay as rapidly in the trilayer, relative to the MoS2/SWCNT bilayer, suggesting that this diffusion-based process may be occurring. Thus, the concomitant depletion of the trion IA and increase of the MoS2 GSB in the trilayer provide evidence for charges escaping the WSe2–SWCNT interface and diffusing to the MoS2–SWCNT interface where they inject into MoS2. When tracking the MoS2 GSB, selective excitation of WSe2 in the trilayer results in a slow rise of MoS2 charges, also supporting this diffusion based process (see Figure S4).
To confirm that charge carrier diffusion plays a role after exciton dissociation and to elaborate on the overall kinetic scheme of this complex system, we turn to a detailed model of the exciton and charge dynamics. Experimental time-dependent spectral contributions of excitons and charges are separated using singular value decomposition (SVD). (38,39) Based on initial TA information, four independent species are used in SVD analysis to extract the decay of charge-related spectral signatures (Figure S5): (1) the initial SWCNT excited state created by excitation at 1000 nm (S65*); (2) electron transfer from SWCNT to MoS2 (S65+ MoS2–); (3) hole transfer from SWCNT to WSe2 (S65– WSe2+); and (4) charge (hole/electron) on s-SWCNT diffusing away to corresponding TMDC (WSe2+/MoS2–), with opposite charges on TMDCs remaining on the respective layer. Figure 4a displays the kinetic scheme that ultimately simulates the 2D TA data most effectively, with the rate equations associated with this scheme provided in Supporting Information secton 1.3. The kinetic scheme and derived equations from the simulation can accurately reproduce the 2D experimental data (Figure 4c,d). Time scales for electron transfer (k11), hole transfer (k22), diffusion (k21 and k31), and each charge recombination (krec) pathway are obtained from the simulated concentration profiles in Figure 4b and are outlined in the visual schematic in Figure 5 (see SI for additional information).

Figure 4

Figure 4. (A) Proposed kinetic scheme, following SWCNT excitatoin at 1000 nm; (B) Concentration profiles for each species generated in the trilayer, with similar color-coding to panel (A); (C) Experimental TA surface plot for the visible region of the trilayer excited at 1000 nm; (D) Simulated TA surface plot from the concentration equations and associated spectra. The color bar to the right specifies intensities of the different signals. Specifically, the GSB for S22 at 575 nm, MoS2 A exciton at 660 nm and WSe2 A excitonat 740 nm can be identified. Color bar to the right specifies intensities of the different signals.

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Figure 5

Figure 5. Kinetic scheme highlighting the different time constants for hole transfer (τHT) to WSe2, electron transfer (τET) to MoS2 and the charge recombination lifetimes (τCR) following selective excitation of SWCNT at 1000 nm.

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Despite a greater predicted thermodynamic driving force for electron transfer (see Figure 1a, ΔGET = −70 meV), we observe faster hole transfer from the SWCNTs to WSe2 (τHT = <0.2 ps) compared to electron transfer from the SWCNTs to MoS2 following selective excitation at 1000 nm (τET = 1.53 ps). Subsequently, diffusion of holes away from the SWCNT/WSe2 interface also occurs at a faster rate (τHT = 5 ps) than diffusion of electrons away from the SWCNT/MoS2 interface (τET = 2.5 ns). Charge recombination of both holes and electrons successfully transferred from SWCNTs to either WSe2 or MoS2 occurs with a time constant of 1.23 μs. The trilayer nearly doubles the charge recombination time, relative to the MoS2/SWCNT bilayer (τrec = 0.73 μs, see also Figure S4). (5) Thus, improved spatial separation in the TMDC/SWCNT/TMDC trilayer leads to long charge recombination lifetimes. (5,8,17)
Our study was motivated by the natural charge transfer cascades driving long-lived charge separation in photosynthesis, as well as a lack of mechanistic understanding of carrier diffusion at TMDC heterointerfaces. The TA analysis on this mixed-dimensionality heterostructure reveals several intriguing features that warrant some discussion. First, the MoS2/SWCNT/WSe2 trilayer allows us to access the HT pathway that is ineffective in the respective bilayer. As prepared, MoS2 tends to be n-type, while WSe2 is typically p-type as-grown. We speculate that having the two oppositely charged layers on either side of the s-SWCNT film may lead to the formation of an electric field or dipole that shifts the band alignment, making HT more favorable in the trilayer than in the bilayer. (2,17) Second, we demonstrate that charges within the s-SWCNT layer can escape the Coulomb attraction of the opposite charge carrier at the SWCNT/TMDC interface. In contrast, charge transfer in similar TMDC/TMDC heterojunctions is typically characterized by spatially confined and tightly bound interfacial excitons with limited mobility away from the heterointerface. The faster time scale for this diffusive process at the WSe2/SWCNT interface, relative to the MoS2/SWCNT interface, may suggest a stronger interlayer Coulomb binding energy at the MoS2/SWCNT interface. We plan to probe these interesting observations in more detail, both experimentally and theoretically, in future studies.

Conclusion

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In conclusion, we have successfully prepared a MoS2/SWCNT/WSe2 heterotrilayer that can separate charges via a charge transfer cascade. By using transient absorption spectroscopy, we have shown that this type of “mixed-dimensionality” Type-II trilayer can yield charge separated lifetimes on the microsecond time scale while significantly increasing charge yield relative to similarly prepared Type-II bilayers. The microsecond charge recombination lifetime demonstrates the significance of SWCNTs as the charge carrier medium and the role that out-of-plane carrier delocalization has on stabilizing long-lived charges. Consistently, the well-resolved SWCNT spectral signatures allowed us to track charge diffusion away from the site of charge generation and demonstrate Coulombically unbound charges moving from one TMDC/SWCNT interface to the other. The observation of a charge transfer pathway in the trilayer that is ineffective in the respective WSe2/SWCNT bilayer suggests that strategic layering in nanoscale heterojunctions provides additional routes for charge movement in specified directions. Ultimately, our results suggest that well-defined charge transfer cascades can result in longer charge separated lifetimes and higher yields of e–/h+ pairs that can escape their mutual Coulombic attraction, positioning these nanoscale model systems as interesting for both fundamental studies and optoelectronic devices.

Methods

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TMDCs

Full coverage CVD grown monolayer MoS2 and WSe2 on c-cut sapphire were purchased from 2D semiconductors. The NREL grown monolayer WSe2 was prepared via chemical vapor deposition (CVD) in a three-temperature zone furnace. An alumina boat positioned 1 in. from zone 1 (upstream, outside of the furnace) contained 500 mg of selenium, and zone 3 contained a Si/SiO2 wafer with a predeposited thin film of 10 mM ammonium metatungstate (AMT) and 10 mM NaOH. 100 sccm aliquot of Ar/2% H2 gas was supplied to the growth chamber to carry the selenium under a growth pressure of 760 Torr. Zone 1, 2, 3 had a temperature ramping rate of 35 °C/min and were heated to 530, 950, and 950 °C, respectively, and maintained for 4 min.

Absorption

Ground state absorption spectra of both neat and bilayer films were measured in air on a Cary 5000 spectrophotometer with baseline correction. The (6,5) SWCNT film thickness was estimated to be ca. 7 nm from the S11 peak optical density at 1000 nm.

Raman and Photoluminescence Spectroscopy

Measurements were taken in an air-free cell using an inVia Renishaw confocal Raman microscope with a 532 and 633 nm laser. (40) Raman scattering was detected using a 1800 lines/mm grating, and photoluminescence (PL) was recorded using a 600 lines/mm grating. A GaInP/AlGaInP sample was used as a reference to account for the day to day changes in laser power. All single Raman and PL spectra, e.g., in Figure 1, are averages of 50–100 spectra taken over a specific area of each sample and then normalized to the GaInP/AlGaInP reference.

Transient Absorption Spectroscopy

Samples for transient absorption measurements were sealed inside a nitrogen glovebox in a double window air-free sample holder to prevent exposure to the atmosphere and avoid sample degradation during TA measurements.

Ultrafast TA

Ultrafast TA was performed on a Coherent Libra Ti/Sapphire laser with a 1 kHz 800 nm output and 150 fs fwhm pulse width. The Coherent Libra output was sent through a beam splitter to generate the pump and probe. The pump was generated with a TOPAS-C optical parametric amplifier. Visible (450–800 nm) and NIR (800–1600 nm) probe pulses were generated by focusing the 800 nm fundamental into a thin or thick sapphire window, respectively. The TA was a pump–probe configuration measurement, contained in a HELIOS optical box from Ultrafast Systems controlled with a HELIOS software package. The probe beam is sent through a beam splitter to measure the reference spectrum to increase signal-to-noise ratio. The probe pulse, an 800 nm fundamental, is delayed with respect to the pump pulse using a mechanical delay stage. Data were chirp corrected and analyzed using a Surface Xplorer from Ultrafast Systems.

Nanosecond–Microsecond (100 ps to 400 μs) TA

Nanosecond–microsecond TA measurements were performed on a Coherent Libra Ti/Sapphire laser with a 1 kHz 800 nm output and 150 fs fwhm pulse width. The fundamental beam was used to generate pump pulses with a TOPAS-C optical parametric amplifier. Visible and NIR probe pulses are generated from an EOS (Ultrafast Systems) apparatus, produced in a diode-laser-pumped photonic crystal fiber, and electronically delayed relative to the pump pulse with a digital delay generator. Data acquisition was performed using the EOS software package from Ultrafast Systems.

SWCNT Preparation

(6,5) Single-walled carbon nanotubes (SWCNTs) dispersions were prepared from SG65i SWCNTs (CoMoCAT, purchased from CHASM) and poly-[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,60-[2,20-bipyridine])] (PFO-bpy) purchased from American Dye Source. Pure (6,5) SWCNTs were extracted from a solution of 0.5 mg/mL SWCNTs and 2 mg/mL of PFO-bpy in toluene via tip sonication (Cole-Palmer CPX 750, 0.5 in. tip) for 15 min at 40% amplitude in a flowing bath of cool water. Immediately after sonication, the dispersion was centrifuged at 13000 rpm for 5 min at 20 °C (Beckman Coulter L-100 XP ultracentrifuge, SW-32 Ti rotor). To remove excess polymer, the supernatant was separated from the pellet and centrifuged for 20 h at 24100 rpm and 20 °C. A compact pellet containing (6,5) SWNCTs and excess PFO-Bpy was redispersed in toluene in an ultrasonic bath sonicator for over an hour. The polymer removal process was then repeated for a second time.

Spray Coating SWCNTs

MoS2/SWCNT and WSe2/SWCNT bilayers were prepared by spray coating the SWCNT ink onto the respective neat substrate. The substrates were placed on a heated metal stage (130 °C) to evaporate the toluene solvent. The ink was applied from an ultrasonic sprayer (SonoTek) directed at the substrate, operated at 0.8 W with a solution flow rate of 0.3 mL/min and a nitrogen flow rate of 7.0 standard liters per minute. Each bilayer film was soaked in their respective hot (78 °C) toluene baths for 10 min to remove additional polymer from the film.

WSe2 Transfer

To transfer the WSe2 monolayer off of its sapphire growth substrate and onto the MoS2/SWCNT bilayer, the WSe2/sapphire substrate was coated with polystyrene (PS) dissolved in toluene (50 mg/mL) via spin coating at 2400 rpm for 45 s. Immediately after spin coating, all 4 sides of the film were scribed using a box cutter to create an opening for etchant to access the WSe2/sapphire interface. The substrate was placed in a hot (70 °C) 2 M NaOH bath for 10 min. The substrate was removed from the NaOH bath and the PS/WSe2 film detached in water by holding the substrate at the air/water interface. The PS/WSe2 film was transferred to the MoS2/SWCNT bilayer by lifting the substrate from underneath the film. The trilayer was then soaked in toluene for 10 min to remove the PS layer. This process was repeated for a second transfer. The film was placed in a spot different from the original transfer to maximize WSe2 surface coverage.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.3c12179.

