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Cite This: J. Phys. Chem. Lett. 2019, 10, 3750−3755
Real-Time Observing Ultrafast Carrier and Phonon Dynamics in Colloidal Tin Chalcogenide van der Waals Nanosheets Xufeng Li,† Nannan Luo,‡ Yuzhong Chen,† Xiaolong Zou,‡ and Haiming Zhu*,†,§ †
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Centre for Chemistry of High-Performance & Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang 310027, China ‡ The Low-Dimensional Materials and Devices Laboratory, Tsinghua-Berkeley Shenzhen Institute, Tsinghua University, Shenzhen, Guangdong 518055, China § State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou, Zhejiang 310027, China S Supporting Information *
ABSTRACT: Because of their earth-abundant, low-cost, and environmentally benign characteristics, two-dimensional (2D) group IV metal chalcogenides (e.g., SnSe2) with layered structures have shown great potential in optoelectronic, photovoltaic, and thermoelectric applications. However, the intrinsic motion of excited carriers and their coupling with lattice photons, which fundamentally dictates device operation and optimization, remain yet to be unraveled. Herein, we directly follow the ultrafast carrier and photon dynamics of colloidal SnSe2 nanosheets in real time using ultrafast transient absorption spectroscopy. We show ∼0.3 ps intervalley relaxation process of photoexcited energetic carriers and ∼3 ps carrier defect trapping process with a long-lived trapped carrier (∼1 ns), highlighting the importance of trapped carriers in optoelectronic devices. In addition, ultrashort laser pulse impulsively drives coherent out-of-plane lattice vibration in SnSe2, indicating strong electron−phonon coupling in SnSe2. This strong electron−phonon coupling could impose a fundamental limit on SnSe2 photovoltaic devices but benefit its thermoelectric applications.
T
transition metal dichalcogenides (TMD, e.g., MoS2, WSe2), which have been extensively investigated.21−27 In this study, we performed ultrafast and broadband transient absorption study on solution-grown SnSe2 colloidal nanosheets (NSs). Compared to mechanical exfoliation or chemical vapor deposition (CVD), colloidal synthesis and solution processing have been extensively applied in SnSe2 optoelectronic and thermoelectric devices because of their facile tunability, low-cost and largescale production.11,14,15,28 Herein, we employed femtosecond laser pulse with duration shorter than phonon period to directly visualize carrier and phonon dynamics in real time. We observed a ∼0.3 ps hot carrier intervallery relaxation process from M valley to Γ valley and a subsequent ∼3 ps carrier defect tapping process with a much longer-lived trapped carrier. In addition, the impulse excitation generated coherent A1g out-ofplane phonon mode with a frequency of 186 cm−1 and a coherence lifetime of ∼3 ps at room temperature, which periodically modulated the optical transitions through strong electron−phonon interaction. SnSe2 NSs were synthesized according to reported one-pot colloidal method with slight modification on the precursor concentration and reaction temperature.29 Briefly, a mixture of SnCl4·5H2O, SeO2, 1,10-phenanthroline, and oleylamine is added into a three-neck flask at room temperature and then
he great success of graphene has triggered intense research interests on two-dimensional (2D) layered materials because of their exotic physical properties arising from anisotropic crystal/electronic structures, weak interlayer van der Waals interactions, and rich and tunable compositions and phases.1−3 Among them, group IV metal chalcogenide semiconductors especially Tin chalcogenides, exhibit favorable properties such as high stability, earth-abundance, and environment-benignity and possess suitable bandgap (1.0− 1.5 eV) with large absorption coefficient.4 Therefore, they have been extensively explored with great potential in photonics,5 optoelectronics,6−10 photovoltaics,11,12 and thermoelectric applications.13−15 In particular, because of its large electron affinity, SnSe2 has been almost exclusively employed as a constituent in 2D (near-) broken gap (type III) heterojunction optoelectronic devices with superior performance.10,16−18 The performance of optoelectronic and thermoelectric devices is ultimately dictated by their intrinsic carrier dynamics, including the relaxation of energetic hot carriers, the recombination of band edge carriers and additionally, and the coupling of these carriers with lattice vibrations (phonons). These carrier and phonon dynamics govern the carrier/energy loss pathways and thus the device efficiencies. Despite numerous reports on material growth and device demonstrations, the intrinsic carrier and phonon dynamics in tin chalcogenides layered materials have been rarely studied,19,20 which has hindered the rational design and optimization of devices. This is in contrast to graphene and group VIB−VI © 2019 American Chemical Society
Received: May 22, 2019 Accepted: June 18, 2019 Published: June 18, 2019 3750
DOI: 10.1021/acs.jpclett.9b01470 J. Phys. Chem. Lett. 2019, 10, 3750−3755
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The Journal of Physical Chemistry Letters heated up to 200 °C under an Ar atmosphere to react for 20 h. The growth of NSs can be attributed to orientated attachment mechanism.29 Details of sample preparation can be found in the Supporting Information. The morphology and lateral size of SnSe2 NSs can be controlled by adjusting the radio of Sn vs Se precursors. (Figure S1) In this study, we focus on NSs with small lateral size (corresponding to Sn/Se ratio = 1/2) for better dispersion and optical study. Figure 1a and inset show representative
characterizations indicate hexagonal single crystalline SnSe2 NSs with uniform thickness and element valence. We first measured the steady state optical absorption spectrum of colloidal SnSe2 NSs dispersed in chloroform from ultraviolet (UV) to near-infrared (NIR) range (Figure 2a). It exhibits a strong absorption covering whole UV−visible
Figure 1. Structural characterization of SnSe2 NSs. (a) Representative TEM image of SnSe2 NSs. The NSs are inclined to stack with each other on TEM grid. Inset: magnification of red circle region, showing an individual NS. (b) Powder XRD pattern of SnSe2 NSs. Inset: top and side views of SnSe2 crystal structure (hexagonal, P3̅m1 space group). (c) HR-TEM image and SAED pattern (inset) of a single SnSe2 NS. (d) XPS spectrum of SnSe2 NSs. The left is the Sn 3d region, and the right is the Se 3d region.
Figure 2. Optical transitions of SnSe2 NSs. (a) Steady-state absorption spectrum of SnSe2 NSs. The red dashed line is the fit of absorption curve by a squareroot dependence, yielding a direct band gap is ∼1.29 eV. Inset: the indirect band gap is determined to be ∼1.1 eV. (b) TA spectrum of SnSe2 NSs after 2.38 eV (520 nm) excitation at 0.4 ps delay time. (c) DFT calculated band structure of thick SnSe2 NSs. The VBM and CBM are located at a point along the Γ-K line and the Γ point, respectively. The arrows indicate two direct transitions (T1 and T2) and lowest energy indirect transition (T0).