  • Additional transient absorption spectroscopy, kinetic analysis of trilayer, ΔG calculation values, and charge transfer yield calculations (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Alexis R. Myers - National Renewable Energy Laboratory, Golden, Colorado 80401, United States;  Department of Chemistry, University of Colorado−Boulder, Boulder, Colorado 80309, United States
    • Zhaodong Li - National Renewable Energy Laboratory, Golden, Colorado 80401, United States;  The Institute of Technological Sciences, Wuhan University, Wuhan, Hubei 430072, China
    • Melissa K. Gish - National Renewable Energy Laboratory, Golden, Colorado 80401, United States;  Orcidhttps://orcid.org/0000-0002-9886-3626
    • Justin D. Earley - National Renewable Energy Laboratory, Golden, Colorado 80401, United States;  Department of Chemistry, University of Colorado−Boulder, Boulder, Colorado 80309, United States;  Orcidhttps://orcid.org/0000-0003-0492-4692
    • Justin C. Johnson - National Renewable Energy Laboratory, Golden, Colorado 80401, United States;  Orcidhttps://orcid.org/0000-0002-8874-6637
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was authored by the National Renewable Energy Laboratory, operated by the Alliance for Sustainable Energy, LLC, for the US Department of Energy(DOE) under Contract No. DE-AC36-08GO28308. This study was supported by the Solar Photochemistry Program, Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. DOE. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government.

References

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    Journal of Physical Chemistry C (2018), 122 (25), 14150-14161CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Nonfullerene electron acceptors have facilitated a recent surge in the efficiencies of org. solar cells, although fundamental studies of the nature of exciton dissocn. at interfaces with nonfullerene electron acceptors are still relatively sparse. Semiconducting single-walled carbon nanotubes (s-SWCNTs), unique 1-dimensional electron donors with mol.-like absorption and highly mobile charges, provide a model system for studying interfacial exciton dissocn. Here, the authors study excited-state photodynamics at the heterojunction between (6,5) s-SWCNTs and two perylene diimide (PDI)-based electron acceptors. Each of the PDI-based acceptors, hPDI2-pyr-hPDI2 and Trip-hPDI2, is deposited onto (6,5) s-SWCNT films to form a heterojunction bilayer. Transient absorption measurements demonstrate that photoinduced hole/electron transfer occurs at the photoexcited bilayer interfaces, producing long-lived sepd. charges with lifetimes exceeding 1.0 μs. Both exciton dissocn. and charge recombination occur more slowly for the hPDI2-pyr-hPDI2 bilayer than for the Trip-hPDI2 bilayer. To explain such differences, the potential roles of the thermodn. charge transfer driving force available at each interface and the different mol. structure and intermol. interactions of PDI-based acceptors are discussed. Detailed photophys. anal. of these model systems can develop the fundamental understanding of exciton dissocn. between org. electron donors and nonfullerene acceptors, which was not systematically studied.
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  17. 17
    Homan, S. B.; Sangwan, V. K.; Balla, I.; Bergeron, H.; Weiss, E. A.; Hersam, M. C. Ultrafast exciton dissociation and long-lived charge separation in a photovoltaic pentacene-MoS2 van der Waals heterojunction. Nano Lett. 2017, 17, 164– 169,  DOI: 10.1021/acs.nanolett.6b03704
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    There is no corresponding record for this reference.
  18. 18
    Mirkovic, T.; Ostroumov, E. E.; Anna, J. M.; van Grondelle, R.; Govindjee; Scholes, G. D. Light absorption and energy transfer in the antenna complexes of photosynthetic organisms. Chem. Rev. 2017, 117, 249– 293,  DOI: 10.1021/acs.chemrev.6b00002
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    18
    Light absorption and energy transfer in the antenna complexes of photosynthetic organisms
    Mirkovic, Tihana; Ostroumov, Evgeny E.; Anna, Jessica M.; van Grondelle, Rienk; Govindjee; Scholes, Gregory D.
    Chemical Reviews (Washington, DC, United States) (2017), 117 (2), 249-293CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. The process of photosynthesis is initiated by the capture of sunlight by a network of light-absorbing mols. (chromophores), which are also responsible for the subsequent funneling of the excitation energy to the reaction centers. Through evolution, genetic drift, and speciation, photosynthetic organisms have discovered many solns. for light harvesting. In this review, we describe the underlying photophys. principles by which this energy is absorbed, as well as the mechanisms of electronic excitation energy transfer (EET). First, optical properties of the individual pigment chromophores present in light-harvesting antenna complexes are introduced, and then we examine the collective behavior of pigment-pigment and pigment-protein interactions. The description of energy transfer, in particular multichromophoric antenna structures, is shown to vary depending on the spatial and energetic landscape, which dictates the relative coupling strength between constituent pigment mols. In the latter half of the article, we focus on the light-harvesting complexes of purple bacteria as a model to illustrate the present understanding of the synergetic effects leading to EET optimization of light-harvesting antenna systems while exploring the structure and function of the integral chromophores. We end this review with a brief overview of the energy-transfer dynamics and pathways in the light-harvesting antennas of various photosynthetic organisms.
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  19. 19
    Ceballos, F.; Ju, M.-G.; Lane, S. D.; Zeng, X. C.; Zhao, H. Highly Efficient and Anomalous Charge Transfer in van der Waals Trilayer Semiconductors. Nano Lett. 2017, 17, 1623– 1628,  DOI: 10.1021/acs.nanolett.6b04815
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    19
    Highly Efficient and Anomalous Charge Transfer in van der Waals Trilayer Semiconductors
    Ceballos, Frank; Ju, Ming-Gang; Lane, Samuel D.; Zeng, Xiao Cheng; Zhao, Hui
    Nano Letters (2017), 17 (3), 1623-1628CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Two-dimensional materials, such as graphene and monolayer transition metal dichalcogenides, allow the fabrication of multilayer structures without lattice matching restriction. A central issue in developing such artificial materials is to understand and control the interlayer electron transfer process, which plays a key role in harnessing their emergent properties. Recent photoluminescence and transient absorption measurements revealed that the electron transfer in heterobilayers occurs on ultrafast time scales. However, there is still a lack of fundamental understanding on how this process can be so efficient at van der Waals interfaces. Here the authors show evidence suggesting the coherent nature of such interlayer electron transfer. In a trilayer of MoS2-WS2-MoSe2, electrons excited in MoSe2 transfer to MoS2 in about one picosecond. Surprisingly, these electrons do not populate the middle WS2 layer during this process. Calcns. showed the coherent nature of the charge transfer and reproduced the measured electron transfer time. The hole transfer from MoS2 to MoSe2 also is efficient and ultrafast. The sepn. of electrons and holes extends their lifetimes to more than one nanosecond, suggesting potential applications of such multilayer structures in optoelectronics.
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  20. 20
    Zereshki, P.; Wei, Y.; Long, R.; Zhao, H. Layer-Coupled States Facilitate Ultrafast Charge Transfer in a Transition Metal Dichalcogenide Trilayer Heterostructure. J. Phys. Chem. Lett. 2018, 9, 5970– 5978,  DOI: 10.1021/acs.jpclett.8b02622
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    20
    Layer-Coupled States Facilitate Ultrafast Charge Transfer in a Transition Metal Dichalcogenide Trilayer Heterostructure
    Zereshki, Peymon; Wei, Yaqing; Long, Run; Zhao, Hui
    Journal of Physical Chemistry Letters (2018), 9 (20), 5970-5978CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Forming van der Waals multilayer structures with two-dimensional materials is a promising new method for material discovery. The weak van der Waals interlayer interaction without at. correspondence relaxes lattice matching requirement and allows formation of high-quality interfaces with virtually any combination of two-dimensional materials. However, the weak nature of the van der Waals interaction also makes it challenging to harness emergent properties of such multilayer materials. Previous studies have indicated that in transition metal dichalcogenide bilayer heterostructures, the interlayer charge and energy transfer is highly efficient. Therefore, it is important to understand interlayer coupling in these materials and its role on charge and energy transfer. Here we show that in a MoSe2/WSe2/WS2 trilayer, the interlayer coupling is strong enough to form layer-coupled states in the conduction band with the electron wave function extends to all three layers. D. functional theory calcns. reveal that the layer-coupled states in Q valley are about 0.1 eV below the individual monolayer states in K valley, which is consistent with photoluminescence measurements. Transient absorption measurements show that these layer-coupled states provide a channel for ultrafast interlayer charge transfer between the top WS2 and the bottom MoSe2 layers. In this process, electrons from the K valley of the individual monolayers are scattered to the layer-coupled states in Q valley. Such a partial charge transfer allows formation of partial-indirect excitons with the holes in one monolayer while electrons shared by three layers. The formation of layer-coupled states is promising for harnessing emergent properties of transition metal dichalcogenide multilayer heterostructures. Our findings also provide new ingredient to understand charge and energy transfer in transition metal dichalcogenide heterobilayers, as the layer-coupled states can play important roles in the efficient transfer obsd. in these systems.
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  21. 21
    Kim, J.; Jin, C.; Chen, B.; Cai, H.; Zhao, T.; Lee, P.; Kahn, S.; Watanabe, K.; Taniguchi, T.; Tongay, S.; Crommie, M. F.; Wang, F. Observation of Ultralong Valley Lifetime in WSe2/MoS2 Heterostrucures. Sci. Adv. 2017, 3, 1– 7,  DOI: 10.1126/sciadv.1700518
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  22. 22
    Wang, Z.; Sun, C.; Xu, X.; Liu, Y.; Chen, Z.; Yang, Y. â.; Zhu, H. Long-Range Hot Charge Transfer Exciton Dissociation in an Organic/2D Semiconductor Hybrid Excitonic Heterostructure. J. Am. Chem. Soc. 2023, 145, 11227– 11235,  DOI: 10.1021/jacs.3c01192
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    22
    Long-Range Hot Charge Transfer Exciton Dissociation in an Organic/2D Semiconductor Hybrid Excitonic Heterostructure
    Wang, Zukun; Sun, Cheng; Xu, Xuehui; Liu, Yanping; Chen, Zeng; Yang, Yang "Michael"; Zhu, Haiming
    Journal of the American Chemical Society (2023), 145 (20), 11227-11235CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Whether and how an electron-hole pair at the donor-acceptor interface separates from their mutual Coulombic interaction has been a long-standing question for both fundamental interests and optoelectronic applications. This question is particularly interesting but yet to be unraveled in the emerging mixed-dimensional org./2D semiconductor excitonic heterostructures where the Coulomb interaction is poorly screened. Here, by tracking the characteristic electroabsorption (Stark effect) signal from sepd. charges using transient absorption spectroscopy, we directly follow the electron-hole pair sepn. process in a model org./2D heterostructure, vanadium oxide phthalocyanine/monolayer MoS2. After sub-100 fs photoinduced interfacial electron transfer, we observe a barrier-less long-range electron-hole pair sepn. to free carriers within 1 ps by hot charge transfer exciton dissocn. Further expt. reveals the key role of the charge delocalization in org. layers sustained by the local crystallinity, while the inherent in-plane delocalization of the 2D semiconductor has a negligible contribution to charge pair sepn. This study reconciles the seemingly contradicting charge transfer exciton emission and dissocn. process and is important to the future development of efficient org./2D semiconductor optoelectronic devices.
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  23. 23
    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– 686,  DOI: 10.1038/nnano.2014.167
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    23
    Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures
    Hong, Xiaoping; Kim, Jonghwan; Shi, Su-Fei; Zhang, Yu; Jin, Chenhao; Sun, Yinghui; Tongay, Sefaattin; Wu, Junqiao; Zhang, Yanfeng; Wang, Feng
    Nature Nanotechnology (2014), 9 (9), 682-686CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Van der Waals heterostructures have recently emerged as a new class of materials, where quantum coupling between stacked atomically thin two-dimensional layers, including graphene, hexagonal-boron nitride and transition-metal dichalcogenides (MX2), give rise to fascinating new phenomena. MX2 heterostructures are particularly exciting for novel optoelectronic and photovoltaic applications, because two-dimensional MX2 monolayers can have an optical bandgap in the near-IR to visible spectral range and exhibit extremely strong light-matter interactions. Theory predicts that many stacked MX2 heterostructures form type II semiconductor heterojunctions that facilitate efficient electron-hole sepn. for light detection and harvesting. Here, the authors report the 1st exptl. observation of ultrafast charge transfer in photoexcited MoS2/WS2 heterostructures using both photoluminescence mapping and femtosecond pump-probe spectroscopy. Hole transfer from the MoS2 layer to the WS2 layer takes place within 50 fs after optical excitation, a remarkable rate for van der Waals coupled two-dimensional layers. Such ultrafast charge transfer in van der Waals heterostructures can enable novel two-dimensional devices for optoelectronics and light harvesting.
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  24. 24
    Guillot, S. L.; Mistry, K. S.; Avery, A. D.; Richard, J.; Dowgiallo, A.-M.; Ndione, P. F.; van de Lagemaat, J.; Reese, M. O.; Blackburn, J. L. Precision printing and optical modeling of ultrathin SWCNT/C60 heterojunction solar cells. Nanoscale 2015, 7, 6556– 6566,  DOI: 10.1039/C5NR00205B
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    24
    Precision printing and optical modeling of ultrathin SWCNT/C60 heterojunction solar cells
    Guillot, Sarah L.; Mistry, Kevin S.; Avery, Azure D.; Richard, Jonah; Dowgiallo, Anne-Marie; Ndione, Paul F.; van de Lagemaat, Jao; Reese, Matthew O.; Blackburn, Jeffrey L.
    Nanoscale (2015), 7 (15), 6556-6566CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising candidates as the active layer in photovoltaics (PV), particularly for niche applications where high IR absorbance and/or semi-transparent solar cells are desirable. Most current fabrication strategies for SWCNT PV devices suffer from relatively high surface roughness and lack nanometer-scale deposition precision, both of which may hamper the reproducible prodn. of ultrathin devices. Addnl., detailed optical models of SWCNT PV devices are lacking, due in part to a lack of well-defined optical consts. for high-purity s-SWCNT thin films. Here, we present an optical model that accurately reconstructs the shape and magnitude of spectrally resolved external quantum efficiencies for ultrathin (7,5) s-SWCNT/C60 solar cells that are deposited by ultrasonic spraying. The ultrasonic spraying technique enables thickness tuning of the s-SWCNT layer with nanometer-scale precision, and consistently produces devices with low s-SWCNT film av. surface roughness (Rq of <5 nm). Our optical model, based entirely on measured optical consts. of each layer within the device stack, enables quant. predictions of thickness-dependent relative photocurrent contributions of SWCNTs and C60 and enables ests. of the exciton diffusion lengths within each layer. These results establish routes towards rational performance improvements and scalable fabrication processes for ultra-thin SWCNT-based solar cells.
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  25. 25
    Sulas-Kern, D. B.; Zhang, H.; Li, Z.; Blackburn, J. L. Interplay between microstructure, defect states, and mobile charge generation in transition metal dichalcogenide heterojunctions. Nanoscale 2021, 13, 8188– 8198,  DOI: 10.1039/D1NR00384D
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    25
    Interplay between microstructure, defect states, and mobile charge generation in transition metal dichalcogenide heterojunctions
    Sulas-Kern, Dana B.; Zhang, Hanyu; Li, Zhaodong; Blackburn, Jeffrey L.
    Nanoscale (2021), 13 (17), 8188-8198CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Two-dimensional transition metal dichalcogenides (2D-TMDCs) have gained attention for their promise in next-generation energy-harvesting and quantum computing technologies, but realizing these technologies requires a greater understanding of TMDC properties that influence their photophysics. To this end, we discuss here the interplay between TMDC microstructure and defects with the charge generation yield, lifetime, and mobility. As a model system, we compare monolayer-only and monolayer-rich MoS2 grown by chem. vapor deposition, and we employ the TMDCs in Type-II charge-sepg. heterojunctions with semiconducting single-walled carbon nanotubes (s-SWCNTs). Our results suggest longer lifetimes and higher yields of mobile carriers in samples contg. a small fraction of defect-rich multilayer islands on predominately monolayer MoS2. Compared to the monolayer-only heterojunctions, the carrier lifetimes increase from 0.73μs to 4.71μs, the hole transfer yield increases from 23% to 34%, and the electron transfer yield increases from 39% to 59%. We reach these conclusions using a unique combination of microwave photocond. (which probes only mobile carriers) along with transient absorption spectroscopy (which identifies spectral signatures unique to each material and type of photoexcited quasiparticle, but does not probe mobility). Our results highlight the substantial changes in photophysics that can occur from small changes in TMDC microstructure and defect d., where the presence of defects does not necessarily preclude improvements in charge generation.
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  26. 26
    Zhao, W.; Ghorannevis, Z.; Amara, K. K.; Pang, J. R.; Toh, M.; Zhang, X.; Kloc, C.; Tan, P. H.; Eda, G. Lattice dynamics in mono- and few-layer sheets of WS2 and WSe2. Nanoscale 2013, 5, 9677– 9683,  DOI: 10.1039/c3nr03052k
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    26
    Lattice dynamics in mono- and few-layer sheets of WS2 and WSe2
    Zhao, Weijie; Ghorannevis, Zohreh; Amara, Kiran Kumar; Pang, Jing Ren; Toh, Minglin; Zhang, Xin; Kloc, Christian; Tan, Ping Heng; Eda, Goki
    Nanoscale (2013), 5 (20), 9677-9683CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Thickness is one of the fundamental parameters that define the electronic, optical, and thermal properties of two-dimensional (2D) crystals. Phonons in Mo disulfide (MoS2) were recently found to exhibit unique thickness dependence due to the interplay between short and long range interactions. Here the authors report Raman spectra of atomically thin sheets of WS2 and WSe2, isoelectronic compds. of MoS2, in the mono- to few-layer thickness regime. Similar to the case of MoS2, the characteristic A1g and E2g1 modes exhibit stiffening and softening with increasing no. of layers, resp., with a small shift of <3 cm-1 due to large mass of the atoms. Thickness dependence is also obsd. in multiphonon bands arising from overtone, combination, and zone edge phonons, whose intensity exhibit significant enhancement in excitonic resonance conditions. Some of these multiphonon peaks are absent only in monolayers. These features provide a unique fingerprint and rapid identification for monolayer flakes.
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  27. 27
    Zhang, X.; Qiao, X.-F.; Shi, W.; Wu, J.-B.; Jiang, D.-S.; Tan, P.-H. Phonon and Raman scattering of two-dimensional transition metal dichalcogenides from monolayer, multilayer to bulk material. Chem. Soc. Rev. 2015, 44, 2757– 2785,  DOI: 10.1039/C4CS00282B
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    27
    Phonon and Raman scattering of two-dimensional transition metal dichalcogenides from monolayer, multilayer to bulk material
    Zhang, Xin; Qiao, Xiao-Fen; Shi, Wei; Wu, Jiang-Bin; Jiang, De-Sheng; Tan, Ping-Heng
    Chemical Society Reviews (2015), 44 (9), 2757-2785CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    Two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets exhibit remarkable electronic and optical properties. The 2D features, sizable bandgaps and recent advances in the synthesis, characterization and device fabrication of the representative MoS2, WS2, WSe2 and MoSe2 TMDs make TMDs very attractive in nanoelectronics and optoelectronics. Similar to graphite and graphene, the atoms within each layer in 2D TMDs are joined together by covalent bonds, while van der Waals interactions keep the layers together. This makes the phys. and chem. properties of 2D TMDs layer-dependent. In this review, we discuss the basic lattice vibrations of 2D TMDs from monolayer, multilayer to bulk material, including high-frequency optical phonons, interlayer shear and layer breathing phonons, the Raman selection rule, layer-no. evolution of phonons, multiple phonon replica and phonons at the edge of the Brillouin zone. The extensive capabilities of Raman spectroscopy in investigating the properties of TMDs are discussed, such as interlayer coupling, spin-orbit splitting and external perturbations. The interlayer vibrational modes are used in rapid and substrate-free characterization of the layer no. of multilayer TMDs and in probing interface coupling in TMD heterostructures. The success of Raman spectroscopy in investigating TMD nanosheets paves the way for expts. on other 2D crystals and related van der Waals heterostructures.
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  28. 28
    Tonndorf, P.; Schmidt, R.; Böttger, P.; Zhang, X.; Börner, J.; Liebig, A.; Albrecht, M.; Kloc, C.; Gordan, O.; Zahn, D. R. T.; de Vasconcellos, S. M.; Bratschitsch, R. Photoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2. Opt. Express 2013, 21, 4908– 4916,  DOI: 10.1364/OE.21.004908
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    28
    Photoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2
    Tonndorf, Philipp; Schmidt, Robert; Boettger, Philipp; Zhang, Xiao; Boerner, Janna; Liebig, Andreas; Albrecht, Manfred; Kloc, Christian; Gordan, Ovidiu; Zahn, Dietrich R. T.; de Vasconcellos, Steffen Michaelis; Bratschitsch, Rudolf
    Optics Express (2013), 21 (4), 4908-4916CODEN: OPEXFF; ISSN:1094-4087. (Optical Society of America)
    We mech. exfoliate mono- and few-layers of the transition metal dichalcogenides molybdenum disulfide, molybdenum diselenide, and tungsten diselenide. The exact no. of layers is unambiguously detd. by at. force microscopy and high-resoln. Raman spectroscopy. Strong photoluminescence emission is caused by the transition from an indirect band gap semiconductor of bulk material to a direct band gap semiconductor in atomically thin form.
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  29. 29
    McCreary, K. M.; Hanbicki, A. T.; Sivaram, S. V.; Jonker, B. T. A- and B-exciton photoluminescence intensity ratio as a measure of sample quality for transition metal dichalcogenide monolayers. APL Materials 2018, 6, na,  DOI: 10.1063/1.5053699
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    Dong, N.; Li, Y.; Feng, Y.; Zhang, S.; Zhang, X.; Chang, C.; Fan, J.; Zhang, L.; Wang, J. Optical limiting and theoretical modelling of layered transition metal dichalcogenide nanosheets. Opt. Express 2015, 5, 1– 10,  DOI: 10.1038/srep14646
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    Tunable lasing on silver island films by coupling to the localized surface plasmon
    Ning, Shuya; Wu, Zhaoxin; Dong, Hua; Yuan, Fang; Ma, Lin; Jiao, Bo; Hou, Xun
    Optical Materials Express (2015), 5 (3), 1-10CODEN: OMEPAX; ISSN:2159-3930. (Optical Society of America)
    Lasing of N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1':4',1''- terphenyl]-4,4''-diamine (BMT-TPD) films on silver island films (SIFs) was investigated. The size of silver nanoparticles (NPs) of SIFs ranged from 8 to 500 nm, which showed the different localized surface plasmon resonance (LSPR). It was found that the lasing wavelength of BMT-TPD was tuned by the LSPR peaks of silver NPs. This was attributed to the coupling between gain medium and plasmonic silver NPs, i.e., the surface plasmon amplification by stimulated emission of radiation which resulted in the lasing at the corresponding wavelengths. This is expected to be a new and easy approach for the tuning of wavelength of lasing.
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    Amani, M.; Taheri, P.; Addou, R.; Ahn, G. H.; Kiriya, D.; Lien, D.-H.; Ager, J. W.; Wallace, R. M.; Javey, A. Recombination Kinetics and Effects of Superacid Treatment in Sulfur- and Selenium-Based Transition Metal Dichalcogenides. Nano Lett. 2016, 16, 2786– 2791,  DOI: 10.1021/acs.nanolett.6b00536
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    31
    Recombination Kinetics and Effects of Superacid Treatment in Sulfur- and Selenium-Based Transition Metal Dichalcogenides
    Amani, Matin; Taheri, Peyman; Addou, Rafik; Ahn, Geun Ho; Kiriya, Daisuke; Lien, Der-Hsien; Ager, Joel W.; Wallace, Robert M.; Javey, Ali
    Nano Letters (2016), 16 (4), 2786-2791CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Optoelectronic devices based on two-dimensional (2D) materials showed tremendous promise over the past few years; however, there are still numerous challenges that need to be overcome to enable their application in devices. These include improving their poor photoluminescence (PL) quantum yield (QY) as well as better understanding of exciton-based recombination kinetics. Recently, the authors developed a chem. treatment technique using an org. superacid, bis(trifluoromethane)sulfonimide (TFSI), which was shown to improve the quantum yield in MoS2 from <1% to over 95%. Here, the authors perform detailed steady-state and transient optical characterization on some of the most heavily studied direct bandgap 2-dimensional materials, specifically WS2, MoS2, WSe2, and MoSe2, over a large pump dynamic range to study the recombination mechanisms present in these materials. The authors then explore the effects of TFSI treatment on the PL QY and recombination kinetics for each case. Results suggest that sulfur-based 2-dimensional materials are amenable to repair/passivation by TFSI, while the mechanism is thus far ineffective on selenium based systems. Also biexcitonic recombination is the dominant nonradiative pathway in these materials and the kinetics for TFSI treated MoS2 and WS2 can be described using a simple two parameter model.
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    Gupta, R.; Singh, B. P.; Singh, V. N.; Gupta, T. K.; Mathur, R. B. Origin of radial breathing mode in multiwall carbon nanotubes synthesized by catalytic chemical vapor deposition. Carbon 2014, 66, 724– 726,  DOI: 10.1016/j.carbon.2013.08.057
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    Origin of radial breathing mode in multiwall carbon nanotubes synthesized by catalytic chemical vapor deposition
    Gupta, Ravi; Singh, Bhanu P.; Singh, Vidya N.; Gupta, Tejendra K.; Mathur, Rakesh B.
    Carbon (2014), 66 (), 724-726CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
    The origin of radial breathing mode (RBM) in the Raman spectra of multiwall carbon nanotubes (MWNCTs) is discussed. In general, RBM is characteristics of single wall carbon nanotube (SWCNT). With the help of transmission electron microscopy (TEM) and Raman spectroscopic studies, it is established that the presence of SWCNT in the cavity of MWCNT is responsible for the appearance of RBM in MWCNT (synthesized by low temp. catalytic chem. vapor deposition technique). The estd. diam. of 8.2 Å (from Raman study) of SWCNT is almost same as that obsd. (∼8.3 Å) in TEM studies.
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  1. Daria D. Blach, Dana B. Sulas-Kern, Bipeng Wang, Run Long, Qiushi Ma, Oleg V. Prezhdo, Jeffrey L. Blackburn, Libai Huang. Long-Range Charge Transport Facilitated by Electron Delocalization in MoS2 and Carbon Nanotube Heterostructures. ACS Nano 2025, 19 (3) , 3439-3447. https://doi.org/10.1021/acsnano.4c12858
  2. Lei Tong, Hui Yan, Chunyan Xu, Weijie Bai, Can Su, Heng Li, Xinyu Wang, Wenhao Fan, Xudong Chen, Zhicheng Zhang, Qingguo Wang, Shougen Yin. Perovskite quantum dots/WSe2 mixed-dimensional van der Waals heterostructure for photoelectric enhancement and polarization sensitivity. Chemical Engineering Journal 2024, 499 , 155861. https://doi.org/10.1016/j.cej.2024.155861
  3. Alexis R. Myers, Dana B. Sulas-Kern, Rao Fei, Debjit Ghoshal, M. Alejandra Hermosilla-Palacios, Jeffrey L. Blackburn. Quantifying carrier density in monolayer MoS2 by optical spectroscopy. The Journal of Chemical Physics 2024, 161 (4) https://doi.org/10.1063/5.0213720
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  • Abstract