TEM images of SnSe2 NSs with a lateral size of 100−200 nm. We characterized the thickness of SnSe2 NSs by AFM (Figure S2), and all NSs exhibit a uniform thickness of ∼9 nm, corresponding to a ∼9 Se−Sn−Se van der Waals layers considering the presence of oleylamine ligands.28,30 The XRD patterns (Figure 1b) elucidate the crystal structure of SnSe2 NSs and all the diffraction peaks can be indexed to the hexagonal SnSe2 (JCPDS No.89-2939) without extra peaks from, e.g., orthorhombic phase. Lattice constants a (in-plane) and c (out-of-plane) can also be determined from XRD results to be 0.383 and 0.613 nm, respectively. The single crystallinity of SnSe2 NSs was further verified by high-resolution transmission electron microscope (HRTEM) (Figure 1c). The 0.332 nm lattice spacing on HRTEM image corresponds to the (100) crystal plane of hexagonal SnSe2 and the lattice constant a = 0.383 nm. The selective-area electron diffraction (SAED) pattern of an individual SnSe2 NS shows a clean singlecrystalline pattern with typical hexagonal symmetry, consistent with XRD results. In addition, we performed X-ray photoelectron spectroscopy (XPS) on SnSe2 NSs (Figure 1d). The Sn 3d3/2 and 3d5/2 doublet peaks with a splitting of 8.45 eV can be assigned to Sn (IV) and 3d5/2 and 3d3/2 peaks of Se (II) have appeared as a board peak at 53.41 eV.31,32 All these
range and extending to NIR. Fitting the absorption spectrum tail at NIR range yields a direct bandgap of ∼1.29 eV (denoted as T1) (red dashed line in Figure 2a) and an indirect bandgap of ∼1.1 eV (denoted as T0) (inset in Figure 2a), consistent with reported bandgap values on similar colloidal SnSe2 NSs11,14 and the indirect bandgap nature of SnSe2. In addition, we also observe a broad featureless band at about ∼2 eV (blue circled region, denoted as T2), suggesting the presence of another high-energy transition. To better visualize these optical transitions, we show a representative TA spectrum at 0.4 ps delay after 2.38 eV photoexcitation (Figure 2b). As a difference spectrum that is more sensitive to optical resonance, the TA spectrum clearly exhibits two bleach peaks at ∼1.24 eV (T1) and ∼1.98 eV (T2), in good agreement with optical transitions on steady state absorption spectra. We did not observe any photoluminescence from SnSe2 nanosheets, which can be ascribed to the indirect bandgap property and ultrafast carrier trapping revealed later. We performed first-principles calculations on SnSe 2 electronic structure to reveal the transition nature of these peaks (see the Supporting Information for calculation details). 3751
DOI: 10.1021/acs.jpclett.9b01470 J. Phys. Chem. Lett. 2019, 10, 3750−3755
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The Journal of Physical Chemistry Letters The calculated lattice constants a (3.83 Å) and c (6.15 Å) of SnSe2 are consistent with previous study33 and the structural characterizations above. The calculated electronic structure of a thick SnSe2 NSs is shown in Figure 2c. The conduction band minimum (CBM) is at Γ point and valence band maximum (VBM) is at a point located along the Γ−K line (close to Γ point), indicating indirect band gap (T0) nature. From calculated band structure, we can also identify two direct transitions at Γ point and M point, corresponding to T1 and T2 optical transitions, respectively. Further orbital analysis indicates T1 and T2 originate from Se-pz to Sn-s and Se-py to Sn-s transitions, respectively. The T0 and T1 transitions share the same CBM and are close in energy while T2 transition is much higher. Optical excitation of T2 transition will lead to carrier intervalley relaxation from T2 to T1/T0. In the following, we carried out broadband femtosecond transient absorption (TA) spectroscopy to investigate the carrier and phonon dynamics in SnSe2 NSs. We excited SnSe2 NSs with a < 40 fs pulse centered at 2.38 eV (520 nm) and 1.45 eV (850 nm) to selectively excite T2 and T1 transitions, respectively, and probed the transmission change (ΔT/T) with a broadband white light pulse (see the Supporting Information). Such a short laser pulse is critical to investigate phonon dynamics in real time. The color plots of TA spectra at 2.38 eV (520 nm) and 1.45 eV (850 nm) excitations are shown in Figure 2a,c. From TA spectra, we clearly observed two positive peaks centered at ∼1.2 and ∼2 eV, respectively, corresponding to the ground state bleach of T1 and T2 transitions. We also observed a broad negative peak centered at ∼1.7 eV, corresponding to photoinduced absorption (PA) of excited state carrier. The TA kinetics of T1, T0, and PA under 2.38 eV (520 nm) and 1.45 eV (850 nm) excitations are shown in Figure 2b,d. Besides population evolution, we also observed prominent oscillations superimposed on TA kinetics, which can be better resolved on TA map with linear delay time (Figure S3). The oscillatory behavior is evident across the entire probe wavelength range, a signature of coherent phonon generation as will be discussed later. Generally, the ground state bleach on TA spectra of semiconductor nanomaterials is dominated by direct phase space filling (band filling) with additional contribution from Columbic interaction (e.g., Stark effect, scattering, screening). The former requires direct carrier occupation while the later does not. The relative contribution of electron and hole occupation to TA signal is determined by their effective mass.34 In SnSe2, the hole effective mass is about an order of magnitude larger than that of the electron (Table S1). Therefore, the bleach signal on SnSe2 TA spectra is dominated by photoexcited electrons in conduction band. We note the fluence dependence of TA signal was always within linear range for all measurements. As shown in Figure 3a,b, after 2.38 eV excitation, which is close to T2 transition, T2 bleach appears instantaneously and then partially recovers in 1 ps. The recovery of T2 bleach is accompanied by formation of T1 bleach. This process is absent under the 1.45 eV excitation condition where we directly excited T1 (Figure 3c,d). Given the electronic states associated with T1 and T2 transition, this indicates photoexcited carriers at the M point relaxes into the Γ point through the intervalley scattering process (Figure 4b). A single exponential fitting on T2 decay and T1 rise yields a relaxation lifetime of ∼0.3 ps, which is on the same order as that in other low-dimensional semiconductor nanomaterials. A power dependent study shows
Figure 3. TA spectra and carrier dynamics of colloidal SnSe2 NSs. Color plot of TA spectra after (a) 2.38 eV (520 nm) and (c) 1.45 eV (850 nm) excitations. Corresponding TA kinetics at T1, T2, and PA peaks after (b) 2.38 eV (520 nm) and (d) 1.45 eV (850 nm) excitations. The blank area in the color plot is due to the scattering pump light.
Figure 4. Trapped carrier decay and scheme of carrier dynamics. (a) Decay kinetics of PA signal at various power, showing negligible power dependence. The marks are experimental data and the solid lines are their exponential fits. (b) Schematic depiction of carrier dynamics of SnSe2 NSs after 2.38 and 1.45 eV excitations.
this intervalley relaxation process does not depend on excitation density (Figure S5), suggesting carrier−phonon interaction or an Auger type of cooling process for dissipating excess energy. After hot carrier relaxation, T1 bleach and the remaining T2 bleach recover together and form a new broad PA feature at ∼1.7 eV. This process can be better visualized from TA spectra at 1.45 eV excitation where we directly excite T1 transition (Figure 3c,d). Under 1.45 eV excitation, T1 bleach and a small amount of T2 bleach appear instantaneously and decay in the same way, accompanied by formation of a broad PA feature. 3752
DOI: 10.1021/acs.jpclett.9b01470 J. Phys. Chem. Lett. 2019, 10, 3750−3755
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The Journal of Physical Chemistry Letters The decays of T1 bleach to PA are similar for both 1.45 and 2.38 eV excitations. Interestingly, we expect no band filling for the T2 transition under direct T1 excitation; thus, the bleach at T2 is likely due to pure Columbic effect. The presence of carrier in the Γ valley can reduce the T2 oscillator strength at the M point through a scattering or screening effect. A single exponential fitting on bleach recovery kinetics of T1 and T2 and formation kinetics of PA yields a conversion lifetime of ∼3 ps. Since the electron in the T1 bleach is already at the lowest energy CBM, the decay of the T1 bleach to PA can only be attributed to the localization process of conduction band electrons by either lattice defect-trapping or lattice deformation induced self-trapping (Figure 4b). The former is extrinsic while the latter is intrinsic even in a perfect crystal lattice. We performed a similar TA study on a mechanically exfoliated SnSe2 single crystal flake (see Figure SI8). The exfoliated