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    Figure 1

    Figure 1. (A) Predicted energy level diagram of MoS2/SWCNT and WSe2/SWCNT bilayers (top); Calculated thermodynamic driving forces for electron and hole transfer at the MoS2/SWCNT and WSe2/SWCNT interfaces (bottom). (B) Raman spectra of (6,5) SWCNT film, MoS2 monolayer, MoS2/SWCNT bilayer, WSe2 monolayer, and the MoS2/SWCNT/WSe2 trilayer. (C) Absorbance spectra for MoS2 monolayer, (6,5) SWCNT film, and the MoS2/SWCNT heterojunction (top); Schematic of MoS2/SWCNT bilayer (bottom). (D) Absorbance spectra of WSe2 monolayer, (6,5) SWCNT film, and the WSe2/SWCNT bilayer (top); Schematic of the WSe2/SWCNT bilayer. (E) Absorbance spectra of (6,5) SWCNT film, MoS2/SWCNT bilayer, and MoS2/SWCNT/WSe2 trilayer (top); Schematic of the MoS2/SWCNT/WSe2 trilayer (bottom).

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    Figure 2

    Figure 2. Transient absorption spectra averaged over 2–5 ns following 1000 nm excitation of (A) SWCNT (black) and MoS2/SWCNT bilayer (purple) and (B) SWCNT (black) and WSe2/SWCNT bilayer (red). Kinetic traces corresponding to the SWCNT trion (X+ or X–, depending on the transferred charge) induced absorption with 1000 nm excitation: (C) SWCNT (black) and MoS2/SWCNT bilayer (purple) and (D) SWCNT (black) and WSe2/SWCNT bilayer (red).

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    Figure 3

    Figure 3. (A) Transient absorption spectra, at varying pump-probe time delays, for the MoS2/SWCNT/WSe2 trilayer, following 1000 nm excitation. (B) Transient absorption spectra averaged over 2–5 ns for the MoS2/SWCNT bilayer (purple), and MoS2/SWCNT/WSe2 trilayer (orange). (C) and (D) Kinetic traces at (C) 1175 nm, corresponding to the SWCNT trion (X+) induced absorption, and (D) 660 nm, corresponding to the ground state bleach of MoS2, following 1000 nm excitation.

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    Figure 4

    Figure 4. (A) Proposed kinetic scheme, following SWCNT excitatoin at 1000 nm; (B) Concentration profiles for each species generated in the trilayer, with similar color-coding to panel (A); (C) Experimental TA surface plot for the visible region of the trilayer excited at 1000 nm; (D) Simulated TA surface plot from the concentration equations and associated spectra. The color bar to the right specifies intensities of the different signals. Specifically, the GSB for S22 at 575 nm, MoS2 A exciton at 660 nm and WSe2 A excitonat 740 nm can be identified. Color bar to the right specifies intensities of the different signals.

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    Figure 5

    Figure 5. Kinetic scheme highlighting the different time constants for hole transfer (τHT) to WSe2, electron transfer (τET) to MoS2 and the charge recombination lifetimes (τCR) following selective excitation of SWCNT at 1000 nm.

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  • References

    This article references 40 other publications.