SnSe2 flake shows a much longer carrier lifetime, which indicates the ∼3 ps carrier localization in colloidal SnSe2 NSs is likely due to the defect trapping process. It is known that colloidal nanocrystals usually have various kinds of defects at the surface or edges, which can trap carriers.35,36 Fitting the decay kinetics of PA yields a trapped carrier lifetime of ∼500 ps (Figure 4a). We also performed a power dependent study on the carrier trapping process and the decay process of trapped carrier. As shown in Figure S6 and Figure 4a, both processes show no dependence on excitation density, confirming first-order carrier trapping and trapped carrier recombination processes. The PA signal originates from the transition of the trapped carrier to even higher energy levels by the probe beam. However, the exact nature of the trap states in colloidal SnSe2 NSs is unclear yet and requires further experimental and theoretical efforts. In addition to population dynamics, we also observed obvious oscillations on TA kinetics across the entire probe range under both 2.38 eV (Figure 5) and 1.46 eV (Figure S7) excitations, indicating strong coherent phonon (CP) generation in SnSe2 NSs under pulse laser excitation even without rigid polarization control. CP generation through femtosecond pump−probe has also been observed in multilayer graphene,27 black phosphorus,37 and transition metal dichalcogenides22,23 2D materials and has been generally ascribed to interlayer vibration mode (e.g., shearing mode, breathing mode). To extract the frequency of vibration mode observed in SnSe2, we converted time domain TA kinetics into a frequency domain spectrum through fast Fourier transformation (FFT) and plotted the frequency spectra as a function of probe wavelength with the mode intensity normalized to maximum. The full FFT map under 2.38 eV excitation is shown in Figure 4a and that of 1.46 eV excitation in Figure S7b. Under both excitation conditions, the FFT map shows a prominent mode with frequency at 185.7 ± 0.5 cm−1 and another rather weak mode at ∼260 cm−1. The latter one originates from ligand molecules, as can be confirmed by neat ligand solution (Figure S7). This coherent vibration suggests SnSe2 NSs can be lightdriven nanoscale oscillators at 5.6 THz, in analogy to nanoelectromechanical systems. To help understand the dominant CP mode at ∼185.7 cm−1, steady-state Raman spectroscopy (with ∼125 cm−1 lower end limited by Raman filter) was performed on colloidal SnSe2 NSs film (Figure 4a right panel). The Raman spectrum shows a distinct peak at 187.4 ± 0.1 cm−1, matching well with the FFT frequency spectrum. This vibration frequency corresponds to the A1g mode with chalcogen atoms stretching out-of-plane
Figure 5. CP dynamics of SnSe2 NSs. (a) 2D color plot of FFT generated frequency spectra as a function of probe energy at 2.38 eV excitation. The right shows the FFT frequency spectrum at 1.65 eV and steady state Raman spectra of SnSe2 NSs. (b) Typical CP oscillation and its damping process after subtracting population decay.
(Figure 4a scheme).33,38 Interestingly, the in-plane Eg vibration mode (∼115 cm−1) of SnSe2, which shows up on steady state Raman spectra33,38 did not appear on FFT frequency spectrum. Depending on the associated electronic state, the CP oscillations under impulsive excitation can arise from two mechanisms: (i) impulsive stimulated Raman scattering (ISRS) or (ii) displacive excitation of coherent phonons (DECP).39,40 In ISRS, a pump pulse impulsively excites ground state Raman-active lattice vibrations, producing phasecoherent phonons while in DECP, photoexcitation to excite states suddenly shifts the potential energy surface, kick-starting the vibrational mode on the electronic excited state coherently. Merlin et al. unified them by rendering DECP as a resonant case of ISRS, i.e., impulsive stimulated resonant Raman scattering.41,42 The generated CP periodically modulates the electric susceptibility thus optical transitions in material via electron−phonon coupling, which is detected by probe pulse. The resulting nuclear oscillation usually follows a sine function of time in ISRS but cosine in DECP mechanism and in particular, only A1 totally symmetric modes can be detected in DECP.39,40 To gain more CP properties, we subtracted the nonoscillatory electronic contribution from TA kinetics (Figure S7c) and obtained the pure oscillatory component (Figure 5b). We fit the damping oscillation with a damped since function A sin(2πf t + φ) exp(−t/τ), where f is the frequency, φ is the oscillation phase, and τ is the phonon lifetime (dephasing time), which is correlated with the Raman line width. Our data can be better described by a cosine wave (φ ∼ 2), together with only symmetric A1g mode observed on FFT frequency spectrum, suggesting DECP mechanism with 3753