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      A review. Increasing interest in using two-dimensional transition metal dichalcogenides (2D TMDCs) in optical energy conversion technologies creates a demand for improving the yields and lifetimes of photogenerated charge carriers. Despite inherently fast photocarrier decay in neat 2D TMDCs, the unique photophysics in these quantum-confined systems motivates continued effort to control the evolution of photoexcited states and create functional devices. An intriguing strategy to accomplish this goal is to employ TMDCs in heterojunctions with appropriate semiconductors, where energy level offsets drive photoinduced charge transfer (PCT) across material interfaces. PCT in TMDC-based systems can be optimized for many different applications, such as driving free carriers to photocatalytic sites for redox reactions like water splitting, extg. charge to perform work in photovoltaics or photodetectors, and manipulating the spin and momentum valley electronic degrees of freedom for quantum computing systems. Here, we review recent strides in optimizing PCT for such applications through greater fundamental understanding of the photophysics that occurs at TMDC/semiconductor interfaces. After giving an overview of isolated TMDC properties, synthetic methods, and the basics of PCT, we discuss TMDCs in heterojunctions with several classes of materials, including other TMDCs, small mol. semiconductors, polymers, single-walled carbon nanotubes, quantum dots, perovskites, and electrolytes. In addn. to highlighting the unique benefits of each materials category, we also identify parallels across common themes, such as the roles of charge-transfer states, spin, electronic coupling, delocalization, interfacial at. morphol., and the precise design of energy landscapes to direct charge and energy motion. We hope to capture a broad range of the valuable work in this fast-paced field to inspire new research directions for employing PCT in targeted TMDC-based systems.
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      Sulas-Kern, D. B.; Zhang, H.; Li, Z.; Blackburn, J. L. Microsecond charge separation at heterojunctions between transition metal dichalcogenide monolayers and single-walled carbon nanotubes. Mater. Horiz. 2019, 6, 2103– 2111,  DOI: 10.1039/C9MH00954J
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      Microsecond charge separation at heterojunctions between transition metal dichalcogenide monolayers and single-walled carbon nanotubes
      Sulas-Kern, Dana B.; Zhang, Hanyu; Li, Zhaodong; Blackburn, Jeffrey L.
      Materials Horizons (2019), 6 (10), 2103-2111CODEN: MHAOBM; ISSN:2051-6355. (Royal Society of Chemistry)
      The use of monolayer transition metal dichalcogenides (TMDCs) for optical-to-elec. or optical-to-chem. energy conversion can be limited by the ultrafast excited state relaxation inherent to neat monolayers. Photoinduced charge sepn. at nanoscale heterojunctions is an important strategy to extend carrier lifetimes, enabling photodetectors, solar cells, and solar fuel prodn. with these ultrathin materials. We demonstrate TMDC/single-walled carbon nanotube (SWCNT) heterojunctions with exceptionally long, microsecond timescale, charge sepn. following sub-picosecond interfacial charge transfer. These carrier lifetimes are orders of magnitude longer-lived than in other monolayer TMDC heterojunctions. We further present two unique methodologies for estg. charge-transfer quantum yields in MoS2 that can be broadly applied and refined for other TMDC systems. Our results highlight the promise of TMDC/SWCNT heterojunctions for advanced (photo)catalytic and optoelectronic systems and devices.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtlGrsL3L&md5=b97d1f4a00eb7a25d5a38fd8b51da0be
    6. 6
      Matsunaga, R.; Matsuda, K.; Kanemitsu, Y. Observation of charged excitons in hole-doped carbon nanotubes using photoluminescence and absorption spectroscopy. Phys. Rev. Lett. 2011, 106, 1– 4,  DOI: 10.1103/PhysRevLett.106.037404
      There is no corresponding record for this reference.
    7. 7
      Ahmed, S.; Yi, J. Two-dimensional transition metal dichalcogenides and their charge carrier mobilities in field-effect transistors. Nano-Micro Lett. 2017, 9, 1– 23,  DOI: 10.1007/s40820-017-0152-6
      There is no corresponding record for this reference.
    8. 8
      Pant, A.; Mutlu, Z.; Wickramaratne, D.; Cai, H.; Lake, R. K.; Ozkan, C.; Tongay, S. Fundamentals of lateral and vertical heterojunctions of atomically thin materials. Nanoscale 2016, 8, 3870– 3887,  DOI: 10.1039/C5NR08982D
      8
      Fundamentals of lateral and vertical heterojunctions of atomically thin materials
      Pant, Anupum; Mutlu, Zafer; Wickramaratne, Darshana; Cai, Hui; Lake, Roger K.; Ozkan, Cengiz; Tongay, Sefaattin
      Nanoscale (2016), 8 (7), 3870-3887CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      At the turn of this century, Herbert Kroemer, the 2000 Nobel Prize winner in Physics, famously commented that "the interface is the device". This statement has since opened up unparalleled opportunities at the interface of conventional three-dimensional (3D) materials (H. Kroemer, Quasi-Elec. and Quasi-Magnetic Fields in Non-Uniform Semiconductors, RCA Rev., 1957, 18, 332-342). More than a decade later, Sir Andre Geim and Irina Grigorieva presented their views on 2D heterojunctions which further cultivated broad interests in the 2D materials field. Currently, advances in two-dimensional (2D) materials enable us to deposit layered materials that are only one or few unit-cells in thickness to construct sharp in-plane and out-of-plane interfaces between dissimilar materials, and to be able to fabricate novel devices using these cutting-edge techniques. The interface alone, which traditionally dominated overall device performance, thus has now become the device itself. Fueled by recent progress in atomically thin materials, we are now at the ultimate limit of interface physics, which brings to us new and exciting opportunities, with equally demanding challenges. This paper endeavors to provide stalwarts and newcomers a perspective on recent advances in synthesis, fundamentals, applications, and future prospects of a large variety of heterojunctions of atomically thin materials.
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    9. 9
      Ihly, R.; Mistry, K. S.; Ferguson, A. J.; Clikeman, T. T.; Larson, B. W.; Reid, O.; Boltalina, O. V.; Strauss, S. H.; Rumbles, G.; Blackburn, J. L. Tuning the driving force for exciton dissociation in single-walled carbon nanotube heterojunctions. Nat. Chem. 2016, 8, 603– 609,  DOI: 10.1038/nchem.2496
      9
      Tuning the driving force for exciton dissociation in single-walled carbon nanotube heterojunctions
      Ihly, Rachelle; Mistry, Kevin S.; Ferguson, Andrew J.; Clikeman, Tyler T.; Larson, Bryon W.; Reid, Obadiah; Boltalina, Olga V.; Strauss, Steven H.; Rumbles, Garry; Blackburn, Jeffrey L.
      Nature Chemistry (2016), 8 (6), 603-609CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
      Understanding the kinetics and energetics of interfacial electron transfer in mol. systems is crucial for the development of a broad array of technologies, including photovoltaics, solar fuel systems, and energy storage. The Marcus formulation for electron transfer relates the thermodn. driving force and reorganization energy for charge transfer between a given donor/acceptor pair to the kinetics and yield of electron transfer. Here the effect is investigated of the thermodn. driving force for photoinduced electron transfer (PET) between single-walled carbon nanotubes (SWCNTs) and fullerene derivs. by employing time-resolved microwave cond. as a sensitive probe of interfacial exciton dissocn. For the first time, the Marcus inverted region (in which driving force exceeds reorganization energy) is obsd. and the reorganization energy is quantified for PET for a model SWCNT/acceptor system. The small reorganization energies (∼ 130 meV, most of which probably arises from the fullerene acceptors) are beneficial in minimizing energy loss in photoconversion schemes.
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    10. 10
      Zorn, N. F.; Zaumseil, J. Charge transport in semiconducting carbon nanotube networks. Appl. Phys. Rev. 2021, 8, 041318  DOI: 10.1063/5.0065730
      10
      Charge transport in semiconducting carbon nanotube networks
      Zorn, Nicolas F.; Zaumseil, Jana
      Applied Physics Reviews (2021), 8 (4), 041318CODEN: APRPG5; ISSN:1931-9401. (American Institute of Physics)
      Efficient and controlled charge transport in networks of semiconducting single-walled carbon nanotubes is the basis for their application in electronic devices, esp. in field-effect transistors and thermoelecs. The recent advances in selective growth, purifn., and sorting of semiconducting and even monochiral carbon nanotubes have enabled field-effect transistors with high carrier mobilities and on/off current ratios that were impossible a few years ago. They have also allowed researchers to examine the microscopic interplay of parameters such as nanotube length, d., diam. distribution, carrier d., intentional and unintentional defects, dielec. environment, etc., and their impact on the macroscopic charge transport properties in a rational and reproducible manner. This review discusses various models that are considered for charge transport in nanotube networks and the exptl. methods to characterize and investigate transport beyond simple cond. or transistor measurements. Static and dynamic absorption, photoluminescence and electroluminescence spectroscopy, as well as scanning probe techniques (e.g., conductive at. force microscopy, Kelvin probe force microscopy), and their unique insights in the distribution of charge carriers in a given nanotube network and the resulting current pathways will be introduced. Finally, recommendations for further optimization of nanotube network devices and a list of remaining challenges are provided. (c) 2021 American Institute of Physics.
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    11. 11
      van der Donck, M.; Zarenia, M.; Peeters, F. M. Excitons, trions, and biexcitons in transition-metal dichalcogenides: Magnetic-field dependence. Phys. Rev. B 2018, 97, 1– 11,  DOI: 10.1103/PhysRevB.97.195408
      There is no corresponding record for this reference.
    12. 12
      Mueller, T.; Malic, E. Exciton physics and device application of two-dimensional transition metal dichalcogenide semiconductors. Npj 2D Mater. Appl. 2018, 2, 1– 12,  DOI: 10.1038/s41699-018-0074-2
      12
      Exciton physics and device application of two-dimensional transition metal dichalcogenide semiconductors
      Mueller, Thomas; Malic, Ermin
      npj 2D Materials and Applications (2018), 2 (1), 1-12CODEN: DMAAAH; ISSN:2397-7132. (Nature Research)
      A review. Two-dimensional group-VI transition metal dichalcogenide semiconductors, such as MoS2, WSe2, and others, exhibit strong light-matter coupling and possess direct band gaps in the IR and visible spectral regimes, making them potentially interesting candidates for various applications in optics and optoelectronics. Here, we review their optical and optoelectronic properties with emphasis on exciton physics and devices. As excitons are tightly bound in these materials and dominate the optical response even at room-temp., their properties are examd. in depth in the first part of this article. We discuss the remarkably versatile excitonic landscape, including bright, dark, localized and interlayer excitons. In the second part, we provide an overview on the progress in optoelectronic device applications, such as elec. driven light emitters, photovoltaic solar cells, photodetectors, and opto-valleytronic devices, again bearing in mind the prominent role of excitonic effects. We conclude with a brief discussion on challenges that remain to be addressed to exploit the full potential of transition metal dichalcogenide semiconductors in possible exciton-based applications.
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    13. 13
      Li, Z.; Attanayake, N. H.; Blackburn, J. L.; Miller, E. M. Carbon dioxide and nitrogen reduction reactions using 2D transition metal dichalcogenide (TMDC) and carbide/nitride (MXene) catalysts. Energy Environ. Sci. 2021, 14, 6242– 6286,  DOI: 10.1039/D1EE03211A
      13
      Carbon dioxide and nitrogen reduction reactions using 2D transition metal dichalcogenide (TMDC) and carbide/nitride (MXene) catalysts
      Li, Zhaodong; Attanayake, Nuwan H.; Blackburn, Jeffrey L.; Miller, Elisa M.
      Energy & Environmental Science (2021), 14 (12), 6242-6286CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      A review. Improving the carbon dioxide and nitrogen redn. reactions (CO2RR and NRR) can reduce anthropogenic greenhouse gas emissions while selectively producing chems. needed for the fuel, plastic, and chem. industries. Efficient CO2RR can be used to replace fossil fuels as well as repurpose captured CO2, while new NRR pathways can be used to supplement or replace the energy intensive Haber-Bosch process for NH3 generation with no CO2 emissions. Therefore, this article focuses on (photo)electrocatalytic and photocatalytic conversion of CO2 and N2 mols. into useful products, such as carbon monoxide, methanol, formic acid, and ammonia, using 2D transition metal dichalcogenides (TMDCs) and metal carbides/nitrides (MXenes). These highly tunable 2D catalysts will be evaluated for their ability to selectively and efficiently undergo CO2RR and NRR by controlling defects, phases, edge sites, interfaces, and functional groups. We first address the CO2RR and NRR challenges, with a particular focus on theor. mechanisms and min. energy pathways. We follow this discussion with a detailed of state-of-the-art 2D TMDC and MXene exptl. catalysts for CO2RR and NRR (photo)electrocatalytic and photocatalytic reactions, and then address areas of opportunity for these catalytic reactions.
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    14. 14
      Kafle, T. R.; Kattel, B.; Yao, P.; Zereshki, P.; Zhao, H.; Chan, W.-L. Effect of the interfacial energy landscape on photoinduced charge generation at the ZnPc/MoS2 interface. J. Am. Chem. Soc. 2019, 141, 11328– 11336,  DOI: 10.1021/jacs.9b05893
      14
      Effect of the Interfacial Energy Landscape on Photoinduced Charge Generation at the ZnPc/MoS2 Interface