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strong electron−phonon interaction at excited state for SnSe2. The CP lifetime τ of ∼2.8 ps matches very well with band edge carrier lifetime. The amplitude profile of CP mode at different probe wavelengths is plotted in Figure 5a top. It shows a smaller amplitude at T1 and T2 transitions and a larger value in between. The amplitude of CP oscillation largely depends on the coupling between electric susceptibility and lattice vibrations of materials. In the pump−probe experiment, impulsive excitation triggers the A1g out-of-plane lattice vibration mode, which modulates the interlayer interaction. As both T1 and T2 optical transitions originate from the Se-p orbital to Sn-s orbital transition, which depends on the interlayer interaction sensitively,33 the A1g vibration mode will translate into the modulation of the T1 and T2 optical transition. This modulation is strongest where the slope of the electronic transition is largest and is reduced at resonance peak position.37,43 The exact amplitude profile will also be strongly affected by sample inhomogeneous broadening. As an interesting comparison, CP is usually too weak to be directly observed for TMDs like MoS2 and WSe2 in the pump−probe experiment.21,23−26 Lastly, it is important to note that the CP is not initiated by the carrier relaxation or trapping as these processes occur on a much longer time scale (approximately picoseconds) compared to the phonon oscillation period (∼180 fs). In conclusion, we have followed directly in time domain the motion of both excited carriers and out-of-plane A1g lattice phonons in colloidal SnSe2 2D van der Waals NSs using ultrafast transient absorption spectroscopy. We found the photoexcited energetic carriers in the high-energy M valley relax to the Γ valley with a lifetime of ∼0.3 ps. After that, the band edge carriers localize to traps with a lifetime of ∼3 ps. The trapped carriers are much-longer lived, relaxing back to the ground state in ∼1 ns. Considering the fast carrier trapping and long-lived trapped carrier, the operation of colloidal SnSe2based photovoltaic and photocatalytic devices reported previously likely involve the transport and conversion of trapped carriers rather than band edge carriers. This calls for reconsidering the design and optimization strategies of SnSe2 devices. Our study also showed an ultrashort laser pulse could impulsively drive coherent out-of-plane lattice motion coupled with electronic transition, indicating strong electron−phonon coupling in SnSe2. This strong electron−phonon coupling could impose a fundamental limit on SnSe2 photovoltaic devices but benefit its thermoelectric applications. The combined carrier and phonon dynamics in real time provide a complete picture of the carrier/energy loss pathways and appear of general interest for other colloidal 2D van der Waals materials and devices.
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Letter
AUTHOR INFORMATION
Corresponding Author
*Email:
[email protected]. ORCID
Xiaolong Zou: 0000-0002-3987-6865 Haiming Zhu: 0000-0001-7747-9054 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the financial support from the National Natural Science Foundation of China (21773208) and National Key Research and Development Program of China (2017YFA0207700, 2017YFB0701600), Shenzhen Basic Research Projects (JCYJ20170407155608882), Guangdong Innovative and Entrepreneurial Research Team Program (2017ZT07C341).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01470. Morphology control of SnSe2 nanoparticle, thickness characterization, calculated effective mass, more data of 2.38 and 1.45 eV excitation and coherent phonon (TA spectra, kinetics power-dependence, fitting lifetimes, associated probe energy-resolved Fourier-transformed maps), experimental details (PDF) 3754
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The Journal of Physical Chemistry Letters
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DOI: 10.1021/acs.jpclett.9b01470 J. Phys. Chem. Lett. 2019, 10, 3750−3755