      Kafle, Tika R.; Kattel, Bhupal; Yao, Peng; Zereshki, Peymon; Zhao, Hui; Chan, Wai-Lun
      Journal of the American Chemical Society (2019), 141 (28), 11328-11336CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Time-resolved and steady-state photoemission spectroscopy are used to study the potential energy landscape, charge transfer, and exciton dynamics at the Zn phthalocyanine (ZnPc)/monolayer (ML) MoS2 and ZnPc/bulk MoS2 interfaces. Although both interfaces have a type-II band alignment and exhibit sub-100 fs CT, the CT excitons formed at the 2 interfaces show drastically different evolution dynamics. The ZnPc/ML-MoS2 behaves like typical donor-acceptor interfaces in which CT excitons dissoc. into electron-hole pairs. Back electron transfer occur at ZnPc/bulk-MoS2, which gave triplet excitons in ZnPc. The difference can be explained by the different amt. of band bending found in the ZnPc film deposited on ML-MoS2 and bulk-MoS2. The potential energy landscape near the interface plays an important role in the charge sepn. behavior. Considering the energy level alignment at the interface alone is not enough for predicting whether free charges can be generated effectively from an interface.
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    15. 15
      Park, S.; Mutz, N.; Schultz, T.; Blumstengel, S.; Han, A.; Aljarb, A.; Li, L.-J.; List-Kratochvil, E. J. W.; Amsalem, P.; Koch, N. Lain-Jong Direct determination of monolayer MoS2 and WSe2 exciton binding energies on insulating and metallic substrates. 2D Mater. 2018, 5, 025003  DOI: 10.1088/2053-1583/aaa4ca
      15
      Direct determination of monolayer MoS2 and WSe2 exciton binding energies on insulating and metallic substrates
      Park, Soohyung; Mutz, Niklas; Schultz, Thorsten; Blumstengel, Sylke; Han, Ali; Aljarb, Areej; Li, Lain-Jong; List-Kratochvil, Emil J. W.; Amsalem, Patrick; Koch, Norbert
      2D Materials (2018), 5 (2), 025003/1-025003/8CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      Understanding the excitonic nature of excited states in two-dimensional (2D) transition-metal dichalcogenides (TMDCs) is of key importance to make use of their optical and charge transport properties in optoelectronic applications. We contribute to this by the direct exptl. detn. of the exciton binding energy (Eb,exc) of monolayer MoS2 and WSe2 on two fundamentally different substrates, i.e. the insulator sapphire and the metal gold. By combining angle-resolved direct and inverse photoelectron spectroscopy we measure the electronic band gap (Eg), and by reflectance measurements the optical excitonic band gap (Eexc). The difference of these two energies is Eb,exc. The values of Eg and Eb,exc are 2.11 eV and 240 meV for MoS2 on sapphire, and 1.89 eV and 240 meV for WSe2 on sapphire. On Au Eb,exc is decreased to 90 meV and 140 meV for MoS2 and WSe2, resp. The significant Eb,exc redn. is primarily due to a redn. of Eg resulting from enhanced screening by the metal, while Eexc is barely decreased for the metal support. Energy level diagrams detd. at the K-point of the 2D TMDCs Brillouin zone show that MoS2 has more p-type character on Au as compared to sapphire, while WSe2 appears close to intrinsic on both. These results demonstrate that the impact of the dielec. environment of 2D TMDCs is more pronounced for individual charge carriers than for a correlated electron-hole pair, i.e. the exciton.
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    16. 16
      Kang, H. S.; Sisto, T. J.; Peurifoy, S.; Arias, D. H.; Zhang, B.; Nuckolls, C.; Blackburn, J. L. Long-Lived Charge Separation at Heterojunctions between Semiconducting Single-Walled Carbon Nanotubes and Perylene Diimide Electron Acceptors. J. Phys. Chem. C 2018, 122, 14150– 14161,  DOI: 10.1021/acs.jpcc.8b01400
      16
      Long-Lived Charge Separation at Heterojunctions between Semiconducting Single-Walled Carbon Nanotubes and Perylene Diimide Electron Acceptors
      Kang, Hyun Suk; Sisto, Thomas J.; Peurifoy, Samuel; Arias, Dylan H.; Zhang, Boyuan; Nuckolls, Colin; Blackburn, Jeffrey L.
      Journal of Physical Chemistry C (2018), 122 (25), 14150-14161CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Nonfullerene electron acceptors have facilitated a recent surge in the efficiencies of org. solar cells, although fundamental studies of the nature of exciton dissocn. at interfaces with nonfullerene electron acceptors are still relatively sparse. Semiconducting single-walled carbon nanotubes (s-SWCNTs), unique 1-dimensional electron donors with mol.-like absorption and highly mobile charges, provide a model system for studying interfacial exciton dissocn. Here, the authors study excited-state photodynamics at the heterojunction between (6,5) s-SWCNTs and two perylene diimide (PDI)-based electron acceptors. Each of the PDI-based acceptors, hPDI2-pyr-hPDI2 and Trip-hPDI2, is deposited onto (6,5) s-SWCNT films to form a heterojunction bilayer. Transient absorption measurements demonstrate that photoinduced hole/electron transfer occurs at the photoexcited bilayer interfaces, producing long-lived sepd. charges with lifetimes exceeding 1.0 μs. Both exciton dissocn. and charge recombination occur more slowly for the hPDI2-pyr-hPDI2 bilayer than for the Trip-hPDI2 bilayer. To explain such differences, the potential roles of the thermodn. charge transfer driving force available at each interface and the different mol. structure and intermol. interactions of PDI-based acceptors are discussed. Detailed photophys. anal. of these model systems can develop the fundamental understanding of exciton dissocn. between org. electron donors and nonfullerene acceptors, which was not systematically studied.
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    17. 17
      Homan, S. B.; Sangwan, V. K.; Balla, I.; Bergeron, H.; Weiss, E. A.; Hersam, M. C. Ultrafast exciton dissociation and long-lived charge separation in a photovoltaic pentacene-MoS2 van der Waals heterojunction. Nano Lett. 2017, 17, 164– 169,  DOI: 10.1021/acs.nanolett.6b03704
      There is no corresponding record for this reference.
    18. 18
      Mirkovic, T.; Ostroumov, E. E.; Anna, J. M.; van Grondelle, R.; Govindjee; Scholes, G. D. Light absorption and energy transfer in the antenna complexes of photosynthetic organisms. Chem. Rev. 2017, 117, 249– 293,  DOI: 10.1021/acs.chemrev.6b00002
      18
      Light absorption and energy transfer in the antenna complexes of photosynthetic organisms
      Mirkovic, Tihana; Ostroumov, Evgeny E.; Anna, Jessica M.; van Grondelle, Rienk; Govindjee; Scholes, Gregory D.
      Chemical Reviews (Washington, DC, United States) (2017), 117 (2), 249-293CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. The process of photosynthesis is initiated by the capture of sunlight by a network of light-absorbing mols. (chromophores), which are also responsible for the subsequent funneling of the excitation energy to the reaction centers. Through evolution, genetic drift, and speciation, photosynthetic organisms have discovered many solns. for light harvesting. In this review, we describe the underlying photophys. principles by which this energy is absorbed, as well as the mechanisms of electronic excitation energy transfer (EET). First, optical properties of the individual pigment chromophores present in light-harvesting antenna complexes are introduced, and then we examine the collective behavior of pigment-pigment and pigment-protein interactions. The description of energy transfer, in particular multichromophoric antenna structures, is shown to vary depending on the spatial and energetic landscape, which dictates the relative coupling strength between constituent pigment mols. In the latter half of the article, we focus on the light-harvesting complexes of purple bacteria as a model to illustrate the present understanding of the synergetic effects leading to EET optimization of light-harvesting antenna systems while exploring the structure and function of the integral chromophores. We end this review with a brief overview of the energy-transfer dynamics and pathways in the light-harvesting antennas of various photosynthetic organisms.
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    19. 19
      Ceballos, F.; Ju, M.-G.; Lane, S. D.; Zeng, X. C.; Zhao, H. Highly Efficient and Anomalous Charge Transfer in van der Waals Trilayer Semiconductors. Nano Lett. 2017, 17, 1623– 1628,  DOI: 10.1021/acs.nanolett.6b04815
      19
      Highly Efficient and Anomalous Charge Transfer in van der Waals Trilayer Semiconductors
      Ceballos, Frank; Ju, Ming-Gang; Lane, Samuel D.; Zeng, Xiao Cheng; Zhao, Hui
      Nano Letters (2017), 17 (3), 1623-1628CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Two-dimensional materials, such as graphene and monolayer transition metal dichalcogenides, allow the fabrication of multilayer structures without lattice matching restriction. A central issue in developing such artificial materials is to understand and control the interlayer electron transfer process, which plays a key role in harnessing their emergent properties. Recent photoluminescence and transient absorption measurements revealed that the electron transfer in heterobilayers occurs on ultrafast time scales. However, there is still a lack of fundamental understanding on how this process can be so efficient at van der Waals interfaces. Here the authors show evidence suggesting the coherent nature of such interlayer electron transfer. In a trilayer of MoS2-WS2-MoSe2, electrons excited in MoSe2 transfer to MoS2 in about one picosecond. Surprisingly, these electrons do not populate the middle WS2 layer during this process. Calcns. showed the coherent nature of the charge transfer and reproduced the measured electron transfer time. The hole transfer from MoS2 to MoSe2 also is efficient and ultrafast. The sepn. of electrons and holes extends their lifetimes to more than one nanosecond, suggesting potential applications of such multilayer structures in optoelectronics.
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    20. 20
      Zereshki, P.; Wei, Y.; Long, R.; Zhao, H. Layer-Coupled States Facilitate Ultrafast Charge Transfer in a Transition Metal Dichalcogenide Trilayer Heterostructure. J. Phys. Chem. Lett. 2018, 9, 5970– 5978,  DOI: 10.1021/acs.jpclett.8b02622
      20
      Layer-Coupled States Facilitate Ultrafast Charge Transfer in a Transition Metal Dichalcogenide Trilayer Heterostructure
      Zereshki, Peymon; Wei, Yaqing; Long, Run; Zhao, Hui
      Journal of Physical Chemistry Letters (2018), 9 (20), 5970-5978CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Forming van der Waals multilayer structures with two-dimensional materials is a promising new method for material discovery. The weak van der Waals interlayer interaction without at. correspondence relaxes lattice matching requirement and allows formation of high-quality interfaces with virtually any combination of two-dimensional materials. However, the weak nature of the van der Waals interaction also makes it challenging to harness emergent properties of such multilayer materials. Previous studies have indicated that in transition metal dichalcogenide bilayer heterostructures, the interlayer charge and energy transfer is highly efficient. Therefore, it is important to understand interlayer coupling in these materials and its role on charge and energy transfer. Here we show that in a MoSe2/WSe2/WS2 trilayer, the interlayer coupling is strong enough to form layer-coupled states in the conduction band with the electron wave function extends to all three layers. D. functional theory calcns. reveal that the layer-coupled states in Q valley are about 0.1 eV below the individual monolayer states in K valley, which is consistent with photoluminescence measurements. Transient absorption measurements show that these layer-coupled states provide a channel for ultrafast interlayer charge transfer between the top WS2 and the bottom MoSe2 layers. In this process, electrons from the K valley of the individual monolayers are scattered to the layer-coupled states in Q valley. Such a partial charge transfer allows formation of partial-indirect excitons with the holes in one monolayer while electrons shared by three layers. The formation of layer-coupled states is promising for harnessing emergent properties of transition metal dichalcogenide multilayer heterostructures. Our findings also provide new ingredient to understand charge and energy transfer in transition metal dichalcogenide heterobilayers, as the layer-coupled states can play important roles in the efficient transfer obsd. in these systems.
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    21. 21
      Kim, J.; Jin, C.; Chen, B.; Cai, H.; Zhao, T.; Lee, P.; Kahn, S.; Watanabe, K.; Taniguchi, T.; Tongay, S.; Crommie, M. F.; Wang, F. Observation of Ultralong Valley Lifetime in WSe2/MoS2 Heterostrucures. Sci. Adv. 2017, 3, 1– 7,  DOI: 10.1126/sciadv.1700518
      There is no corresponding record for this reference.
    22. 22
      Wang, Z.; Sun, C.; Xu, X.; Liu, Y.; Chen, Z.; Yang, Y. â.; Zhu, H. Long-Range Hot Charge Transfer Exciton Dissociation in an Organic/2D Semiconductor Hybrid Excitonic Heterostructure. J. Am. Chem. Soc. 2023, 145, 11227– 11235,  DOI: 10.1021/jacs.3c01192
      22
      Long-Range Hot Charge Transfer Exciton Dissociation in an Organic/2D Semiconductor Hybrid Excitonic Heterostructure
      Wang, Zukun; Sun, Cheng; Xu, Xuehui; Liu, Yanping; Chen, Zeng; Yang, Yang "Michael"; Zhu, Haiming
      Journal of the American Chemical Society (2023), 145 (20), 11227-11235CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Whether and how an electron-hole pair at the donor-acceptor interface separates from their mutual Coulombic interaction has been a long-standing question for both fundamental interests and optoelectronic applications. This question is particularly interesting but yet to be unraveled in the emerging mixed-dimensional org./2D semiconductor excitonic heterostructures where the Coulomb interaction is poorly screened. Here, by tracking the characteristic electroabsorption (Stark effect) signal from sepd. charges using transient absorption spectroscopy, we directly follow the electron-hole pair sepn. process in a model org./2D heterostructure, vanadium oxide phthalocyanine/monolayer MoS2. After sub-100 fs photoinduced interfacial electron transfer, we observe a barrier-less long-range electron-hole pair sepn. to free carriers within 1 ps by hot charge transfer exciton dissocn. Further expt. reveals the key role of the charge delocalization in org. layers sustained by the local crystallinity, while the inherent in-plane delocalization of the 2D semiconductor has a negligible contribution to charge pair sepn. This study reconciles the seemingly contradicting charge transfer exciton emission and dissocn. process and is important to the future development of efficient org./2D semiconductor optoelectronic devices.
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    23. 23
      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– 686,  DOI: 10.1038/nnano.2014.167
      23
      Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures
      Hong, Xiaoping; Kim, Jonghwan; Shi, Su-Fei; Zhang, Yu; Jin, Chenhao; Sun, Yinghui; Tongay, Sefaattin; Wu, Junqiao; Zhang, Yanfeng; Wang, Feng
      Nature Nanotechnology (2014), 9 (9), 682-686CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Van der Waals heterostructures have recently emerged as a new class of materials, where quantum coupling between stacked atomically thin two-dimensional layers, including graphene, hexagonal-boron nitride and transition-metal dichalcogenides (MX2), give rise to fascinating new phenomena. MX2 heterostructures are particularly exciting for novel optoelectronic and photovoltaic applications, because two-dimensional MX2 monolayers can have an optical bandgap in the near-IR to visible spectral range and exhibit extremely strong light-matter interactions. Theory predicts that many stacked MX2 heterostructures form type II semiconductor heterojunctions that facilitate efficient electron-hole sepn. for light detection and harvesting. Here, the authors report the 1st exptl. observation of ultrafast charge transfer in photoexcited MoS2/WS2 heterostructures using both photoluminescence mapping and femtosecond pump-probe spectroscopy. Hole transfer from the MoS2 layer to the WS2 layer takes place within 50 fs after optical excitation, a remarkable rate for van der Waals coupled two-dimensional layers. Such ultrafast charge transfer in van der Waals heterostructures can enable novel two-dimensional devices for optoelectronics and light harvesting.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhsVSqtLfN&md5=50397cdfd0123a649f5dae13efa6bc70
    24. 24
      Guillot, S. L.; Mistry, K. S.; Avery, A. D.; Richard, J.; Dowgiallo, A.-M.; Ndione, P. F.; van de Lagemaat, J.; Reese, M. O.; Blackburn, J. L. Precision printing and optical modeling of ultrathin SWCNT/C60 heterojunction solar cells. Nanoscale 2015, 7, 6556– 6566,  DOI: 10.1039/C5NR00205B
      24
      Precision printing and optical modeling of ultrathin SWCNT/C60 heterojunction solar cells
      Guillot, Sarah L.; Mistry, Kevin S.; Avery, Azure D.; Richard, Jonah; Dowgiallo, Anne-Marie; Ndione, Paul F.; van de Lagemaat, Jao; Reese, Matthew O.; Blackburn, Jeffrey L.
      Nanoscale (2015), 7 (15), 6556-6566CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising candidates as the active layer in photovoltaics (PV), particularly for niche applications where high IR absorbance and/or semi-transparent solar cells are desirable. Most current fabrication strategies for SWCNT PV devices suffer from relatively high surface roughness and lack nanometer-scale deposition precision, both of which may hamper the reproducible prodn. of ultrathin devices. Addnl., detailed optical models of SWCNT PV devices are lacking, due in part to a lack of well-defined optical consts. for high-purity s-SWCNT thin films. Here, we present an optical model that accurately reconstructs the shape and magnitude of spectrally resolved external quantum efficiencies for ultrathin (7,5) s-SWCNT/C60 solar cells that are deposited by ultrasonic spraying. The ultrasonic spraying technique enables thickness tuning of the s-SWCNT layer with nanometer-scale precision, and consistently produces devices with low s-SWCNT film av. surface roughness (Rq of <5 nm). Our optical model, based entirely on measured optical consts. of each layer within the device stack, enables quant. predictions of thickness-dependent relative photocurrent contributions of SWCNTs and C60 and enables ests. of the exciton diffusion lengths within each layer. These results establish routes towards rational performance improvements and scalable fabrication processes for ultra-thin SWCNT-based solar cells.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXksVamsL4%253D&md5=26adee6fb034cf0d90a6342414ddf261
    25. 25
      Sulas-Kern, D. B.; Zhang, H.; Li, Z.; Blackburn, J. L. Interplay between microstructure, defect states, and mobile charge generation in transition metal dichalcogenide heterojunctions. Nanoscale 2021, 13, 8188– 8198,  DOI: 10.1039/D1NR00384D
      25
      Interplay between microstructure, defect states, and mobile charge generation in transition metal dichalcogenide heterojunctions
      Sulas-Kern, Dana B.; Zhang, Hanyu; Li, Zhaodong; Blackburn, Jeffrey L.
      Nanoscale (2021), 13 (17), 8188-8198CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Two-dimensional transition metal dichalcogenides (2D-TMDCs) have gained attention for their promise in next-generation energy-harvesting and quantum computing technologies, but realizing these technologies requires a greater understanding of TMDC properties that influence their photophysics. To this end, we discuss here the interplay between TMDC microstructure and defects with the charge generation yield, lifetime, and mobility. As a model system, we compare monolayer-only and monolayer-rich MoS2 grown by chem. vapor deposition, and we employ the TMDCs in Type-II charge-sepg. heterojunctions with semiconducting single-walled carbon nanotubes (s-SWCNTs). Our results suggest longer lifetimes and higher yields of mobile carriers in samples contg. a small fraction of defect-rich multilayer islands on predominately monolayer MoS2. Compared to the monolayer-only heterojunctions, the carrier lifetimes increase from 0.73μs to 4.71μs, the hole transfer yield increases from 23% to 34%, and the electron transfer yield increases from 39% to 59%. We reach these conclusions using a unique combination of microwave photocond. (which probes only mobile carriers) along with transient absorption spectroscopy (which identifies spectral signatures unique to each material and type of photoexcited quasiparticle, but does not probe mobility). Our results highlight the substantial changes in photophysics that can occur from small changes in TMDC microstructure and defect d., where the presence of defects does not necessarily preclude improvements in charge generation.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXosFKku7s%253D&md5=daf35cb5b0403e1647883af83b035064
    26. 26
      Zhao, W.; Ghorannevis, Z.; Amara, K. K.; Pang, J. R.; Toh, M.; Zhang, X.; Kloc, C.; Tan, P. H.; Eda, G. Lattice dynamics in mono- and few-layer sheets of WS2 and WSe2. Nanoscale 2013, 5, 9677– 9683,  DOI: 10.1039/c3nr03052k
      26
      Lattice dynamics in mono- and few-layer sheets of WS2 and WSe2
      Zhao, Weijie; Ghorannevis, Zohreh; Amara, Kiran Kumar; Pang, Jing Ren; Toh, Minglin; Zhang, Xin; Kloc, Christian; Tan, Ping Heng; Eda, Goki
      Nanoscale (2013), 5 (20), 9677-9683CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Thickness is one of the fundamental parameters that define the electronic, optical, and thermal properties of two-dimensional (2D) crystals. Phonons in Mo disulfide (MoS2) were recently found to exhibit unique thickness dependence due to the interplay between short and long range interactions. Here the authors report Raman spectra of atomically thin sheets of WS2 and WSe2, isoelectronic compds. of MoS2, in the mono- to few-layer thickness regime. Similar to the case of MoS2, the characteristic A1g and E2g1 modes exhibit stiffening and softening with increasing no. of layers, resp., with a small shift of <3 cm-1 due to large mass of the atoms. Thickness dependence is also obsd. in multiphonon bands arising from overtone, combination, and zone edge phonons, whose intensity exhibit significant enhancement in excitonic resonance conditions. Some of these multiphonon peaks are absent only in monolayers. These features provide a unique fingerprint and rapid identification for monolayer flakes.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhsFalsbfO&md5=e2bb5d7bfcd79b172f26b663d7122d92
    27. 27
      Zhang, X.; Qiao, X.-F.; Shi, W.; Wu, J.-B.; Jiang, D.-S.; Tan, P.-H. Phonon and Raman scattering of two-dimensional transition metal dichalcogenides from monolayer, multilayer to bulk material. Chem. Soc. Rev. 2015, 44, 2757– 2785,  DOI: 10.1039/C4CS00282B
      27
      Phonon and Raman scattering of two-dimensional transition metal dichalcogenides from monolayer, multilayer to bulk material
      Zhang, Xin; Qiao, Xiao-Fen; Shi, Wei; Wu, Jiang-Bin; Jiang, De-Sheng; Tan, Ping-Heng
      Chemical Society Reviews (2015), 44 (9), 2757-2785CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      Two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets exhibit remarkable electronic and optical properties. The 2D features, sizable bandgaps and recent advances in the synthesis, characterization and device fabrication of the representative MoS2, WS2, WSe2 and MoSe2 TMDs make TMDs very attractive in nanoelectronics and optoelectronics. Similar to graphite and graphene, the atoms within each layer in 2D TMDs are joined together by covalent bonds, while van der Waals interactions keep the layers together. This makes the phys. and chem. properties of 2D TMDs layer-dependent. In this review, we discuss the basic lattice vibrations of 2D TMDs from monolayer, multilayer to bulk material, including high-frequency optical phonons, interlayer shear and layer breathing phonons, the Raman selection rule, layer-no. evolution of phonons, multiple phonon replica and phonons at the edge of the Brillouin zone. The extensive capabilities of Raman spectroscopy in investigating the properties of TMDs are discussed, such as interlayer coupling, spin-orbit splitting and external perturbations. The interlayer vibrational modes are used in rapid and substrate-free characterization of the layer no. of multilayer TMDs and in probing interface coupling in TMD heterostructures. The success of Raman spectroscopy in investigating TMD nanosheets paves the way for expts. on other 2D crystals and related van der Waals heterostructures.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXislKju74%253D&md5=a3fce32cfaafd83fc4f0dbba89f90061
    28. 28
      Tonndorf, P.; Schmidt, R.; Böttger, P.; Zhang, X.; Börner, J.; Liebig, A.; Albrecht, M.; Kloc, C.; Gordan, O.; Zahn, D. R. T.; de Vasconcellos, S. M.; Bratschitsch, R. Photoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2. Opt. Express 2013, 21, 4908– 4916,  DOI: 10.1364/OE.21.004908
      28
      Photoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2
      Tonndorf, Philipp; Schmidt, Robert; Boettger, Philipp; Zhang, Xiao; Boerner, Janna; Liebig, Andreas; Albrecht, Manfred; Kloc, Christian; Gordan, Ovidiu; Zahn, Dietrich R. T.; de Vasconcellos, Steffen Michaelis; Bratschitsch, Rudolf
      Optics Express (2013), 21 (4), 4908-4916CODEN: OPEXFF; ISSN:1094-4087. (Optical Society of America)
      We mech. exfoliate mono- and few-layers of the transition metal dichalcogenides molybdenum disulfide, molybdenum diselenide, and tungsten diselenide. The exact no. of layers is unambiguously detd. by at. force microscopy and high-resoln. Raman spectroscopy. Strong photoluminescence emission is caused by the transition from an indirect band gap semiconductor of bulk material to a direct band gap semiconductor in atomically thin form.
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    29. 29
      McCreary, K. M.; Hanbicki, A. T.; Sivaram, S. V.; Jonker, B. T. A- and B-exciton photoluminescence intensity ratio as a measure of sample quality for transition metal dichalcogenide monolayers. APL Materials 2018, 6, na,  DOI: 10.1063/1.5053699
      There is no corresponding record for this reference.
    30. 30
      Dong, N.; Li, Y.; Feng, Y.; Zhang, S.; Zhang, X.; Chang, C.; Fan, J.; Zhang, L.; Wang, J. Optical limiting and theoretical modelling of layered transition metal dichalcogenide nanosheets. Opt. Express 2015, 5, 1– 10,  DOI: 10.1038/srep14646
      30
      Tunable lasing on silver island films by coupling to the localized surface plasmon
      Ning, Shuya; Wu, Zhaoxin; Dong, Hua; Yuan, Fang; Ma, Lin; Jiao, Bo; Hou, Xun
      Optical Materials Express (2015), 5 (3), 1-10CODEN: OMEPAX; ISSN:2159-3930. (Optical Society of America)
      Lasing of N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1':4',1''- terphenyl]-4,4''-diamine (BMT-TPD) films on silver island films (SIFs) was investigated. The size of silver nanoparticles (NPs) of SIFs ranged from 8 to 500 nm, which showed the different localized surface plasmon resonance (LSPR). It was found that the lasing wavelength of BMT-TPD was tuned by the LSPR peaks of silver NPs. This was attributed to the coupling between gain medium and plasmonic silver NPs, i.e., the surface plasmon amplification by stimulated emission of radiation which resulted in the lasing at the corresponding wavelengths. This is expected to be a new and easy approach for the tuning of wavelength of lasing.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXpvFyktb8%253D&md5=b342bfa668cb52596dd2366177e594f2
    31. 31
      Amani, M.; Taheri, P.; Addou, R.; Ahn, G. H.; Kiriya, D.; Lien, D.-H.; Ager, J. W.; Wallace, R. M.; Javey, A. Recombination Kinetics and Effects of Superacid Treatment in Sulfur- and Selenium-Based Transition Metal Dichalcogenides. Nano Lett. 2016, 16, 2786– 2791,  DOI: 10.1021/acs.nanolett.6b00536
      31
      Recombination Kinetics and Effects of Superacid Treatment in Sulfur- and Selenium-Based Transition Metal Dichalcogenides
      Amani, Matin; Taheri, Peyman; Addou, Rafik; Ahn, Geun Ho; Kiriya, Daisuke; Lien, Der-Hsien; Ager, Joel W.; Wallace, Robert M.; Javey, Ali
      Nano Letters (2016), 16 (4), 2786-2791CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Optoelectronic devices based on two-dimensional (2D) materials showed tremendous promise over the past few years; however, there are still numerous challenges that need to be overcome to enable their application in devices. These include improving their poor photoluminescence (PL) quantum yield (QY) as well as better understanding of exciton-based recombination kinetics. Recently, the authors developed a chem. treatment technique using an org. superacid, bis(trifluoromethane)sulfonimide (TFSI), which was shown to improve the quantum yield in MoS2 from <1% to over 95%. Here, the authors perform detailed steady-state and transient optical characterization on some of the most heavily studied direct bandgap 2-dimensional materials, specifically WS2, MoS2, WSe2, and MoSe2, over a large pump dynamic range to study the recombination mechanisms present in these materials. The authors then explore the effects of TFSI treatment on the PL QY and recombination kinetics for each case. Results suggest that sulfur-based 2-dimensional materials are amenable to repair/passivation by TFSI, while the mechanism is thus far ineffective on selenium based systems. Also biexcitonic recombination is the dominant nonradiative pathway in these materials and the kinetics for TFSI treated MoS2 and WS2 can be described using a simple two parameter model.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xkt1CgtrY%253D&md5=6226077ca663b3f186cc2a6346f3e1ea
    32. 32
      Gupta, R.; Singh, B. P.; Singh, V. N.; Gupta, T. K.; Mathur, R. B. Origin of radial breathing mode in multiwall carbon nanotubes synthesized by catalytic chemical vapor deposition. Carbon 2014, 66, 724– 726,  DOI: 10.1016/j.carbon.2013.08.057
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      Origin of radial breathing mode in multiwall carbon nanotubes synthesized by catalytic chemical vapor deposition
      Gupta, Ravi; Singh, Bhanu P.; Singh, Vidya N.; Gupta, Tejendra K.; Mathur, Rakesh B.
      Carbon (2014), 66 (), 724-726CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
      The origin of radial breathing mode (RBM) in the Raman spectra of multiwall carbon nanotubes (MWNCTs) is discussed. In general, RBM is characteristics of single wall carbon nanotube (SWCNT). With the help of transmission electron microscopy (TEM) and Raman spectroscopic studies, it is established that the presence of SWCNT in the cavity of MWCNT is responsible for the appearance of RBM in MWCNT (synthesized by low temp. catalytic chem. vapor deposition technique). The estd. diam. of 8.2 Å (from Raman study) of SWCNT is almost same as that obsd. (∼8.3 Å) in TEM studies.
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    33. 33
      Fang, N.; Chang, Y. R.; Fujii, S.; Yamashita, D.; Maruyama, M.; Gao, Y.; Fong, C. F.; Kozawa, D.; Otsuka, K.; Nagashio, K.; Okada, S.; Kato, Y. K. Room-temperature quantum emission from interface excitons in mixed-dimensional heterostructure. arXiv:2307.15399 [cond-mat.mes-hall] 2023, na,  DOI: 10.48550/arXiv.2307.15399
      There is no corresponding record for this reference.
    34. 34
      Ma, D.; Shi, J.; Ji, Q.; Chen, K.; Yin, J.; Lin, Y.; Zhang, Y.; Liu, M.; Feng, Q.; Song, X.; Guo, X.; Zhang, J.; Zhang, Y.; Liu, Z. A universal etching-free transfer of MoS2 films for applications in photodetectors. Nano Research 2015, 8, 3662– 3672,  DOI: 10.1007/s12274-015-0866-z
      34
      A universal etching-free transfer of MoS2 films for applications in photodetectors
      Ma, Donglin; Shi, Jianping; Ji, Qingqing; Chen, Ke; Yin, Jianbo; Lin, Yuanwei; Zhang, Yu; Liu, Mengxi; Feng, Qingliang; Song, Xiuju; Guo, Xuefeng; Zhang, Jin; Zhang, Yanfeng; Liu, Zhongfan
      Nano Research (2015), 8 (11), 3662-3672CODEN: NRAEB5; ISSN:1998-0000. (Springer GmbH)
      Transferring MoS2 films from growth substrates onto target substrates is a crit. issue for their practical applications. Moreover, it remains a great challenge to avoid sample degrdn. and substrate destruction, because the current transfer method inevitably employs a wet chem. etching process. We developed an etching-free transfer method for transferring MoS2 films onto arbitrary substrates by using ultrasonication. Briefly, the collapse of ultrasonication-generated microbubbles at the interface between polymer-coated MoS2 film and substrates induce sufficient force to delaminate the MoS2 films. Using this method, the MoS2 films can be transferred from all substrates (silica, mica, strontium titanate, and sapphire) and retains the original sample morphol. and quality. This method guarantees a simple transfer process and allows the reuse of growth substrates, without involving any hazardous etchants. The etching-free transfer method is likely to promote broad applications of MoS2 in photodetectors. [Figure not available: see fulltext.].
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhs12qs7zJ&md5=a964c933fccb39943d7c0cd707209fc2
    35. 35
      Rose, A. H.; Dunklin, J. R.; Zhang, H.; Merlo, J. M.; van de Lagemaat, J. A universal etching-free transfer of MoS2 films for applications in photodetectors. ACS Photonics 2020, 7, 1129– 1134,  DOI: 10.1021/acsphotonics.0c00233
      35
      Plasmon-Mediated Coherent Superposition of Discrete Excitons under Strong Exciton-Plasmon Coupling in Few-Layer MoS2 at Room Temperature
      Rose, Aaron H.; Dunklin, Jeremy R.; Zhang, Hanyu; Merlo, Juan M.; van de Lagemaat, Jao
      ACS Photonics (2020), 7 (5), 1129-1134CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      We demonstrate room temp. coherent hybridization of the A- and B-excitons in few-layer MoS2, mediated by simultaneous strong coupling to surface plasmon polaritons. Few-layer MoS2 was placed on a tunable plasmonic structure and the system's dispersion was measured by tuning the plasmon energy across the exciton energies. Strong coupling was obsd. as double Rabi splitting at the A- and B-excitons of 81 and 93 meV, resp. A coupled harmonic oscillator model sheds light on the nature of the interaction, revealing a quantum superposition of the A- and B-excitons, mediated by the plasmon interaction. This observation suggests the possibility of room temp. intra- or intervalley quantum information transport and/or spin entanglement. The expt. confirms a previous theor. prediction of room temp. exciton-exciton hybridization in two-dimensional MoS2. Further, through modeling we find that room temp. strong coupling is a general phenomenon among two-dimensional transition metal dichalcogenide exciton-plasmon systems.
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    36. 36
      Xu, Z.-Q.; Zhang, Y.; Lin, S.; Zheng, C.; Zhong, Y. L.; Xia, X.; Li, Z.; Sophia, P. J.; Fuhrer, M. S.; Cheng, Y.-B.; Bao, Q. Synthesis and Transfer of Large-Area Monolayer WS2 Crystals: Toward the Recyclable Use of Sapphire Substrates. ACS Nano 2015, 9, 6178– 6187,  DOI: 10.1021/acsnano.5b01480
      36
      Synthesis and Transfer of Large-Area Monolayer WS2 Crystals: Moving Toward the Recyclable Use of Sapphire Substrates
      Xu, Zai-Quan; Zhang, Yupeng; Lin, Shenghuang; Zheng, Changxi; Zhong, Yu Lin; Xia, Xue; Li, Zhipeng; Sophia, Ponraj Joice; Fuhrer, Michael S.; Cheng, Yi-Bing; Bao, Qiaoliang
      ACS Nano (2015), 9 (6), 6178-6187CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Two-dimensional layered transition metal dichalcogenides (TMDs) show intriguing potential for optoelectronic devices due to their exotic electronic and optical properties. Only a few efforts were dedicated to large-area growth of TMDs. Practical applications will require improving the efficiency and reducing the cost of prodn., through (1) new growth methods to produce large size TMD monolayer with less-stringent conditions, and (2) nondestructive transfer techniques that enable multiple reuse of growth substrate. The authors report to employ atm. pressure CVD (APCVD) for the synthesis of large size (>100 μm) single crystals of atomically thin tungsten disulfide (WS2), a member of TMD family, on sapphire substrate. More importantly, the authors demonstrate a polystyrene (PS) mediated delamination process via capillary force in water which reduces the etching time in base soln. and imposes only minor damage to the sapphire substrate. The transferred WS2 flakes are of excellent continuity and exhibit comparable electron mobility after several growth cycles on the reused sapphire substrate. The photoluminescence emission from WS2 grown on the recycled sapphire is much higher than that on fresh sapphire, possibly due to p-type doping of monolayer WS2 flakes by a thin layer of water intercalated at the at. steps of the recycled sapphire substrate. The growth and transfer techniques described here are expected to be applicable to other atomically thin TMD materials.
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    37. 37
      Dowgiallo, A.-M.; Mistry, K. S.; Johnson, J. C.; Blackburn, J. L. Ultrafast Spectroscopic Signature of Charge Transfer between Single-Walled Carbon Nanotubes and C60. ACS Nano 2014, 8, 8573– 8581,  DOI: 10.1021/nn503271k
      37
      Ultrafast Spectroscopic Signature of Charge Transfer between Single-Walled Carbon Nanotubes and C60
      Dowgiallo, Anne-Marie; Mistry, Kevin S.; Johnson, Justin C.; Blackburn, Jeffrey L.
      ACS Nano (2014), 8 (8), 8573-8581CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The time scales for interfacial charge sepn. and recombination play crucial roles in detg. efficiencies of excitonic photovoltaics. Near-IR photons are harvested efficiently by semiconducting single-walled carbon nanotubes (SWCNTs) paired with appropriate electron acceptors, such as fullerenes (e.g., C60). However, little is known about crucial photochem. events that occur on femtosecond to nanosecond time scales at such heterojunctions. Here, we present transient absorbance measurements that utilize a distinct spectroscopic signature of charges within SWCNTs, the absorbance of a trion quasiparticle, to measure both the ultrafast photoinduced electron transfer time (τpet) and yield (.vphi.pet) in photoexcited SWCNT-C60 bilayer films. The rise time of the trion-induced absorbance enables the detn. of the photoinduced electron transfer (PET) time of τpet ≤ 120 fs, while an exptl. detd. trion absorbance cross section reveals the yield of charge transfer (.vphi.pet ≈ 38 ± 3%). The extremely fast electron transfer times obsd. here are on par with some of the best donor:acceptor pairs in excitonic photovoltaics and underscore the potential for efficient energy harvesting in SWCNT-based devices.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtFKms7rI&md5=a3f7be6f8713a7a82a7fcf4b00f21168
    38. 38
      Jaumot, J.; Gargallo, R.; de Juan, A.; Tauler, R. A graphical user-friendly interface for MCR-ALS: a new tool for multivariate curve resolution in MATLAB. Chemometr. Intell. Lab. Syst. 2005, 76, 101– 110,  DOI: 10.1016/j.chemolab.2004.12.007
      38
      A graphical user-friendly interface for MCR-ALS: a new tool for multivariate curve resolution in MATLAB
      Jaumot, Joaquim; Gargallo, Raimundo; de Juan, Anna; Tauler, Roma
      Chemometrics and Intelligent Laboratory Systems (2005), 76 (1), 101-110CODEN: CILSEN; ISSN:0169-7439. (Elsevier B.V.)
      A new graphical user-friendly interface for Multivariate Curve Resoln. using Alternating Least Squares has been developed as a freely available MATLAB toolbox. Through the use of this new easy-to-use graphical interface, the selection of the type of data anal. (either individual expts. giving a single data matrix or the more powerful simultaneous anal. of several expts. using one or more techniques) and the selection of the appropriate constraints can be performed in an intuitive and easy way, with the help of the options in the graphical interface. Different examples of use of this interface are given.
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    39. 39
      Jaumot, J.; de Juan, A.; Tauler, R. MCR-ALS GUI 2.0: New features and applications. Chemometr. Intell. Lab. Syst. 2015, 140, 1– 12,  DOI: 10.1016/j.chemolab.2014.10.003
      There is no corresponding record for this reference.
    40. 40
      Zhang, H.; Dunklin, J. R.; Reid, O. G.; Yun, S. J.; Nanayakkara, S. U.; Lee, Y. H.; Blackburn, J. L.; Miller, E. M. Disentangling oxygen and water vapor effects on optoelectronic properties of monolayer tungsten disulfide. Nanoscale 2020, 12, 8344– 8354,  DOI: 10.1039/C9NR09326E
      40
      Disentangling oxygen and water vapor effects on optoelectronic properties of monolayer tungsten disulfide
      Zhang, Hanyu; Dunklin, Jeremy R.; Reid, Obadiah G.; Yun, Seok Joon; Nanayakkara, Sanjini U.; Lee, Young Hee; Blackburn, Jeffrey L.; Miller, Elisa M.
      Nanoscale (2020), 12 (15), 8344-8354CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      By understanding how the environmental compn. impacts the optoelectronic properties of transition metal dichalcogenide monolayers, we demonstrate that simple photoluminescence (PL) measurements of tungsten disulfide (WS2) monolayers can differentiate relative humidity environments. In this paper, we examine the PL and photocond. of chem. vapor deposition grown WS2 monolayers under three carefully controlled environments: inert gas (N2), dry air (O2 in N2), and humid nitrogen (H2O vapor in N2). The WS2 PL is measured as a function of 532 nm laser power and exposure time and can be decompd. into the exciton, trion, and lower energy state(s) contributions. Under continuous illumination in either O2 or H2O vapor environment, we find dramatic (and reversible) increases in PL intensity relative to the PL in an inert environment. The PL bathochromically shifts in an O2 environment and is dominated by increased trion emission and diminished exciton emission. In contrast, the WS2 PL increase in a H2O environment results from an overall increase in emission from all spectral components where the exciton contribution dominates. The drastic increases in PL are anticorrelated with corresponding decreases in photocond., as measured by time-resolved microwave cond. The results suggest that both O2 and H2O react photochem. with the WS2 monolayer surface, modifying the optoelectronic properties, but do so via distinct pathways. Thus, we use these optoelectronic differences to differentiate the amt. of humidity in the air, which we show with 0%, 40%, and 80% relative humidity environments. This deeper understanding of how ambient conditions impact WS2 monolayers enables novel humidity sensors as well as a better understanding of the correlation between TMDC surface chem., light emission, and photocond. Moreover, these WS2 measurements highlight the importance of considering the impact of the local environment on reported results.
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